DESCRIPTIONThe EUP2410 is a highly efficient, synchronous, fixedfrequency, current-mode step-up converter with output toinput disconnect. When EUP2410 is disabled, the internalconduction path from SW to OUT is fully blocked and theOUT pin is isolated from the battery. This output disconnect feature reduces the shutdown current to typically only 50nA. The 500KHz switching frequency allows for smaller external components producing a compact solution for a wide range of load currents. Highly integration and internalcompensation network minimizes as 5 external components,N-Channel switch and P-Channel Synchronous switchintegration will greatly improve converter efficiency. Internal soft-start function also reduce inrush current. The EUP2410 regulates the output voltage up to 6V from either a 2-cell NiMH/NiCd or a single-cell Li-Ion battery.The EUP2410 is offered in a thin SOT23-5 package.Typical Application CircuitFEATURESz Over 90% Efficiencyz 1.6A Typical Switch Current Limitz 500KHz Fixed Switching Frequencyz Output to Input Disconnect at Shutdown Mode z Internal Synchronous Rectifierz Internal Soft-Startz Internal Compensation z 50nA Shutdown Current z Thermal Shutdownz 5-Pin TSOT-23 Packagez RoHS compliant and 100% lead(Pb)-freeAPPLICATIONS z GPS PND z Handheld Digital Audioz Digital Still and Video Camerasz White LED FlashFigure1.DS2410 Ver 1.0 Apr. 2009 2Pin ConfigurationsPackage TypePin ConfigurationsTSOT23-5Pin DescriptionPIN NAME DESCRIPTION1 FBRegulation Feedback Input. Connect to an external resistive voltage divider from theoutput to FB to set the output voltage.2 GNDGround.3 OUT Supply Input for the EUP2410. Connect to the output of the converter.4 SW Output Switching Node. SW is the drain of the internal low-side N-Channel MOSFETand high-side P-Channel MOSFET. Connect the inductor to SW to complete thestep-up converter.5 EN Regulator On/Off Control Input. A logic high input (VEN > 1.4V) turns on theregulator. A logic low input (VEN < 0.4V) puts the EUP2410 into low currentshutdown mode.Block DiagramFigure 2.Ordering InformationOrder Number Package Type Marking Operating Temperature Range EUP2410OIR1 TSOT23-5 n0XXXX -40 °C to +85°CEUP2410Lead Free Code1: Lead Free 0: LeadPackingR: Tape & ReelOperating temperature rangeI: Industry StandardPackage TypeO: TSOT3DS2410 Ver 1.0 Apr. 2009Absolute Maximum RatingsFB, OUT, EN to GND --------------------------------------------------------------------------------------- -0.3V to 6VSW Current (Note 1) ------------------------------------------------------------------------------------- -1.6A to +1.6ASupply V oltage V IN -------------------------------------------------------------------------------------------- 2.2V to 6VOutput V oltage V OUT ------------------------------------------------------------------------------------------ 2.5V to 6VOperating Temperature Rang --------------------------------------------------------------------------- -40°C to +85°CStorage Temperature Rang, T stg ----------------------------------------------------------------------- -65°C to +150°CThermal ResistanceθJA (TSOT) ------------------------------------------------------------------------------------------------ --- 220°C/WNote1: SW has internal clamp diodes between GND-SW and SW-OUT. Applications that forward bias these diodesshould take care not to exceed the IC's package power dissipation limits.Electrical CharacteristicsV IN = 2.4V, V OUT = 3.5V, C IN=10uF, C OUT = 10µF, L1=4.7µH, R1 =178KΩ, R2 =100KΩ, T A= -40°C to +85°C, unlessotherwise noted. Typical values are at T A = +25°C.)EUP2410UnitParameter ConditionsMin Typ Max.Supply V oltage 2.2 5 VOutput V oltage Range 2.5 6 V0.05µA1=5VSupply Current (Shutdown) V EN=V OUT=0V, V SW0.39mA Supply Current V FB= 1.3VFeedback V oltage 1.2 1.25 1.3 VnA50Feedback Input Current V FB= 1.2VSwitching Frequency 310 500 690 KHzMaximum Duty Cycle 80 85 90 %EN Input Low V oltage 0.4 VEN Input High V oltage 1.4 VEN Pull Down Resistor 1 MΩLow-Side On Resistance V OUT=3.3V by design 450 mΩLow-Side Current Limit 1 1.6 2 AHigh-Side On Resistance V OUT=3.3V by design 650 mΩThermal Shutdown Note 2 160 °CThermal Shutdown Hysteresis Note 2 30 °CNote 2: Guaranted By Design .4DS2410 Ver 1.0 Apr. 2009DS2410 Ver 1.0 Apr. 20095Typical Operating CharacteristicsOperating Conditions¡G V IN = 2.4V , V OUT = 3.5V , C IN =10uF, C OUT = 10µF, L1=4.7µH, R1 =178K Ω, R2 =100K ΩI LOAD =400mAContinuous Mode OperationI LOAD =20mATransient ResponseI LOAD =40mA to 400mA StepVSWVOUT IINUCTOR VSWVOUTIINUCTORILOADVOUTDS2410 Ver 1.0 Apr. 20096Typical Operating Characteristics (continued)Operating Conditions¡G V IN = 2.4V , V OUT = 3.5V , C IN =10uF, C OUT = 10µF, L1=4.7µH, R1 =178K Ω, R2 =100K ΩStartupR LOAD =16ΩFeedback Voltage vs TemperatureVEN VSWVOUTDS2410 Ver 1.0 Apr. 2009 7OperationThe EUP2410 uses a 500KHz fixed-frequency, current-mode regulation architecture to regulate the output voltage. The EUP2410 measures the output voltage through an external resistive voltage divider and compares that to the internal 1.25V reference to generate the error voltage. The current-mode regulator compares the error voltage to the inductor current to regulate the output voltage. The use of current-mode regulation improves transient response and control loop stability.When the EUP2410 is disabled (EN = Low), both power switches are off. There is no current path from SW to OUT. Therefore, the output voltage discharges to ground. When the EUP2410 is enabled (EN = High), a limited start-current charges the output capacitor through the P-Channel MOSFET until the output voltage rising to SW, then the part operates in force PWM mode for regulating the output voltage to the target value.At the beginning of each cycle, the N-Channel MOSFET switch is turned on, forcing the inductor current to rise. The current at the source of the switch is internally measured and converted to a voltage by the current sense amplifier. That voltage is compared to the error voltage. When the inductor current rises sufficiently, the PWM comparator turns off the switch, forcing the inductor current to the output capacitor through the internal P-Channel MOSFET rectifier, which forces the inductor current to decrease. The peak inductor current is controlled by the error voltage, which in turn is controlled by the output voltage. Thus the output voltage controls the inductor current to satisfy the load .Soft-StartThe EUP2410 includes a soft-start timer that limits the voltage at the error