AN4140 Application note STLD41 output power capability Introduction This application note is dedicated to the output power capability of the STLD41, which is a white LED driver for display backlight. The schematic, functional description and external components selection are also discussed in this application note. Detailed behavior, if different LED counts on the output and different currents through LEDs are used, is also shown. August 2012 Doc ID 023412 Rev 1 1/17 www.st.com Contents AN4140 Contents 1 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3 Application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4 Component calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5 6 7 2/17 4.1 CIN selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.2 COUT selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.3 RSENSE value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.4 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Output power capability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5.1 With the LQH6PPN470M3 inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5.2 With the MSS1038473MLB inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.3 With the LPS6235473MLB inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.1 With the LQH6PPN470M3 inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.2 With the MSS1038473MLB inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 6.3 With the LPS6235473MLB inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Doc ID 023412 Rev 1 AN4140 1 Description Description The STLD41 is a boost converter that operates from 3.0 V to 21 V and can provide an output voltage as high as 38 V. It can drive up to 40 white LEDs connected in 4 strings of 10 LEDs in series. The total output current capability is 120 mA at an output voltage of 38 V. Regulation is performed by the internal error amplifier which works with the feedback voltage from the sensing resistor. The device can be turned ON/OFF by way of the logic signal connected to the EN pin. The Enable pin allows the device to be turned off, so reducing the current consumption to less than 1 µA. The LEDs can be dimmed by applying a PWM signal to the PWM pin. Soft-start with controlled inrush current limit, thermal shutdown and overvoltage protection are integrated functions of the device. Doc ID 023412 Rev 1 3/17 Applications 2 AN4140 Applications ● Mini PCs ● PMP & PND ● Printers ● Game consoles. Figure 1. 4/17 Package DFN8 (3 x 3 mm) Doc ID 023412 Rev 1 AN4140 3 Application circuit Application circuit Figure 2. Table 1. Application schematic List of external components Comp. Manufacturer Part number Value L CoilCraft Murata CoilCraft LPS6235473MLB LQH6PPN470M43 MSS1038473MLB 47 µH CIN Murata GRM32ER71H106K 10 µF 1210 ±10%, X7R, 50 V COUT Murata GRM31CR61H225KA88L 2.2 µF 1206 ±15%, X5R, 50 V D STMicroelectronics STPS1L40M 1A 3.75 x 1 x 1.9 mm 40 V 0.4 Ω – 1.6 Ω 0603 RSENSE Doc ID 023412 Rev 1 Size Ratings 6 x 6 x 3.5 mm I = 1.2 A, DCR = 0.245 Ω 6 x 6 x 4.3 mm I = 1.25 A, DCR = 0.23 Ω 10 x 10 x 3.8 mm I = 2.22 A, DCR = 0.13 Ω 5/17 Component calculation AN4140 4 Component calculation 4.1 CIN selection It is recommended to use 10 µF as the input capacitor to achieve good stability of the device and low noise on the VIN track. 