C310

LED DRIVER SOLUTION FOR LCD BACKLIGHTINGTYPICAL APPLICATIONS CIRCUITDEVICE MARKINGSC310Package SOT23-5DEVICE DESCRIPTIONThe 310is a single or multi cell LED driverdesigned for LCD backlighting applications.The input voltage range of the device is between 0.8V and 8V.This means the 310is compatible with singleNiMH,NiCd or Alkaline cells,as well as multi-cell or LiIon batteries.The device features a shutdown control,resulting in a standby current less than 5µA,and an output capable of driving serial or parallel LED’s.The circuit generates constant power output,which are ideal for driving single or multiple LED’s over a wide range of operating voltages.These features make the device ideal for driving LED’s particularly in LCD backlight applications for Digital Still cameras and PDA’s.FEATURES94%efficiencyMinimum operating input voltage 0.8V Maximum operating input voltage 8V Standby current less than 5µAProgrammable output current Series or parallel LED configuration Low saturation voltage switching transistorSOT23-5packageThe 310is a PFM DC-DC controller IC that drives an external switching transistor with a very low saturation resistance.These transistors are the best switching devices available for this type of conversion enabling high efficiency conversion with low input voltages.The drive output of the 310LED drivergenerates a dynamic drive signal for the switching transistor.The circuit can start up under full load and operates down to an input voltage of 0.8volts.The solution configuration ensures optimum efficiency over a wider range of load currents;several circuit configurations are possible depending on battery life versus brightness considerations.The 310is offered in the SOT23-5package which,when combined with a SOT23switching transistor,generates a high efficiency small size circuit solution.The IC and discrete combination offers the ultimate cost Vs performance solution for LED backlight applications.ABSOLUTE MAXIMUM RATINGS:Supply Voltage-0.3 to 10VMaximum Voltage other pins -0.3 to V CC +0.3V Power Dissipation 450mWOperating Temperature -40 to 85°C Storage Temperature-55 to 150°CELECTRICAL CHARACTERISTICS:=25OPERATING CONDITIONS1 Application dependent, see reference designs2 These parameters guaranteed by Design and characteristics3 Total supply current =I Q +I VDRIVE , see typical characteristics4 Operating frequency is application circuit dependent. See applications section.618For the circu its descri bed in the applica tions section 618is the recomme nded pass tra nsistor.For the ma x imu m brightness circuit descri bed in the a p p lic a tio n s s e c tio n is th e recomme nded Sch ottky diode.T he follow ing indica tes outline data for the device,m ore detailed informa tion ca n be found in the surface mo unt products data book or on WebT he follow ing indica tes outline data for the m ore detailed informa tion ca n be found on WebELECTRICAL CHARACTERISTICS:=255Measured under pulse conditions. Pulse width=300µs. Duty cycle 2%ELECTRICAL CHARACTERISTICS:=2510001000DEVICE DESCRIPTIONThe310is PFM,controller IC which,when combined with a high performance external transistor, enables the production of a high efficiency boost converter for LED driving applications.A block diagram is shown for the 310 below.The on chip comparator forces the driver circuit and therefore the external switching transistor off if the voltage at I SENSE exceeds19mV.An internal reference circuit and divider set this threshold.The voltage at I SENSE is taken from a current sense resistor connected in series with the emitter of the switching transistor.A monostable following the output of the comparator forces the turn-off time of the output stage to be typically1.7us.This ensures that there is sufficient time to discharge the inductor coil before the next on period.With every on pulse the switching transistor is kept on until the voltage across the current-sense resistor exceeds the threshold of the I SENSE input.The on-pulse length,and therefore the switching frequency,is determined by the programmed peak current,the input voltage and the input to output voltage differential.See applications section for details.The driver circuit supplies the external switching transistor with a fixed drive current.To maximise efficiency the external transistor switched quickly, typically being forced off within 30ns.REFERENCE DESIGNSThree typical LED driving applications are shown.Firstly a typical LCDbacklight circuit,then maximum brightness LED driving solution and lastly an optimised battery life LED driving solution.This application shows the 310in a typical LCDbacklight application for Digital Still Cameras and PDA’s.The input voltage for these backlight circuits are usually fixed from the main system power,typically 3.3V or 5V.The LED’sare connected serially so that the light is distributed uniformly in each LED.The current provided to the LED’s can either be pulsed or DC.The DC current is programmable via a sense resistor,R SENSE ,and is set to an optimum LEDcurrent of 20mA for the reference designs.DC current is achieved by adding a Schottky rectifying diode and an output capacitor, as shown in the reference design below.LCD backlightcircuitRef Value Part Number Manufacture CommentsU1N/A 310 PlC SOT23-5Q1N/A 618 PlC Low V CE(sat )NPN,SOT23D1N/A 1000 PlC 1A Schottky diode,SOT23C12.2␮F Generic Various 0805Size R16150m ⍀Generic Various 1206Size R17250m ⍀GenericVarious1206SizeL1868µHSurface mount inductorMaterials list6Used for 3.3V input,I LED set to 20mA Ϯ10%.7Used for 5V input,I LED to 20mA Ϯ10%.8See Application section.