amplifier output during startup to prevent excessive current at the input. This prevents premature termination of the source voltage at startup due to inrush current. This also limits the inductor current at startup, forcing the input current to rise slowly to the amount required to regulate the output voltage during soft-start.Application InformationComponent SelectionSetting the Output VoltageSet the output voltage by selecting the resistive voltage divider ratio. The voltage divider drops the output voltage to the 1.25V feedback voltage. Use a 100k Ω resistor for R2 of the voltage divider. Determine the high-side resistor R1 by the equation:Where V OUT is the output voltage, V FB is the 1.25V feedback voltage and R2=100k Ω.Selecting the Input CapacitorAn input capacitor is required to supply the AC ripple current to the inductor, while limiting noise at the input source. Multi-layer ceramic capacitors are the best choice as they have extremely low ESR and are available in small footprints. Use an input capacitor value of 4.7µF or greater. This capacitor must be placed physically close to the device.Selecting the Output CapacitorA single 4.7µF to 10µF ceramic capacitor usually provides sufficient output capacitance for most applications. Larger values up to 22µF may be used to obtain extremely low output voltage ripple and improve transient response. The impedance of the ceramic capacitor at the switching frequency is dominated by the capacitance, and so the output voltage ripple is mostly independent of the ESR. The output voltage ripple V RIPPLE is calculated as:Where V IN is the input voltage, I LOAD is the load current, C2 is the capacitance of the output capacitor and f SW is the 500KHz switching frequency.Selecting the InductorThe inductor is required to force the output voltage higher while being driven by the lower input voltage. A good rule for determining the inductance is to allow the peak-to-peak ripple current to be approximately 30%-50% of the maximum input current. Make sure that the peak inductor current is below the minimum current limit at the duty cycle used (to prevent loss of regulation due to the current limit variations).Calculate the required inductance value L using the equations:−=2R V V V 1R FB FBOUT ()SW OUT IN OUT LOAD RIPPLE f 2C V V V IV ××−=()I f V V V V L sw OUT IN OUT IN∆××−×=η××=IN )MAX (LOAD OUT )MAX (IN V I V I )MAX (IN I %)50%30(I −=∆Where I LOAD(MAX) is the maximum load current, ∆I is the peak-to-peak inductor ripple current and b is efficiency. For the EUP2410, typically, 4.7µH is recommended for most applications. Choose an inductor that does not saturate at the peak switch current as calculated above with additional margin to cover heavy load transients and extreme startup conditions. Layout ConsiderationsHigh frequency switching regulators require very careful layout for stable operation and low noise. All components must be placed as close to the IC as possible. All feedback components must be kept close to the FB pin to prevent noise injection on the FB pin trace. The ground return of C1 and C2 should be tied close to the GND pin. See the EUP2410 demo board layout for reference.Figure 3. 5V Typical Application CircuitDS2410 Ver 1.0 Apr. 20098Package InformationTSOT23-5MILLIMETERS INCHES SYMBOLSMIN. MAX. MIN. MAX.A - 1.00 - 0.039A1 0.00 0.15 0.000 0.006D 2.90 0.114E1 1.60 0.063E 2.60 3.00 0.102 0.118L 0.30 0.60 0.012 0.024b 0.30 0.50 0.012 0.020e 0.95 0.0379DS2410 Ver 1.0 Apr. 2009。
RT2101A®DS2101A-08 May 2022Copyright 2022 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.©Design Tools Sample &BuyReference Design General DescriptionThe RT2101A is a high efficiency step-down converter and capable of delivering 3A output current over a wide input voltage range from 2.95V to 6V.The RT2101A provides accurate regulation for a variety of loads with ±2.5% accuracy. For reducing inductor size, it provides up to 2MHz switching frequency. The efficiency is maximized through the integrated 45m Ω MOSFETs and 550μA typical supply current.Under-voltage lockout voltage of the RT2101A is 2.7V , and it also provides external setting by a resistor network on the enable pin.The RT2101A provides protections such as inductor current limit under-voltage lockout and thermal shutdown.The over-temperature threshold is 145°C.The RT2101A is available in WQFN-16L 3x3 package.2.95V to 6V Input, 3A Output, 2MHz, Synchronous Step-Down ConverterFeatures●AEC-Q100 Grade 2 Qualified ●Integrated 45m Ω MOSFETs ●Input Range : 2.95V to 6V●Adjustable PWM Frequency : 700kHz to 2MHz ●Output Current : 3A ●95% Efficiency●Adjustable Soft-Start ●Power Good Indicator ●Enable Control●Under-Voltage Lockout ●Current Limit ●Thermal ShutdownApplications●Low-Voltage, High-Density Power Systems ●Distributed Power Systems ●Point-of-Load ConversionsSimplified Application CircuitVOUTMarking Information72= : Product CodeYMDNN : Date CodeOrdering InformationNote :Richtek products are :❝ RoHS compliant and compatible with the current require-ments of IPC/JEDEC J-STD-020.❝ Suitable for use in SnPb or Pb-free soldering processes.RT2101AG : Green (Halogen Free and Pb Free)RT2101A©Copyright 2022 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.Functional Block DiagramPin Configuration(TOP VIEW)WQFN-16L 3x3GNDGND VIN VIN SW SW SS/TRSW A G N D C O M P F B R T /S Y N CI N N O O TG O O DDS2101A-08 May 2022RT2101A©Copyright 2022 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.OperationThe RT2101A is a synchronous step-down DC-DC converter with two integrated power MOSFETs. It can deliver up to 3A output current from a 2.95V to 6V input supply. The RT2101A's current mode architecture allows the transient response to be optimized over a wider input voltage and load range. Cycle-by-cycle current limit provides protection against shorted outputs and soft-start eliminates input current surge during start-up. The RT2101A is synchronizable to an external clock with frequency ranging from 700kHz to 2MHz. The RT2101A is available in WQFN-16L 3x3 package.High-side MOSFET peak current is measured by internal sensing resistor. The Current Signal is where Slope Compensator works together with sensing voltage sensing resistor. The error amplifier adjusts COMP voltage by comparing the feedback signal (V FB ) from the output voltage with the internal 0.827V reference. When the load current increases, it causes a drop in the feedback voltage relative to the reference, the COMP voltage then rises to