4.2 COUT selection It is recommended to use 2.2 µF as the optimal value of the output capacitor to get the best compromise between output voltage ripple and load transient response. 4.3 RSENSE value RSENSE = VREF / ILED RSENSE sensing resistor VREF reference voltage = 160 mV typical ILED total LED output current PRSENSE = RSENSE * ILED² PRSENSE power dissipation of the sensing resistor Table 2. 4.4 RSENSE calculated values ILED 100 150 200 250 300 350 400 RSENSE 1.6 Ω 1.1 Ω 0.8 Ω 0.65 Ω 0.55 Ω 0.45 Ω 0.4 Ω Inductor selection A thin shielded inductor with a low DC series resistance of winding is recommended for this application. To achieve a good efficiency in step-up mode, it is recommended to use an inductor with a DC series resistance RDCL = RD / 10 [Ω, Ω, 1], where RD is the dynamic resistance of the LED. Equation 1 6/17 Doc ID 023412 Rev 1 AN4140 Component calculation where: IPEAK is peak inductor current IOUT is current sourced at the VOUT pin η is the efficiency of the STLD41 VOUT is output voltage at the VOUT pin VIN is input voltage at the VBAT pin L is inductance value of the inductor f is the switching frequency. Doc ID 023412 Rev 1 7/17 Output power capability 5 AN4140 Output power capability All measurements were made at ambient temperature (24 °C) and 3 different inductors. 5.1 With the LQH6PPN470M3 inductor Figure 3. Current capability with the LQH6PPN470M3 AM12752v1 40 38 36 34 32 30 4.2 7 28 3.8 26 VOUT-MAX [V] 8 6 24 10 Iout = 100 mA 8 6 4.2 Iout = 150 mA 20 Iout = 200 mA 3.8 3 18 20 10 4.6 22 14 8 5 3.4 12 4.6 16 Iout = 250 mA 7 Iout = 300 mA 14 3.4 12 8 3 Iout = 350 mA 6 4.6 10 7 5 4.2 Iout = 400 mA 3.8 4.2 6 2.5 8/17 5.0 VIN [V] Doc ID 023412 Rev 1 10.0 20.0 AN4140 With the MSS1038473MLB inductor Figure 4. Current capability with the MSS1038473MLB AM12753v1 40 38 36 4.6 34 32 5 3.8 12 8 7 10 14 20 12 30 10 6 28 VOUT- MAX [V] 5.2 Output power capability 3.4 26 4.6 24 8 5 4.2 Iout = 100 mA 22 18 3 7 4.6 3.8 20 6 4.2 3.4 Iout = 150 mA Iout = 200 mA 7 Iout = 250 mA 16 4.6 3.8 14 Iout = 300 mA Iout = 350 mA 12 5 3 10 3 8 3.4 Iout = 400 mA 3.8 4.2 6 2.5 5.0 10.0 20.0 VIN [V] Doc ID 023412 Rev 1 9/17 Output power capability 5.3 AN4140 With the LPS6235473MLB inductor Figure 5. Current capability with the LPS6235473MLB AM12754v1 40 38 36 4.6 34 32 8 4.2 7 30 5 28 VOUT - MAX [V] 20 4.6 3.8 26 24 6 4.2 Iout = 100 mA 22 3.4 20 Iout = 150 mA 5 Iout = 200 mA 18 3 3.8 4.6 16 3.4 14 12 3 4.2 3.4 10 3 8 6 2.5 5.0 10.0 VIN [V] 10/17 Doc ID 023412 Rev 1 20.0 AN4140 Efficiency 6 Efficiency 6.1 With the LQH6PPN470M3 inductor Eff [ %] Figure 6. IOUT = 100 mA with the LQH6PPN470M3 AM12755v1 95.00 93.00 91.00 89.00 87.00 85.00 83.00 81.00 79.00 77.00 75.00 73.00 71.00 69.00 67.00 65.00 63.00 61.00 59.00 57.00 55.00 Vout = 12 V Vout = 14 V Vout = 17 V Vout = 20 V Vout = 23 V Vout = 25 V Vout = 28 V Vout = 31 V Vout = 33 V Vout = 36 V 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 VIN [V ] IOUT = 200 mA with the LQH6PPN470M3 Eff [%] Figure 7. AM12756v1 97.00 95.00 93.00 91.00 89.00 87.00 85.00 83.00 81.00 79.00 77.00 75.00 73.00 71.00 69.00 67.00 65.00 63.00 61.00 59.00 57.00 55.00 Vout = 12 V Vout = 14 V Vout = 17 V Vout = 20 V Vout = 23 V Vout = 26 V Vout = 28 V Vout = 31 V Vout = 34 V Vout = 36 V 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 VIN [V] Doc ID 023412 Rev 1 11/17 Efficiency AN4140 Eff [%] Figure 8. IOUT = 300 mA with the LQH6PPN470M3 AM12757v1 99.00 97.00 95.00 93.00 91.00 89.00 87.00 85.00 83.00 81.00 79.00 77.00 75.00 73.00 71.00 69.00 67.00 65.00 63.00 61.00 59.00 57.00 55.00 Vout = 12 V Vout = 14 V Vout = 17 V Vout = 20 V Vout = 23 V Vout = 25 V Vout = 28 V Vout = 31 V Vout = 33 V Vout = 36 V 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 VIN [V] Eff [%] Figure 9. 