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1、Driving high power and high brightness LEDsApplication Note 5310Figure 1. Series connection Total LEDs = n(a) Series of two LEDs(b) Series of n LEDs.Series connectionFigure 1 shows the series connection. The LED current in the series is the same throughout the series. Its disadvan-tage is that if on
2、e LED is opened, no LED will light up. The total VF across the series LED is higher, but the current re-quirement is lower. The LED driver circuit used must be capable of producing output LED voltage greater than the series total VF. Generally, the closer the rated LED output voltage to the series L
3、ED total VF, the better is the efficiency.Common anode or common cathode parallel connectionThe common anode or common cathode parallel con-nection is used if each individual LED current is set inde-pendently. The advantage of this connection is that if one LED is opened, the other LEDs are unaffect
4、ed. The disad-vantage of this connection is that a higher current rating is needed. Common anode and common cathode parallel connectionThe difference between the LED forward voltages will cause current hogging. In cases where this is unavoid-able, use the smallest number of LEDs with matched electri
5、cal, thermal and operating lifetime characteristic. The matched LEDs are equivalent to a single larger LED with a higher current rating. However, the matched LEDs may drift apart due to different degradation and thermal characteristics.IntroductionThis paper describes the various methods of driving
6、the high power and high brightness LED. The task of designing an LED based system involves the following selection. I. number of LED used and the connection scheme between LEDsII. linear or switch mode LED driver III. power supply source i.e. DC voltage or AC mains or batteryIV. optical subsystem, e
7、.g. lens, filter cover etc.The number of LED needed depends on the luminance required and the current the LEDs are driven. The optical subsystem is not covered here as it is beyond the scope of this paper. LED connection schemeIf there is more than one LED, the LED connection scheme must be decided.
8、There is no hard and fast rule in selecting the connection scheme. Sometimes it is a matter of preference. In some cases, the LED drivers chosen will decide the connection scheme. At times, the available power supply and effi-ciency required may influence the connection scheme.Typically, the connect
9、ion between LED is classified into three main configurations.a) Seriesb) ParallelThis is subdivided intoi) Common anodeii) Common cathodeiii) Common anode and common cathodec) Mixed series paralleli) Series of two LEDsII) Series of n LEDs名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整
10、理 - - - - - - - 第 1 页，共 16 页 - - - - - - - - - Mixed series - parallel connectionFigure 3 shows some examples of mixed series parallel connection. This is usually selected as a trade off between the total VF and the total current required, so that it will fit with available led drivers.Figure 3. Mix
11、ed series parallel connection exampleFigure 2. Parallel connection exampleLED driversThe connection scheme will influence the number of drivers channel needed. Most LED driver typically has a single channel. However, there are some multi channel LED drivers available. Each channel can drive only a s
12、ingle series connected LEDs or a single branch of the parallel connected LED in common anode or common cathode configuration. The following condition must be met to be able to drive the series connected LEDs. a) Output LED driver voltage must be greater than the total VF of the series connected LEDb
13、) LED driver constant current output must be higher or equal to the desired LED current. If the current is higher, use PWM dimming or modify the circuit, for example the sense resistor.In general, LED drivers are classified into:a) Linear LED driverb) Switch mode LED driverLinear LED driver are less
14、 efficient and generally occupy a larger space. Switch mode LED driver is more efficient and generally smaller. However, they have electrical and radiated noise and are complicated to design. When the input power supply is lower than the LEDs total VF, switch mode LED driver must be used. The select
15、ion between linear or switch mode is generally decided by simplicity, the available power supply and efficiency.CommonCommonCommon anode &anodeCathodecommon cathode名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 2 页，共 16 页 - - - - - - - - - Converting a voltage regula
16、tor to LED driverA typical voltage regulator has the following pins, VIN, GND, VOUT and FB. The FB pin sense a voltage to control the voltage regulator output voltage. For a constant current LED driver, current sense is necessary. By placing a resistor in the LED current path, the LED current is con
