AND8171/D NCP1421/2 Reference Designs for High−Power White LED Flash Applications http://onsemi.com Prepared by: Jim Hill ON Semiconductor APPLICATION NOTE Abstract profile, small sized inductor and output capacitor to be used. Also an integrated disconnect switch provides “true cutoff” by isolating the output from the battery during shutdown. The NCP1421 comes in the 3x5 mm Micro−8 package, and the NCP1422 comes in the 3x3 mm DFN package. Because of these features the NCP1421/2 are well suited to provide current regulation for biasing high current white LED’s in portable flash applications. Figure 1 illustrates this circuit. In summary the reference voltage is split between the current sense resistor, R4, and a divided down voltage from the white LED with resistors R2 and R3. This helps remove some of the dependence of the NCP1421/2’s output voltage, and thus current, on the LED’s forward voltage, VF. This also helps prevent lot−to−lot VF variation affecting the LED brightness. Figure 1 shows a typical circuit which, with the Bill of Materials shown in Table 1, can provide LED currents of 200, 600 and 800 mA. The 200 mA design uses the NCP1422 because of its smaller footprint, and the 600 mA and 800 mA designs use the NCP1421 and NCP1422 respectively to showcase the load current limits of each device. The higher currents (600 and 800 mA) assume that the LED will be pulsed and not run at steady state. 50 ms pulses on the LBI/EN were used in the analysis of these circuits. The NCP1421/2 takes 1.5 ms (nominal) to turn on after the LBI/EN pin is driven high. The attached design illustrates how the NCP1421/2 boost converters can be configured as a current regulator for biasing high current white LED’s. Typical boost converters, such as these, have a reference voltage of 1.2 V. Since this is a current sourcing application, the more straightforward approach of directly sensing the boost converter’s reference voltage (Vref), which is 1.2 V, across a sense resistor would dissipate too much power at the currents required to drive high−power White LED’s. Also, the lot−to−lot forward voltage variation is too high to simply regulate at a fixed voltage with a current limiting resistor. Therefore, this paper describes a technique that reduces both the power loss in the sense resistor and the lot−to−lot variation effect of the LED. This applications shows two implementations of this concept. Figure 1 shows a simple boost converter configured at various current levels and uses the Lumileds LXHL− WW06 white LED. Figure 5 shows a circuit that switches between a low current for focus lighting and high current for the flash and uses the Lumileds LXCL−PWF1 white LED. Overview The NCP1421 and NCP1422 are monolithic boost converter IC’s uniquely suited to power higher current portable applications (600 − 800 mA maximum). Their high switching frequency (up to 1.2 MHz) allows for a low C3 22 F R2 NCP1422 D1 1 FB VOUT 8 2 LBI/EN LX 7 R3 R1 100k 3 LBO 4 REF L1 6.8 H GND 6 BAT 5 C1 220 nF U1 C2 22 F VIN R4 ON OFF 50 ms Pulse Figure 1. NCP1422 Configured to Drive High Current White LED Semiconductor Components Industries, LLC, 2004 November, 2004 − Rev. 0 1 Publication Order Number: AND8171/D AND8171/D 100 1000 95 800 mA 700 600 mA 600 500 400 300 200 mA 200 85 600 mA 80 800 mA 75 70 65 60 100 0 3.0 200 mA 90 800 EFFICIENCY (%) OUTPUT CURRENT (mA) 900 55 VF = 3.5 V @ 600 mA 3.2 3.4 3.6 3.8 4.0 VF = 3.5 V @ 600 mA 50 3.0 4.2 3.2 3.4 3.6 3.8 4.0 4.2 INPUT VOLTAGE (V) INPUT VOLTAGE (V) Figure 2. Output Current vs. Input Voltage Figure 3. Converter Efficiency vs. Input Voltage 100 95 EFFICIENCY (%) 90 85 200 mA 80 75 600 mA 70 65 60 800 mA 55 50 3.0 VF = 3.5 V @ 600 mA 3.2 3.4 3.6 3.8 4.0 4.2 INPUT VOLTAGE (V) Figure 4. Electrical to Optical Efficiency vs. Input Voltage Design Steps input voltage is assumed to be 3.6 V and has been optimized around this point. Step 10: Determine output voltage. Output voltage will be VF + VR4 = 4.1 V One can use the 3.6 V as Vin chosen above because this circuit decreases LED current as VF increases from the designed value. This is shown by the following equation: ID = 1/R4*(Vref − VF*(R3/R2 + R3)) Conversely it increases current as VF decreases from the designed value, but then the difference between Vin and Vout is less, so the peak current is reduced. Step 11: Use the NCP1421 or NCP1422 datasheet to