(Notes)　Th i s circu it provides a co ntinuous cu rre nt output to the LED by re ctifying and buffering the DC-D C output.Th isensu re s ma x imu m LED brightness.Maximum brightness solution(Notes)(Notes)To ensure optimum efficiency,and therefore maximum battery life,the LED is supplied with apulsed current.Maximum efficiency is ensured withthe removal of rectifier losses experienced in the maximum brightness solution.APPLI CAT I ONS I NFORMAT I ONT he follow ing se ction is a design guide for optimu m co nverter performa nce.Sw itching tra nsistor se lectionT he choice of sw itching tra nsistor has a ma jor imp act on the co nverter efficiency.For optimu m performa nce ,a bipolar tra nsistor w ith low V CE (SAT)and high gain is re quire d.T he 618is an ideal choice of tra nsistor,having a low sa tura tion voltage.A data sh eet for the 618is available on w eb site or throughyour loca l sa les office.Outline informa tion isincluded in the characteristics section of this data sh eet.Sch ottky diode se lectionFor the m ax im um battery life solution a Schottky re ctifier diode is not re quire d.As w ith the sw itching transistor the Schottky rectifier diode has a m ajor imp act on the co nverter efficiency.A Sch ottky diode w ith a low forw ard voltage and fast re co very time sh ould be use d for this applica tion.T he diode sh ould be se lected so that the ma x imu m forw ard cu rre nt is gre ater or equal to the ma x imu m peak cu rre nt in the inductor,and the ma x imu m re verse voltage is gre ater or equal to the output voltage.T he 1000me ets these needs.Datash eets for the ZH Se ries are available on w eb site or thro u g h y o u r lo c a l s a le s o ffic e.Ou tlineinforma tion is included in the chara cteristics se ction of this data sh eet.For the ma x imu m brightness so lution a pulse d cu rre nt is su pplied to the LED and thus a Sch ottky re ctifier diode is re quire d.I nductor se lectionThe ind u c to r v a lu e m u s t b e c ho s e n to s a tis fy performa nce ,co st and size re quire me nts of the overa ll so lution.For the LCD backlight re fere nce design w e re co mme nd an inductor value of 68uH w ith a co re sa tura tion cu rre nt ra ting gre ater than the co nverter peak cu rre nt value and low se ries re sistance.I nductor selection has a significant im pact on the co nv e rte r p e rfo rm a nce.Fo r a p p lica tio ns w he re efficiency is critica l,an inductor w ith a se ries re sistance of 500m or less should be used.A list of recommended inductors is shown in the table below:ISSUE 2 - MARCH 2004Peak current definitionThe peak current rating is a design parameter whose value is dependent upon the overall application.For the high brightness reference designs,a peak current of was chosen to ensure that the converter could provide the required output power to the LED.In general,the I PK value must be chosen to ensure that the switching transistor,Q1,is in full saturation with maximum output power conditions,assuming worse-case input voltage and transistor gain under all operating temperature extremes.Once I PK is decided the value of R SENSE can be determined by:A selection guide of sense resistor and inductor values for given input voltages,output currents and number of LED connected in series is provided in the table below.R V ISENSEISENSEPK=I I V V T LMIN PK OUT IN OFF=−−×()F T T ON OFF =+1I I I AV PK MIN=+2()T T V V V ON OFF OUT IN IN=−P V V I T T T OUT OUT IN AV OFFON OFF=−××+()Output Power CalculationBy making the above assumptions for inductance and peak current the output power can be determined by:Note:V OUT =output voltage +Schottky rectifier voltage drop.WhereT OFF ≅1.7us (internally set by 310)and andWhere Operating frequency can be derived by:Capacitor selectionFor pulsed operation,as in the maximum battery life solution,no capacitors are required at the output to the LED.For rectified operation,as in the maximum brightness solution,a small value ceramic capacitor is required, typically 2.2uF.Generally an input capacitor is not required,but a small ceramic capacitor may be added to aid EMC,typically 470nF to 1uF.(notes)Top SilkDrill FileShutdown ControlThe 310offers a shutdown mode that produces astandby current of less than 5uA when in operation.When the voltage at the S TDN pin is 0.7V or higher the 310is enabled,hence the driver is in normaloperation.When the voltage at the S TDN pin is 0.1V or lower the 310is disabled,hence the driver is inshutdown mode.If the S TDN pin is open circuit the 310 is also enabled.Layout of LCD backlighting solutionDemonstration boardA demonstration board for the LCD backlighting solution,is available upon request.These can be obtained through your local office or through web pages.For all reference designs Gerber files and bill of materials can be supplied.STDN VCC Gnd ISENSEVDRIVE 12345Top ViewPINOUT DIAGRAMPin No.Name Description1V CCSupply voltage,generally Alkaline,NiMH or NiCd single cell2G nd Ground3S TDNShutdown 4I SENSE Inductor current sense input.Internal threshold voltage set to 19mV.Connect external sense resistor 5V DRIVEDrive output for external switching transistor.Connect to base ofexternal switching transistor.PIN DESCRIPTIONSDIM Millimeters InchesMIN MAX MIN MAX A 0.90 1.450.0350.057A10.000.150.000.006A20.90 1.30.0350.051b 0.350.500.0140.020C 0.090.200.00350.008D 2.80 3.000.1100.118E 2.60 3.000.1020.118E1 1.501.750.0590.069e 0.95 REF 0.037 REF e1 1.90 REF 0.075 REF L 0.100.600.0040.024a°1010SOT23-5PACKAGE DIMENSIONS。