allow higher inductor current to match the load current.EN ComparatorThe RT2101A is enable when EN pin is higher than 1.25V.It is disable when EN pin lower than 1.18V. There is an internal pull-high current source to charge the EN pin to high when the EN pin is floating.Oscillator (OSC)The internal oscillator that provides switching frequency from 700kHz to 2MHz. It is adjusted using an external timing resistor. It also can be synchronized by an external clock in the range between 700kHz and 2MHz from RT/SYNC pin.PGOOD ComparatorWhen the feedback voltage (V FB ) rises above 93% or falls below 107% of reference voltage the PGOOD open drain output will be high impedance. The PGOOD open drain output will be internally pulled low when the feedback voltage (V FB ) falls below 88% or rises above 113% of reference voltage.Soft-Start (SS)An internal current source (2.2μA) charges an external capacitor to build the soft-start ramp voltage (V SS ). The V FB voltage will track the V SS during soft-start interval.The Soft-Start setting capacitor (C SS ) for the Soft-Start time (t SS ) can be easily calculated by the following equation :Over-Temperature Protection (OTP)The RT2101A implement an internal over-temperature protection. When junction temperature is higher than 145°C, it will stop switching. Until the junction temperature decreases below 125°C, the RT2101A will re-soft-start from initial condition.SS SS t (ms) 2.2 (A)C (nF) =0.827 (V)⨯μRT2101A©Copyright 2022 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.Electrical Characteristics(V IN= 5V, C IN = 10μF, T A = T J = −40°C to 105°C, unless otherwise specified)Absolute Maximum Ratings (Note 1)●Supply Input Voltage, VIN ----------------------------------------------------------------------------------------- −0.3V to 7V●Switch Node Voltage, SW ----------------------------------------------------------------------------------------- −1V to (V IN + 0.3V)<100ns ----------------------------------------------------------------------------------------------------------------- −5V to (V IN + 5V)●BOOT to SW --------------------------------------------------------------------------------------------------------- −0.3V to 7V●Other Pins------------------------------------------------------------------------------------------------------------- −0.3V to (V IN + 0.3V)●Power Dissipation, P D @ T A = 25°CWQFN-16L 3x3------------------------------------------------------------------------------------------------------3.33W●Package Thermal Resistance (Note 2)WQFN-16L 3x3, θJA -------------------------------------------------------------------------------------------------30°C/W WQFN-16L 3x3, θJC ------------------------------------------------------------------------------------------------7.5°C/W ●Junction T emperature -----------------------------------------------------------------------------------------------150°C ●Lead Temperature (Soldering, 10 sec.)-------------------------------------------------------------------------260°C●Storage T emperature Range -------------------------------------------------------------------------------------- −65°C to 150°C ●ESD Susceptibility (Note 3)HBM (Human Body Model)----------------------------------------------------------------------------------------2kVRecommended Operating Conditions (Note 4)●Supply Input Voltage ------------------------------------------------------------------------------------------------2.95V to 6V ●Junction T emperature Range -------------------------------------------------------------------------------------- −40°C to 125°C ●Ambient T emperature Range -------------------------------------------------------------------------------------- −40°C t o 105°CDS2101A-08 May 2022RT2101A©Copyright 2022 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.Note 1. Stresses beyond those listed “Absolute Maximum Ratings ” may cause permanent damage to the device. These arestress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may affect device reliability.Note 2. θJA is measured at T A = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC ismeasured at the exposed pad of the package.Note 3. Devices are ESD sensitive. Handling precautions are recommended.Note 4. The device is not guaranteed to function outside its operating conditions.Note 5. Guarantee by Design .RT2101A©Copyright 2022 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.Typical Application CircuitTable 1. Recommended Component SelectionV V OUT 1.8VDS2101A-08 May 2022RT2101A©Copyright 2022 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.Typical Operating CharacteristicsReference Voltage vs. Temperature0.800.810.820.830.840.85-50-25255075100125Temperature (°C)R e f e r e n c e V o l t a g e (V )Frequency vs. Temperature9009209409609801000-50-25255075100125Temperature (°C)F r e q u e n c y (k H z )Frequency vs. Input Voltage0.920.930.940.950.960.970.980.991.001.011.0233.544.555.56Input Voltage (V)F r e q u e n c y (M H z )Efficiency vs. Output Current1020304050607080901000.0010.010.1110Output Current (A)E f f i c i e n c y (%)Efficiency vs. Output Current1020304050607080901000.0010.010.1110Output Current (A)E f f i c i e n c y (%)Output Voltage vs. Output Current1.801.811.821.831.841.850.511.522.53Output Current (A)O u t p u t V o l t a g e (V)RT2101A©Copyright 2022 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.Output Ripple Time (500ns/Div)I L (1A/Div)V OUT (10mV/Div)V SW(5V/Div)V IN = 3.3V, V OUT = 1.8V, I OUT = 1ACurrent Limit vs. Temperature6.06.67.27.88.49.0-50-25255075100125Temperature (°C)C u r r e n t L i m i t (A )Current Limit vs. Input Voltage6.06.57.07.58.08.59.033.544.555.56Input Voltage (V)C u r r e n t L i m i t(A )Output RippleTime (500ns/Div)I L (2A/Div)V OUT (10mV/Div)V SW (5V/Div)V IN = 3.3V, V OUT = 1.8V, I OUT = 3AUVLO vs. Temperature2.32.42.52.62.72.82.9-50-25255075100125Temperature (°C)U V L O (V)Enable Threshold Voltage vs. Temperature1.001.081.161.241.321.40-50-25255075100125Temperature (°C)E n a b l e T h r e s h o l d V o l t a g e (V )DS2101A-08 May 2022RT2101A©Copyright 2022 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.Load Transient ResponseI OUT (1A/Div)V OUT(200mV/Div)V IN = 3.3V, V OUT = 1.8V, I OUT = 0A to 3ATime (100μs/Div)Switching Frequency vs. R RT3004005006007008009001000R RT (k )S w i t c h i n g F r e q u e n c y (k H z )ΩSwitching Frequency vs. R RT6080100120140160180200R RT (k )S w i t c h i n g F r e q u e n c y (k H z )ΩLoad Transient ResponseTime (100μs/Div)I OUT (1A/Div)V OUT(200mV/Div)V IN = 5V, V OUT = 3.3V, I OUT = 0A to 3ART2101A©Copyright 2022 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.Application InformationThe basic RT2101A application circuit is shown in Typical Application Circuit. External component selection is determined by the maximum load current and begins with the selection of the inductor value and operating frequency followed by C IN and C OUT . The switching frequency range from 700kHz to 2MHz. It is adjusted by using a resistor to ground on the RT/SYNC pin.Output Voltage SettingThe resistive divider allows the FB pin to sense the output voltage as shown in Figure 1.Figure 1. Setting the Output VoltageThe output voltage setting range is 0.827V to 3.6V and the set by an external resistive divider is according to the following equation :⎛⎫+ ⎪⎝⎭OUT FB R1V = V 1R2where V FB is the feedback reference voltage 0.827V (typ.).Inductor SelectionFor a given input and output voltage, the inductor value and operating frequency determine the ripple current. The ripple current ΔI L increases with higher V IN and decreases with higher inductance :⎡⎤⎡⎤∆⨯-⎢⎥⎢⎥⨯⎣⎦⎣⎦OUT OUT L OSC IN V V I =1f L V Having a lower ripple current reduces the ESR losses in the output capacitors and the output voltage ripple. Highest efficiency operation is achieved at low frequency with small ripple current. This, however, requires a large inductor. A reasonable starting point for selecting the ripple current is ΔI L = 0.4 (IMAX). The largest ripple current occurs at the highest V IN . To guarantee that the ripple current stays below a specified maximum, the inductor value should be chosen according to the following equation :⎡⎤⎡⎤⨯-⎢⎥⎢⎥⨯∆⎣⎦⎣⎦OUT OUT OSC L(MAX)IN(MAX)V V L =1f I V Input and Output Capacitors SelectionThe input capacitance, C IN , is needed to filter the trapezoidal current at the source of the top MOSFET. A low ESR input capacitor with larger ripple current rating should be used for the maximum RMS current. RMScurrent is given by :RMS OUT(MAX)I = I This formula has a maximum at V IN = 2V OUT , where I RMS =I OUT / 2. This simple worst case condition is commonly used for design because even significant deviations do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based on only 2000hours of life, which makes it advisable to either further derate the capacitor or choose a capacitor rated at a higher temperature than required. Several capacitors may also be placed in parallel to meet size or height requirements in the design. The selection of C OUT is determined by the effective series resistance (ESR) that is required to minimize voltage ripple, load step transients, and the amount of bulk capacitance that is necessary to ensure that the control loop is stable. Loop stability can be examined by viewing the load transient response as described in a later section. The output ripple, ΔV OUT , is determined by :⎡⎤∆≤∆+⎢⎥⎣⎦OUT L OSC OUT 1V I ESR 8f C Using Ceramic Input and Output CapacitorsHigher values, lower cost ceramic capacitors are nowbecoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. However, care must be taken when these capacitors are used at the input and output. When a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, V IN . At best, this ringing can couple with the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at V IN large enough to damage the part.V OUT11DS2101A-08 May 2022RT2101A©Copyright 2022 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.Thermal ConsiderationsFor continuous operation, do not exceed absolutemaximum junction temperature. The maximum power dissipation depends on the thermal resistance of the IC package, PCB layout, rate of surrounding airflow, and difference between junction and ambient temperature. The maximum power dissipation can be calculated by the following formula :P D(MAX) = (T J(MAX) − T A ) / θJAwhere T J(MAX) is the maximum junction temperature, T A is the ambient temperature, and θJA is the junction to ambient thermal resistance.For recommended operating condition specifications, the maximum junction temperature is 125°C. The junction to ambient thermal resistance, θJA , is layout dependent. For WQFN-16L 3x3 package, the thermal resistance, θJA , is 30°C/W on a standard JEDEC 51-7 four-layer thermal test board. The maximum power dissipation at T A = 25°C can be calculated by the following formula :P D(MAX) = (125°C − 25°C) / (30°C/W) = 3.33W for WQFN-16L 3x3 packageThe maximum power dissipation depends on the operating ambient temperature for fixed T J(MAX) and thermal resistance, θJA . The derating curve in Figure 2 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation.Figure 2. Derating Curve of Maximum Power DissipationLayout ConsiderationsFor the best performance of the RT2101A, the following guidelines must be strictly followed.❝The input capacitor should be placed as close as possible to the device pins (VIN and GND).❝The RT/SYNC pin is sensitive. The RT resistor should be located as close as possible to the IC and minimal lengths of trace.❝The SW node is with high frequency voltage swing. It should be kept at a small area.❝Place the feedback components as close as possible to the IC and keep away from the noisy devices.❝The GND and AGND should be connected to a strong ground plane for heat sinking and noise protection.0.00.40.81.21.62.02.42.83.23.60255075100125Ambient Temperature (°C)M a x i m u m P o w e r D i s s i p a t i o n (W )©Copyright 2022 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.12DS2101A-08 May 2022 RT2101AFigure 3. PCB Layout GuideW-Type 16L QFN 3x3 PackageRichtek Technology Corporation14F, No. 8, Tai Yuen 1st Street, Chupei CityHsinchu, Taiwan, R.O.C.Tel: (8863)5526789Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.DS2101A-08 May 13。