determine the appropriate L1, C1, and C2. For this application, 6.8 H, 22 F, and 22 F were found to work well over the load and line range. Step 12: Determine the inductor saturation current. For this circuit Vin min = 3 V: ILavg = Iout / (1−D) where D = (1−Vin/Vout). Therefore ILavg = 600 mA/(1−(1−Vin/Vout)) = 840 mA Step 13: Add 20% margin to this ILavg and pick an inductor with an Isat > 1.0 A. The following steps show how to determine the critical components for this circuit. (R2, R3, R4, L1) This shows the 600 mA version as an example: Step 1: Let LED current = ID = 600 mA Step 2: From the LED datasheet, let VF = 3.5 V (Find value of VF at 600 mA). Step 3: Let R3 = 100 k Step 4: Let VR4 = 0.5 * Vref which is 0.6 V. This places equal dependence on VF variation and tolerance of the reference and R4. One could increase the output voltage by making the voltage across R4 (VR4) larger or decrease power dissipation in R4 by lowering VR4. Step 5: For ID = 600 mA and VR4 = 0.6 V, R4 = 1.0 . Step 6: Now, VR4 plus the divided voltage off of the LED must equal 1.2 V, and that is 0.6 V Step 7: So, R2 = (VF/(Vref − VR4)) * R3 − R3 = (3.5/0.6) * 100 k − 100 k = 483 k Step 8: Then choose a standard value of R2 which is close to the above calculated value. Choose R2 = 475 k. Step 9: Pick input voltage range. These circuits assume a one−cell Li−ion battery pack or a 3−cell NiMH pack so the http://onsemi.com 2 AND8171/D Finally, Figure 5 shows a Focus/Flash application where the NCP1422 drives one LED at 200 and 600 mA. An C3 22 F external MOSFET changes the R4 resistance to vary the LED current. 50 ms pulses were used for this design. R2 NCP1422 1 FB VOUT 8 2 LBI/EN LX 7 D1 R3 3 LBO 4 REF R1 100k C1 220 nF R4a L1 6.8 H GND 6 BAT 5 U1 Enable Signal Q1 ON OFF R4b 50 ms Pulse Figure 5. 200/600 mA Focus/Flash Application VIN = 3.6 V Figure 6. LED Current and Vin Ripple Voltage with 200/600 mA Focus/Flash Pulse (CH2 = Vin, ac−coupled @ 50 mV/div; CH4 = ILED @ 200 mA/div) http://onsemi.com 3 C2 22 F VIN AND8171/D Table 1. Bill of Materials for Figure 1 Ref Part Number Description PCB Footprint Manufacturer 200 mA Design U1 NCP1422MNR2 NCP1422 Boost Converter D1 LXHL−WW06 White LED DFN−10 (3 x 3 mm) ON Semiconductor L1 VLP5610T−6R8 6.8 H Inductor (5.6 x 5.0 x 1.0 mm) TDK R1 CRCW0402104…. 100 k 0402 Vishay R2 CRCW04025603…. 560 k 0402 Vishay R3 CRCW04021503…. 150 k 0402 Vishay R4 DCRCW12062R70... 2.7 1206 Vishay C1 C1608X5R1A224K 220 nF 0603 TDK C2 C2012X5R0J226M 22 F / 6.3 V (X5R Ceramic) 0805 TDK C3 C2012X5R0J226M 22 F / 6.3 V (X5R Ceramic) 0805 TDK Micro−8 (3 x 5 mm) ON Semiconductor Lumileds 600 mA Design U1 NCP1421DMR2 NCP1421 Boost Converter D1 LXHL−WW06 White LED Lumileds L1 VLP6214T−6R8 6.8 H Inductor (6.2 x 5.8 x 1.4 mm) TDK / Coilcraft R1 CRCW0402104…. 100 k 0402 Vishay R2 CRCW04025603…. 475 k 0402 Vishay R3 CRCW04021503…. 100 k 0402 Vishay R4 CRCW12061R00... 1.0 1206 Vishay C1 C1608X5R1A224K 220 nF 0603 TDK C2 C2012X5R0J226M 22 F / 6.3 V (X5R Ceramic) 0805 TDK C3 C2012X5R0J226M 22 F / 6.3 V (X5R Ceramic) 0805 TDK DFN−10 (3 x 3 mm) ON Semiconductor 800 mA Design U1 NCP1422DMR2 NCP1422 Boost Converter D1 LXHL−WW06 White LED Lumileds L1 VLP6214T−6R8 6.8 H Inductor (6.2 x 5.8 x 1.4 mm) TDK R1 CRCW0402104…. 100 k 0402 Vishay R2 CRCW04025603…. 750 k 0402 Vishay R3 CRCW04021503…. 150 k 0402 Vishay R4 CRCW12061R50...* 0.75 1206 Vishay C1 C1608X5R1A224K 220 nF 0603 TDK C2 C2012X5R0J226M 22 F / 6.3 V (X5R Ceramic) 0805 TDK C3 C2012X5R0J226M 22 F / 6.3 V (X5R Ceramic) 0805 TDK *2 − 1.5 resistors were used in parallel. http://onsemi.com 4 AND8171/D Table 2. Bill of Materials for Figure 5 200/600 mA Design U1 NCP1422MNR2 NCP1422 Boost Converter DFN−10 (3 x 3 mm) ON Semiconductor D1 LXCL−PWF1 White LED (1.64 x 2.04 x 0.9 mm) Lumileds Q1 NTJS3157N N−Channel MOSFET SC−88 ON Semiconductor L1 VLP5610−6R8 6.8 H Inductor (5.6 x 5.0 x 1.0 mm) TDK R1 CRCW0402104…. 100 k 0402 Vishay R2 CRCW04025603…. 475 k 0402 Vishay R3 CRCW04021503…. 100 k 0402 Vishay R4a CRCW12062R00...* 1.0 1206 Vishay R4b CRCW12062R00... 2.0 1206 Vishay C1 C1608X5R1A224K 220 nF 0603 TDK C2 C2012X5R0J226M 22 F / 6.3 V (X5R Ceramic) 0805 TDK C3 C2012X5R0J226M 22 F / 6.3 V (X5R Ceramic) 0805 TDK *2 − 2.0 resistors were used in parallel. http://onsemi.com 5 AND8171/D ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. 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