99.00 97.00 95.00 93.00 91.00 89.00 87.00 85.00 83.00 81.00 79.00 77.00 75.00 73.00 71.00 69.00 67.00 65.00 63.00 61.00 59.00 57.00 55.00 IOUT = 400 mA with the LQH6PPN470M3 AM12758v1 Vout = 9 V Vout = 12 V Vout = 15 V Vout = 17 V Vout = 20 V Vout = 23 V Vout = 26 V Vout = 28 V Vout = 31 V Vout = 34 V 2 3 4 5 6 7 8 9 10 11 12 13 VIN [V] 12/17 Doc ID 023412 Rev 1 14 15 16 17 18 19 20 21 AN4140 With the MSS1038473MLB inductor Eff [%] Figure 10. IOUT = 100 mA with the MSS1038473MLB 95.00 93.00 91.00 89.00 87.00 85.00 83.00 81.00 79.00 77.00 75.00 73.00 71.00 69.00 67.00 65.00 63.00 61.00 59.00 57.00 55.00 AM12759v1 Vout = 9 V Vout = 12 V Vout = 16 V Vout = 19 V Vout = 22 V Vout = 25 V Vout = 27 V Vout = 30 V Vout = 33 V Vout = 36 V 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 VIN [V] Figure 11. IOUT = 200 mA with the MSS1038473MLB Eff [%] 6.2 Efficiency 99.00 97.00 95.00 93.00 91.00 89.00 87.00 85.00 83.00 81.00 79.00 77.00 75.00 73.00 71.00 69.00 67.00 65.00 63.00 61.00 59.00 57.00 55.00 AM12760v1 Vout = 10 V Vout = 13 V Vout = 16 V Vout = 19 V Vout = 22 V Vout = 25 V Vout = 27 V Vout = 30 V Vout = 33 V Vout = 36 V 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 VIN [V] Doc ID 023412 Rev 1 13/17 Efficiency AN4140 Eff [%] Figure 12. IOUT = 300 mA with the MSS1038473MLB 99.00 97.00 95.00 93.00 91.00 89.00 87.00 85.00 83.00 81.00 79.00 77.00 75.00 73.00 71.00 69.00 67.00 65.00 63.00 61.00 59.00 57.00 55.00 AM12761v1 Vout = 10 V Vout = 13 V Vout = 17 V Vout = 20 V Vout = 23 V Vout = 26 V Vout = 28 V Vout = 31 V Vout = 33 V Vout = 36 V 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 VIN [V] Eff [%] Figure 13. IOUT = 400 mA with the MSS1038473MLB 99.00 97.00 95.00 93.00 91.00 89.00 87.00 85.00 83.00 81.00 79.00 77.00 75.00 73.00 71.00 69.00 67.00 65.00 63.00 61.00 59.00 57.00 55.00 AM12762v1 Vout = 10 V Vout = 13 V Vout = 17 V Vout = 20 V Vout = 23 V Vout = 26 V Vout = 28 V Vout = 30 V Vout = 33 V Vout = 36 V 2 3 4 5 6 7 8 9 10 11 12 13 VIN [V] 14/17 Doc ID 023412 Rev 1 14 15 16 17 18 19 20 21 AN4140 With the LPS6235473MLB inductor Figure 14. IOUT = 100 mA with the LPS6235473MLB AM12763v1 96.00 94.00 92.00 90.00 88.00 Vout = 10 V 86.00 Vout = 13 V Eff [%] 84.00 82.00 Vout = 16 V 80.00 Vout = 19 V 78.00 Vout = 22 V 76.00 74.00 Vout = 25 V 72.00 Vout = 28 V 70.00 68.00 Vout = 30 V 66.00 Vout = 33 V 64.00 Vout = 36 V 62.00 60.00 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 VIN [V] Figure 15. IOUT = 200 mA with the LPS6235473MLB AM12764v1 98.00 96.00 94.00 92.00 90.00 Vout = 10 V 88.00 86.00 Vout = 13 V 84.00 Vout = 16 V 82.00 Eff [%] 6.3 Efficiency Vout = 19 V 80.00 78.00 Vout = 22 V 76.00 Vout = 25 V 74.00 Vout = 28 V 72.00 70.00 Vout = 30 V 68.00 66.00 Vout = 33 V 64.00 Vout = 36 V 62.00 60.00 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 VIN [V] Doc ID 023412 Rev 1 15/17 Revision history 7 AN4140 Revision history Table 3. 16/17 Document revision history Date Revision 28-Aug-2012 1 Changes Initial release. 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