13972fStep-Down Switching Regulatorwith 75µA Quiescent CurrentThe L T ®3972 is an adjustable frequency (200kHz to 2.4MHz) monolithic buck switching regulator that accepts input voltages up to 33V (62V Maximum). A high effi ciency 95mΩ switch is included on the die along with a boost Schottky diode and the necessary oscillator, control, and logic circuitry. Current mode topology is used for fast transient response and good loop stability. Low ripple Burst Mode operation maintains high effi ciency at low output currents while keeping output ripple below 15mV in a typical application. In addition, the L T3972 can fur-ther enhance low output current effi ciency by drawing bias current from the output when V OUT is above 3V. Shutdown reduces input supply current to less than 1μA while a resistor and capacitor on the RUN/SS pin provide a controlled output voltage ramp (soft-start). A power good fl ag signals when V OUT reaches 91% of the programmed output voltage. The L T3972 is available in 10-Pin MSOP and 3mm × 3mm DFN packages with exposed pads for low thermal resistance.■Automotive Battery Regulation ■ Distributed Supply Regulation ■ Industrial Supplies ■ Wall T ransformer Regulation■Wide Input Range:Operation from 3.6V to 33VOver-Voltage Lockout Protects Circuits Through 62V T ransients ■ 3.5A Maximum Output Current■ Low Ripple (<15mV P-P ) Burst Mode ® Operation:I Q = 75μA at 12V IN to 3.3V OUT■ Adjustable Switching Frequency: 200kHz to 2.4MHz ■ Low Shutdown Current: I Q < 1μA ■ Integrated Boost Diode■ Synchronizable Between 250kHz to 2MHz ■ Power Good Flag■ Saturating Switch Design: 95mΩ On-Resistance ■ Output Voltage: 0.79V to 30V ■ Thermal Protection ■ Soft-Start Capability■ Small 10-Pin Thermally Enhanced MSOP and (3mm × 3mm) DFN Packages5V Step-Down ConverterV OUTEffi ciencyBurst Mode is a registered trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners.OUTPUT CURRENT (A)0.550E F F I C I E N C Y (%)7010012 2.53680 TA01a6090801.533.5T YPICAL APPLICATIOND ESCRIPTIONF EATURESA PPLICATIONS23972fELECTRICAL CHARACTERISTICS V IN , RUN/SS Voltage (Note 5) ...................................62V BOOST Pin Voltage ...................................................56V BOOST Pin Above SW Pin .........................................30V FB, RT, V C Voltage .......................................................5V PG, BD, SYNC Voltage . (30V)(Note 1)PARAMETER CONDITIONSMIN TYP MAX UNITSMinimum Input Voltage ●3 3.6V V IN Overvoltage Lockout ●333537V Quiescent Current from V INV RUN/SS = 0.2V 0.010.5μA V BD = 3V, Not Switching ●3065μA V BD = 0, Not Switching120160μA Quiescent Current from BDV RUN/SS = 0.2V0.010.5μAThe ● denotes the specifi cations which apply over the full operatingtemperature range, otherwise specifi cations are at T A = 25°C. V IN = 10V, V RUN/SS = 10V, V BOOST = 15V, V BD = 3.3V unless otherwise noted. (Note 2)Operating Junction Temperature Range (Note 2)L T3972E .............................................–40°C to 125°C L T3972I ..............................................–40°C to 125°C Storage Temperature Range ...................–65°C to 150°C Lead Temperature (Soldering, 10 sec)(MSE Only) .......................................................300°CTOP VIEWDD PACKAGE10-LEAD (3mm s 3mm) PLASTIC DFN1096784531121RT V C FB PG SYNCBD BOOST SW V IN RUN/SSθJA = 45°C/W , θJC = 10°C/WEXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB12345BD BOOST SW V IN RUN/SS109876RT V C FB PG SYNCTOP VIEWMSE PACKAGE10-LEAD PLASTIC MSOP 11θJA = 45°C/W , θJC = 10°C/WEXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCBPIN CONFIGURATIONORDER INFORMATIONLEAD FREE FINISH TAPE AND REEL PART MARKING*PACKAGE DESCRIPTIONTEMPERATURE RANGE L T3972EDD#PBF L T3972EDD#TRPBF LDXR 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C L T3972IDD#PBF L T3972IDD#TRPBF LDXR 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C L T3972EMSE#PBF L T3972EMSE#TRPBF L TDXS 10-Lead Plastic MSOP –40°C to 125°C L T3972IMSE#PBFL T3972IMSE#TRPBFL TDXS10-Lead Plastic MSOP–40°C to 125°CConsult L TC Marketing for parts specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed by a label on the shipping container .Consult L TC Marketing for information on non-standard lead based fi nish parts.For more information on lead free part marking, go to: http://www.linear .com/leadfree/ For more information on tape and reel specifi cations, go to: http://www.linear .com/tapeandreel/A BSOLUTE MAXIMUM RATINGSPARAMETER CONDITIONS MIN TYP MAX UNITSV BD = 3V, Not Switching●90130μAV BD = 0, Not Switching15μA Minimum Bias Voltage (BD Pin) 2.73VFeedback Voltage●780775790790800805mVmVFB Pin Bias Current (Note 3)V FB = 0.8V, V C = 1.2V●1040nA FB Voltage Line Regulation4V < V IN < 33V0.0020.01%/V Error Amp g m500μMho Error Amp Gain2000V C Source Current60μA V C Sink Current60μA V C Pin to Switch Current Gain 5.3A/V V C Clamp Voltage2VSwitching Frequency R T = 8.66kR T = 29.4kR T = 187k 2.21.02002.451.12302.71.25260MHzMHzkHzMinimum Switch Off-Time●60150nS Switch Current Limit Duty Cycle = 5% 4.6 5.4 6.2A Switch V CESAT I SW = 3.5A335mV Boost Schottky Reverse Leakage V BD = 0V0.022μA Minimum Boost Voltage (Note 4)● 1.52V BOOST Pin Current I SW = 1A3550mA RUN/SS Pin Current V RUN/SS = 2.5V58μA RUN/SS Input Voltage High 2.5V RUN/SS Input Voltage Low0.2V PG Threshold Offset from Feedback Voltage V FB Rising65mV PG Hysteresis10mV PG Leakage V PG = 5V0.11μA PG Sink Current V PG = 0.4V●200800μA SYNC Low Threshold0.5V SYNC High Threshold0.7V SYNC Pin Bias Current V SYNC = 0V0.1μANote 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime.Note 2: The L T3972E is guaranteed to meet performance specifi cations from 0°C to 125°C. Specifi cations over the –40°C to 125°C operating temperature range are assured by design, characterization and correlation with statistical process controls. The L T3972I specifi cations are guaranteed over the –40°C to 125°C temperature range.Note 3: Bias current fl ows out of the FB pin.Note 4: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch.Note 5: Absolute Maximum at V IN and RUN/SS Pins is 62V for non-repetitive 1 minute transients, and 40V for continuous operation.The● denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at T A = 25°C. V IN = 10V, V RUN/SS = 10V, V BOOST = 15V, V BD = 3.3V unless otherwise noted. (Note 2)ELECTRICAL CHARACTERISTICS33972f43972fINPUT VOLTAGE (V)S U P P L Y C U R R E N T (μA )153680 G04503051020101301109070253035DUTY CYCLE (%)0S W I T C H C U R R E N T L I M I T (A )403680 G084.520603.53.06.05.55.04.080100TEMPERATURE (°C)–50S U P P L Y C U R R E N T (μA )350253680 G05200100–25505040030025015075100150125OUTPUT CURRENT (A)0.550E F F I C I E N C Y (%)7010012 2.53680 G01609080 1.533.5OUTPUT CURRENT (A)0.550E F F I C I E N C Y (%)7010012 2.53680 G02609080 1.533.5INPUT VOLTAGE (V)5L O A D C U R R E N T (A )153680 G074.510203.53.05.55.04.02530INPUT VOLTAGE (V)5L O A D C U R R E N T (A )153680 G064.010203.02.55.55.04.53.52530TEMPERATURE (°C)S W I T C H C U R R E N T L I M I T (A )4.04.55.55.03680 G093.53.02.02.56.56.0–5025–255075100150125Effi ciencyEffi ciencyNo Load Supply CurrentMaximum Load CurrentSwitch Current LimitSwitch Current LimitMaximum