17、verted to a sense voltage at the FB pin. The resistor is usually placed at the LED cathode side for low side current sensing. Alternatively, the resistor is placed on the LED anode side for high side current sensing. A differential amplifer with high common mode rejection ratio is needed to sense th
18、e voltage across the resistor.Figure 4. Converting a voltage regulator to LED driverDC to DC converterVOUTFBRVINVoltageRegulatorDC to DC converterVoltageRegulatorVOUTFBRVINThe resistor value for low side current sensing is LEDFBIVHowever, for high side current sensing, the resistor value required is
19、 VLEDFBAIV*Note:VFB is the regulated feedback voltage at the FB pin. ILED is the desired LED current.AV is the gain of the differential amplifier.For linear LED driver, the number of LEDs that can be driven in series isN = FRDOINVVVV-For the switch mode LED driver, the number of LEDs is N =FRORVVV-N
20、ote:VIN is the input voltage supply.VDO is the drop out voltage.VR is the sense voltage across the resistor.VOR is the rated designed output voltage of the series connected LED string.VF is the typical forward voltage of a single LED.(a) Low side current sensing(b) High side current sensing名师资料总结 -
21、- -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 3 页，共 16 页 - - - - - - - - - Linear LED driver examples1. Simple resistive current limiting.A resistor is used to limit the LED current from a DC power supply. Figure 6. LM317HV LED driverThe LM317HV regulates a fixed 1.23V betwe
22、en the ADJ and the OUT pin. The LED current is given by the equation, LED = R1.23IThe advantage of this circuit over the simple resistive driver is that the LED current is constant despite the LED forward voltage drift. Figure 5. Resistor as a current limiterThe resistance is given by R =FFINIVV-Not
23、es:VF - total VF of the series connected LED string.IF - desired LED current.VIN input voltage supply.The advantage of simple resistive limiting is the simplicity. However, the LED current is not stable due to LED forward voltage drift.LM317HVR0.1uFINVADJVINVOUTDC Power supplyRVIN2. LM317 or LM317HV
24、Figure 6 showed the LM317HV voltage regulator used as an LED driver.名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 4 页，共 16 页 - - - - - - - - - Figure 8. LT3021 LED driverIf the LED Vf is 3.6V, the number of series connected LED is two.Figure 7. LM2941 LED driver3. L
25、M2941 Another driver similar to the LM317 is the LM2941. The LM2941 voltage regulator has a maximum input voltage of 26V. The LM2941 regulates 1.275V across the ADJ and GND terminal. Figure 7 show the LM2941 as an LED driver providing 354mA.470nFON / OFF3.6LM2941INADJGNDOUTVIN4. LT3021The LT3021 is
26、another linear voltage regulator with maximum input voltage at 10V and a maximum current rating of 500mA. The LT3021 regulates the output to maintain 0.2V across the ADJ and GND terminal. The LED current is R0.2 The drop out voltage is 160mV.VINR3.3uFADJGNDLT3021OUTSHDNIN名师资料总结 - - -精品资料欢迎下载 - - - -
27、 - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 5 页，共 16 页 - - - - - - - - - 5. TLE4242GVREF is regulated to 177mV between ADJ and GND pins. The maximum input voltage rating is 42V with a dropout voltage of 0.7V. The LED current in the circuit is In the circuit shown in figure 9, R = 5.1ohm. The
28、 LED current is 347 mA. Figure 9. Infineon TLE4242G LED driverTLE4242GINREF5.1OUTSTDVGNDINRVREF6. AS3691 and AS3692The AS3691 has a voltage rating of 15V with maximum LED current of 400mA. The AS3692 is similar to AS3691. The difference is the AS3692 has a voltage rating up to 50V, but the maximum c
29、urrent is 200mA. Contact Austriamicrosystem for latest information on availability and application notes on AS3691 and AS3692.名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 6 页，共 16 页 - - - - - - - - - Figure 10. Maxim 16800 LED driverFor LED color management applica
30、tion with the ADJD-J823 and the HDJD-J822, the PWM output from the ADJD-J823/HDJD-J822 is connected to the EN input of the MAX16800.RSOUTVINPWMRVOUT0.1uF0.1uFMAX16800GNDCS+CS-ENINOUTV57. MAX16800 LED driver.This LED driver has a voltage rating of 40V. The driver regulates a fixed voltage across the
31、CS+ and CS- The current is set by an external resistor, Notes:ILED is the desired LED current VSENSE is typically 0.204V across CS+ and CS- pins.RS = LEDSENSEIV名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 7 页，共 16 页 - - - - - - - - - Figure 11. HV9911 in boost topo
32、logyNotes: 1. VREF is typically 1.25V. 2. The maximum LED current for this circuit is 350mA.The LED current is related to the equation, 10131213*RIRRRVLEDREF=+)(Switch mode LED driverThe switch mode LED driver is related to the switching voltage regulator topologies. The switching voltage regulator
33、maintains a constant voltage at various current loads. The switch mode LED driver however, deliver constant current to LED at whatever VF the LEDs required, provided that the over voltage protection or power rating is not exceeded. Some common examples of switch mode LED driver topologies are a) BUC