Load CurrentNo Load Supply CurrentEffi ciencyT A = 25°C unless otherwise noted.OUTPUT CURRENT (A)0.550E F F I C I E N C Y (%)TOTAL POWER LOSS (W)7010012 2.53680 G036090800.51.53.01.02.52.01.533.5T YPICAL PERFORMANCE CHARACTERISTICS53972fBOOST DIODE CURRENT (A)B O O S T D I O D E V F (V )0.81.01.2 2.03680 G180.60.400.5 1.0 1.50.21.4RUN/SS PIN VOLTAGE (V)0S W I T C H C U R R E N T L I M I T (A )1.53680 G16420.512107653 2.533.5FB PIN VOLTAGE (mV)S W I T C H I N G F R E Q U E N C Y (k H z )800100012006003680 G14600400200400800500100300700900200TEMPERATURE (°C)M I N I M U M S W I T C H O N T I M E (n s )801001203680 G156040200140–5025–255075100150125RUN/SS PIN VOLTAGE (V)R U N /S S P I N C U R R E N T (μA )8101215253680 G17645102030352SWITCH CURRENT (A)B O O S T P I NC U R R E N T (m A )154560751203680 G113090105031245TEMPERATURE (°C)F E E D B A C K V O L T AG E (m V )8003680 G12760840780820–5025–255075100150125TEMPERATURE (°C)F R E Q U E N C Y (M H z )1.001.103680 G130.900.801.200.951.050.851.15–5025–255075100150125SWITCH CURRENT (A)40050070033680 G1030020012451000600V O L T A G E D R O P (m V)Boost Pin CurrentFeedback VoltageSwitching FrequencyFrequency FoldbackMinimum Switch On-TimeSoft-StartRUN/SS Pin CurrentBoost DiodeSwitch Voltage DropT A = 25°C unless otherwise noted.T YPICAL PERFORMANCE CHARACTERISTICS63972fFB PIN ERROR VOLTAGE (mV)–200–50V C P I N C U R R E N T (μA )–200200200503680 G19–40–1001004010–1030–30Error Amp Output CurrentTEMPERATURE (°C)V C V O L T A G E (V )1.502.002.503680 G221.000.50–5025–255075100150125LOAD CURRENT (mA)1I N P U T V O L T A G E (V )3.03.5100003680 G202.52.01010010005.04.54.0110000101001000LOAD CURRENT (mA)I N P U T V O L T A G E (V )5.05.53680 G214.54.06.56.03680 G24I L0.2A/DIVV SW 5V/DIVV OUT 10mV/DIV5μs/DIVV IN = 12V V OUT = 3.3V I LOAD = 10mATEMPERATURE (°C)T H R E S H O L D V O L T A G E (%)8590953680 G238075–5025–2550751001501253680 G25I L0.2A/DIV V SW 5V/DIVV OUT10mV/DIVV IN = 12V V OUT = 3.3V I LOAD = 110mA1μs/DIV3680 G26I L0.5A/DIVV SW 5V/DIVV OUT10mV/DIVV IN = 12V V OUT = 3.3V I LOAD = 1A1μs/DIVMinimum Input VoltageMinimum Input VoltageV C VoltagesPower Good Threshold Switching Waveforms; T ransition from Burst Mode to Full FrequencySwitching Waveforms; Full Frequency Continuous OperationSwitching Waveforms; Burst ModeT A = 25°C unless otherwise noted.T YPICAL PERFORMANCE CHARACTERISTICS73972fBD (Pin 1): This pin connects to the anode of the boost Schottky diode. BD also supplies current to the internal regulator.BOOST (Pin 2): This pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch.SW (Pin 3): The SW pin is the output of the internal power switch. Connect this pin to the inductor, catch diode and boost capacitor.V IN (Pin 4): The V IN pin supplies current to the L T3972’s internal regulator and to the internal power switch. This pin must be locally bypassed.RUN/SS (Pin 5): The RUN/SS pin is used to put the L T3972 in shutdown mode. Tie to ground to shut down the L T3972. Tie to 2.5V or more for normal operation. If the shutdown feature is not used, tie this pin to the V IN pin. RUN/SS also provides a soft-start function; see the Applications Information section.SYNC (Pin 6): This is the external clock synchronization input. Ground this pin for low ripple Burst Mode operation at low output loads. Tie to a clock source for synchronization. Clock edges should have rise and fall times faster than 1μs. Tie Pin to GND if not used. See Synchronization section in Applications Information.PG (Pin 7): The PG pin is the open collector output of an internal comparator. PG remains low until the FB pin is within 9% of the fi nal regulation voltage. PG output is valid when V IN is above 3.6V and RUN/SS is high.FB (Pin 8): The L T3972 regulates the FB pin to 0.790V. Connect the feedback resistor divider tap to this pin.V C (Pin 9): The V C pin is the output of the internal error amplifi er. The voltage on this pin controls the peak switch current. Tie an RC network from this pin to ground to compensate the control loop.RT (Pin 10): Oscillator Resistor Input. Connecting a resistor to ground from this pin sets the switching frequency.Exposed Pad (Pin 11): Ground. The Exposed Pad must be soldered to PCB.+–V OUTB LOCK DIAGRAMP IN FUNCTIONSThe L T3972 is a constant frequency, current mode step-down regulator. An oscillator, with frequency set by RT, enables an RS flip-flop, turning on the internal power switch. An amplifi er and comparator monitor the current fl owing between the V IN and SW pins, turning the switch off when this current reaches a level determined by the voltage at V C. An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the V C pin. If the error amplifi er’s output increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp on the V C pin provides current limit. The V C pin is also clamped to the voltage on the RUN/SS pin; soft-start is implemented by generating a voltage ramp at the RUN/SS pin using an external resistor and capacitor.An internal regulator provides power to the control circuitry. The bias regulator normally draws power from the V IN pin, but if the BD pin is connected to an external voltage higher than 3V bias power will be drawn from the external source (typically the regulated output voltage). This improves effi ciency. The RUN/SS pin is used to place the L T3972 in shutdown, disconnecting the output and reducing the input current to less than 0.5μA.The switch driver operates from either the input or from the BOOST pin. An external capacitor and diode are used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to fully saturate the internal bipolar NPN power switch for effi cient opera-tion.To further optimize effi ciency, the L T3972 automatically switches to Burst Mode operation in light load situations. Between bursts, all circuitry associated with controlling the output switch is shut down, reducing the input supply current to 75μA in a typical application.The oscillator reduces the L T3972’s operating frequency when the voltage at the FB pin is low. This frequency foldback helps to control the output current during startup and overload.The L T3972 contains a power good comparator which trips when the FB pin is at 91% of its regulated value. The PG output is an open-collector transistor that is off when the output is in regulation, allowing an external resistor to pull the PG pin high. Power good is valid when the L T3972 is enabled and V IN is above 3.6V.The L T3972 has an overvoltage protection feature which disables switching action when the V IN goes above 35V typical (33V minimum). When switching is disabled, the L T3972 can safely sustain input voltages up to 62V.OPERATION83972f93972fFB Resistor NetworkThe output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resis-tors according to:R R V V OUT 120791=⎛⎝⎜⎞⎠⎟.–Reference designators refer to the Block Diagram.Setting the Switching FrequencyThe L T3972 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 2.4MHz by using a resistor tied from the RT pin to ground. A table showing the necessary RT value for a desired switching frequency is in Figure 1.SWITCHING FREQUENCY (MHz)R T VALUE (kΩ)0.20.30.40.50.60.70.80.91.01.21.41.61.82.02.22.421514010078.763.453.645.339.23426.722.118.21512.710.79.09Figure 1. Switching Frequency vs. R T ValueOperating Frequency T radeoffsSelection of the operating frequency is a tradeoff between effi ciency, component size, minimum dropout voltage, and maximum input voltage. The advantage of high frequency operation is that smaller inductor and capacitor values may be used. The disadvantages are lower effi ciency, lower maximum input voltage, and higher dropout voltage. The highest acceptable switching frequency (f SW(MAX)) for a given application can be calculated as follows:f V V t V V V SW MAX D OUTON MIN D IN SW ()()=++()–where V IN is the typical input voltage, V OUT is the output voltage, V D is the catch diode drop (~0.5V) and V SW is the internal switch drop (~0.5V at max load). This equation shows that slower switching frequency is necessary to safely accommodate high V IN /V OUT ratio. Also, as shown in the next section, lower frequency allows a lower dropout voltage. The reason input voltage range depends on the switching frequency is because the L T3972 switch has fi nite minimum on and off times. The switch can turn on for a minimum of ~150ns and turn off for a minimum of ~150ns. Typical minimum on time at 25°C is 80ns. This means that the minimum and maximum duty cycles are:DC f t DC f t MIN SW ON MIN MAX SW OFF MIN ==()()1–where f SW is the switching frequency, the t ON(MIN) is the minimum switch on time (~150ns), and the t OFF(MIN) is the minimum switch off time (~150ns). These equations show that duty cycle range increases when switching frequency is decreased.A good choice of switching frequency should allow ad-equate input voltage range (see next section) and keep the inductor and capacitor values small. Input Voltage RangeThe maximum input voltage for L T3972 applications de-pends on switching frequency, Absolute Maximum Ratings of the V IN and BOOST pins, and the operating mode.The L T3972 can operate from input voltages of up to 33V, and withstand voltages up to 62V. Note that while V IN is above 35V typical (33V minimum and 37V maximum) the part will keep the switch off and the output will not be in regulation.To safely allow inputs of up to 62V, be sure to choose a Schottky diode, inductor size, and switching frequency to allow safe operation at 37V according to the following discussions.While the output is in start-up, short-circuit, or other overload conditions, the switching frequency should be chosen according to the following equation:APPLICATIONS INFORMATION103972fV V Vf t V V IN MAX OUT D SW ON MIN D SW()()=++–where V IN(MAX) is the maximum operating input voltage,V OUT is the output voltage, V D is the catch diode drop (~0.5V), V SW is the internal switch drop (~0.5V at max load), f SW is the switching frequency (set by R T ), and t ON(MIN) is the minimum switch on time (~100ns). Note that a higher switching frequency will depress the maximum operating input voltage. Conversely, a lower switching frequency will be necessary to achieve safe operation at high input voltages.If the output is in regulation and no short-circuit, start-up, or overload events are expected, then input voltage transients of up to 33V are acceptable regardless of the switching frequency. In this mode, the L T3972 may enter pulse skipping operation where some switching pulses are skipped to maintain output regulation. In this mode the output voltage ripple and inductor current ripple will be higher than in normal operation.The minimum input voltage is determined by either the L T3972’s minimum operating voltage of ~3.6V or by its maximum duty cycle (see equation in previous section). The minimum input voltage due to duty cycle is:V V V f t V VIN MIN OUT DSW OFF MIN D SW()()=++1––where V IN(MIN) is the minimum input voltage, and t OFF(MIN) is the minimum switch off time (150ns). Note that higher switching frequency will increase the minimum input voltage. If a lower dropout voltage is desired, a lower switching frequency should be used.Inductor SelectionFor a given input and output voltage, the inductor value and switching frequency will determine the ripple current. The ripple current ΔI L increases with higher V IN or V OUT and decreases with higher inductance and faster switch-ing frequency. A reasonable starting point for selectingthe ripple current is: ΔI L = 0.4(I OUT(MAX))where I OUT(MAX) is the maximum output load current. To guarantee suffi cient output current, peak inductor current must be lower than the L T3972’s switch current limit (I LIM ). The peak inductor current is: I L(PEAK) = I OUT(MAX) + ΔI L /2where I L(PEAK) is the peak inductor current, I OUT(MAX) is the maximum output load current, and ΔI L is the inductor ripple current. The L T3972’s switch current limit (I LIM ) is 5.5A at low duty cycles and decreases linearly to 4.5A at DC = 0.8. The maximum output current is a function of the inductor ripple current: I OUT(MAX) = I LIM – ΔI L /2Be sure to pick an inductor ripple current that provides suffi cient maximum output current (I OUT(MAX)).The largest inductor ripple current occurs at the highest V IN . To guarantee that the ripple current stays below the specifi ed maximum, the inductor value should be chosen according to the following equation:L V V f I V V V OUT D SW L OUT D IN MAX =+⎛⎝⎜⎞⎠⎟+⎛⎝⎜⎜⎞Δ1–()⎠⎟⎟where V D is the voltage drop of the catch diode (~0.4V), V IN(MAX) is the maximum input voltage, V OUT is the output voltage, f SW is the switching frequency (set by RT), and L is in the inductor value.The inductor’s RMS current rating must be greater than the maximum load current and its saturation current should be about 30% higher. For robust operation in fault conditions (start-up or short circuit) and high input voltage (>30V), the saturation current should be