34、K. These are step down regulator which the output voltage is generally lower than the input voltage.b) BOOST. These are step up regulator which the output voltage is higher than the input.c) BUCK-BOOST. These are step down / step up regulator which the output voltage is inverted. d) SEPIC. This is s
35、imilar to buck boost except the output voltage is not inverted. e) FLYBACK. These are step up or step down regulator with a transformer instead of an inductor. Designers should obtain information on the IC datasheets and application notes from the manufacturer and work closely with the LED driver ma
36、nufacturer. Before turning on a switch mode LED driver, the LEDs should be connected. An open circuit in the switch mode LED will cause the output voltage to rise to maximum limit and may exceed the LED driver maximum voltage rating.Switch mode LED driver examples 1. HV9911 in boost topologyThe inpu
37、t voltage (21-27V) is provided between VIN and GND. This boost topology circuit provides 80V maximum output voltage. I.e. the number of InGaN LEDs that can be driven is 12 to 20 LEDs. If the PWM is not used, it should be connected to VDD to enable the LED driver.名师资料总结 - - -精品资料欢迎下载 - - - - - - - -
38、- - - - - - - - - - 名师精心整理 - - - - - - - 第 8 页，共 16 页 - - - - - - - - - Figure 12. HV9911 in buck topology2. HV9911 in buck topologyThis buck topology circuit accepts 130 to 200 V DC input between VIN and GND. It can provide 20 to 100V LED output voltage. The high side current sensing limit the LED
39、current to 350mA. If the PWM is not used, it should be connected to VDD to enable the LED driver.The LED current is related to the equation, Notes:1. VREF is typically 1.25V. 2. The maximum LED current for this circuit is 350mA.10131213*RIRRRVLEDREF=+)(名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - -
40、- - - - - 名师精心整理 - - - - - - - 第 9 页，共 16 页 - - - - - - - - - Figure 13. HV9910 in buck topology3. HV9910This buck topology circuit is not isolated from line voltage. The LEDs must be connected to the driver before turning on the AC supply. Table 1 shows the possible VAC input range with this circui
41、t. The components L1 and R4 values are selected based on the operating LED current. Assuming that the LED forward voltage is 3.6V, the maximum number of LED that can be driven is approximately eleven.Table 1. Component selection.VAC Input range VAC Input settingL1L1 Part *R4 ohmLED current mAOutput
42、LED driver voltageminmaxPCH-0.PCH-0.0.*Notes:* The inductor part numbers are from Coilcraft.* Implemented by two 0.27 ohm resistors connected in parallel. The components values for row 1 and 2 are shown in figure 13.The control PWM signal must be isolated from mains. If the PWM is not used, it shoul
43、d be connected to VDD to enable the LED driver. If any measuring instrument needs to be used, they must also be isolated using isolating transformer. 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 10 页，共 16 页 - - - - - - - - - Figure 14. L6902D LED driver4. ST L6902T
44、his is a buck LED driver. Table 2 shows the various components values needed to obtain the corresponding LED current.Table 2. Resistor selectionR1R3R4R5R6RsLED current9.1k5101k8.2k27k0.33350mA9.1k5109102.4k20k0.2700mANote: The component values for the first row are shown in figure 15 to provide 350m
45、A LED current.The Vdim pad provides an input for linear dimming or inverted logic PWM dimming. At 0V, the LED current is maximum. As the voltage increase from 0V to 3.3V, the LED current will reduce linearly from maximum to zero. Resistor R1 and R3 provide over voltage protection at 23.3V in case an
46、 open circuit occur at the LED output. Assuming InGaN LEDs with VF = 3.6V, the maximum number of LEDs that can be connected in series is 6 InGaN LEDs.名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 11 页，共 16 页 - - - - - - - - - 5. ST L4973This is a buck topology LED d
47、river with 48V input. A maximum of 12 InGaN LEDs can be connected. Resistors R1, R2 and the internal 5.1V supply reduced the sense voltage to 0.5V.Figure 15. L4973V3.3 LED driverFigure 16. LTC3490 driving a single LED6. LTC3490This circuit is able to drive one InGaN LED from a single battery cell. A
48、n internal sense resistor limits the LED output current to 350mA. If the LED pin is open, the output voltage is internally limited to 4.7V.名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 12 页，共 16 页 - - - - - - - - - Figure 17. LTC3783 LED driver7. LTC3783This flyback
49、 LED driver can provide 150mA to a series connected LEDs. The over voltage protection is trigger at 130V and deactivate at 120V. The number of series LEDs that can be connected is 120/3.6 = 33 LEDs. PWM signal provide the dimming control.Figure 18. LM3402 LED driver8. LM3402This is a step down buck
50、LED driver. It has a constant on time architecture and the circuit is designed for a single INGAN LED. Refer to the web site below for other LM3402 reference design for other LED output voltage. http:/ - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 13 页，共 16 页 - - - - - - -