above 5A. To keep the effi ciency high, the series resistance (DCR) should be less than 0.1Ω, and the core material should be intended for high frequency applications. Table 1 lists several vendors and suitable types.APPLICATIONS INFORMATIONTable 1. Inductor VendorsVENDOR URL PART SERIES TYPE Murata LQH55D Open TDK SLF10145ShieldedToko D75CD75F Shielded OpenSumida CDRH74CR75CDRH8D43Shielded Open ShieldedNEC MPLC073MPBI0755Shielded ShieldedOf course, such a simple design guide will not always re-sult in the optimum inductor for your application. A larger value inductor provides a slightly higher maximum load current and will reduce the output voltage ripple. If your load is lower than 3.5A, then you can decrease the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor, or one with a lower DCR resulting in higher effi ciency. There are several graphs in the Typical Performance Characteristics section of this data sheet that show the maximum load current as a function of input voltage and inductor value for several popular output voltages. Low inductance may result in discontinuous mode operation, which is okay but further reduces maximum load current. For details of maximum output current and discontinuous mode opera-tion, see Linear Technology Application Note 44. Finally, for duty cycles greater than 50% (V OUT/V IN > 0.5), there is a minimum inductance required to avoid subharmonic oscillations. See AN19.Input CapacitorBypass the input of the L T3972 circuit with a ceramic capacitor of X7R or X5R type. Y5V types have poor performance over temperature and applied voltage, and should not be used. A 10μF to 22μF ceramic capacitor is adequate to bypass the L T3972 and will easily handle the ripple current. Note that larger input capacitance is required when a lower switching frequency is used. If the input power source has high impedance, or there is signifi cant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a lower performance electrolytic capacitor. Step-down regulators draw current from the input sup-ply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the L T3972 and to force this very high frequency switching current into a tight local loop, minimizing EMI.A 10μF capacitor is capable of this task, but only if it is placed close to the L T3972 and the catch diode (see the PCB Layout section). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the L T3972. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the L T3972 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the L T3972’s voltage rating. This situation is easily avoided (see the Hot Plugging Safety section).For space sensitive applications, a 4.7μF ceramic capaci-tor can be used for local bypassing of the L T3972 input. However, the lower input capacitance will result in in-creased input current ripple and input voltage ripple, and may couple noise into other circuitry. Also, the increased voltage ripple will raise the minimum operating voltage of the L T3972 to ~3.7V.Output Capacitor and Output RippleThe output capacitor has two essential functions. Along with the inductor, it fi lters the square wave generated by the L T3972 to produce the DC output. In this role it determines the output ripple, and low impedance at the switching frequency is important. The second function is to store energy in order to satisfy transient loads and stabilize the L T3972’s control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is:CV fOUTOUT SW=100where f SW is in MHz, and C OUT is the recommended output capacitance in μF. Use X5R or X7R types. This choice will provide low output ripple and good transient response. T ransient performance can be improved with a higher value capacitor if the compensation network is also adjusted to maintain the loop bandwidth. A lowerAPPLICATIONS INFORMATIONvalue of output capacitor can be used to save space and cost but transient performance will suffer. See the Fre-quency Compensation section to choose an appropriate compensation network.When choosing a capacitor, look carefully through the data sheet to fi nd out what the actual capacitance is under operating conditions (applied voltage and temperature). A physically larger capacitor, or one with a higher voltage rating, may be required. High performance tantalum or electrolytic capacitors can be used for the output capacitor. Low ESR is important, so choose one that is intended for use in switching regulators. The ESR should be specifi ed by the supplier, and should be 0.05Ω or less. Such a capacitor will be larger than a ceramic capacitor and will have a larger capacitance, because the capacitor must be large to achieve low ESR. Table 2 lists several capacitor vendors.Catch DiodeThe catch diode conducts current only during switch off time. Average forward current in normal operation can be calculated from:I D(AVG) = I OUT (V IN – V OUT )/V INwhere I OUT is the output load current. The only reason to consider a diode with a larger current rating than necessary for nominal operation is for the worst-case condition of shorted output. The diode current will then increase to thetypical peak switch current. Peak reverse voltage is equal to the regulator input voltage. Use a Schottky diode with a reverse voltage rating greater than the input voltage. The overvoltage protection feature in the L T3972 will keep the switch off when V IN > 35V which allows the use of 40V rated Schottky even when V IN ranges up to 62V. Table 3 lists several Schottky diodes and their manufacturers.Table 3. Diode VendorsPART NUMBER V R (V)I AVE (A)V F AT 3A (mV)On Semiconductor MBRA340403500Diodes Inc.PDS340B340A B340LA404040333500500450Ceramic CapacitorsCeramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the L T3972 due to their piezoelectric nature. When in Burst Mode operation, the L T3972’s switching frequency depends on the load current, and at very light loads the L T3972 can excite the ceramic capaci-tor at audio frequencies, generating audible noise. Since the L T3972 operates at a lower current limit during Burst Mode operation, the noise is nearly silent to a casual ear. If this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output.VENDOR PHONE URLPART SERIES COMMANDS Panasonic(714) 373-7366Ceramic,Polymer ,Tantalum EEF SeriesKemet (864) Ceramic,Tantalum T494, T495Sanyo(408) 749-9714Ceramic,Polymer ,Tantalum POSCAPMurata (408) Ceramic AVX Ceramic,Tantalum TPS Series Taiyo Yuden(864) CeramicTable 2. Capacitor VendorsAPPLICATIONS INFORMATION。
