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SC4505EVB Datasheet(PDF) 8 Page - Semtech Corporation |
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SC4505EVB Datasheet(HTML) 8 Page - Semtech Corporation |
8 / 15 page 8 2007 Semtech Corp. www.semtech.com SC4505 POWER MANAGEMENT 1.5A 2 1 81% S µ 1 H µ 1.5 2.7V 0.8A I PK = × × × + = When using tiny inductors for size-sensitive applications, only a few limited selections are available that fulfill this need, and all are within the low µH range of inductance. We select a Coiltronics 1.5 µH SD3114 series, presently the only 3mmx3mmx1.5mm inductor that can provide the required current in the microminiature size required in space-constrained applications. Table 4 shows a list of several low profile inductor manufacturers. Please consult the manufacturers for detailed information on their entire selection of power inductors. Output Capacitor Selection The next task in SC4505 design is targeting the proper amount of ripple voltage due to the constant-current LED loads. The two error amplifiers that control the PWM converter sense the delta between requested current and actual current in each output current regulator. On a cycle-by-cycle basis, a small amount of output ripple ensures good sensing and tight regulation, while the output current regulators keep each LED current at a fixed value. Overall, this allows usage of small output caps while ensuring precision LED current regulation. Although the mechanics of regulation and frequency dependence may be complex, actual selection of output capacitor can be simplified to two boundary conditions, minimum output current and maximum output current. Output capacitor is chosen to keep ripple voltage between 10mV & 200mV under all loads. Design example for Backlight=20mA, Flashlight=100mA: Minimum Load Current: 20mA (Backlight Only) Maximum Load Current: 120mA (Backlight and Flash) All other cases (Torch mode only, Flash mode only, Backlight and Torch) fall within these two boundary conditions, so they are automatically satisfied by the selected output capacitor. Since the load is a constant current, the capacitor equation ∆ ∆ ⋅ = ∆ T V C I can be solved for the output ripple. At 1MHz switching frequency and with the assumption of the worse case analysis (D=0), an even simpler relationship can be applied: where C OUT is in µF. For worse case analysis, We see that our typical case of 20mA, 120mA can be immediately converted into its corresponding ripple relationships of: ∆V OUTMIN = 20mV/COUT where C OUT is in µF. ∆V OUTMAX = 120mV/COUT where C OUT is in µF. For the example, if 1 µF output capacitor were used, the 20mV/120mV boundary conditions are well within the suggested guidelines. Recommended ceramic capacitor manufacturers are listed in Table 5. Applications Information Table 4. Recommended Inductors D) - 1 S µ 1 C I V T OU OUT OUT ( × × = ∆ S µ 1 C I V T OU OUT OUT × = ∆ PART L ( µH) MAX DCR (Ω) MAX HEIGHT (mm) VENDOR SD3112-1R0 SD3114-1R5 SD3114-4R7 1.0 1.5 4.7 0.069 0.057 0.147 1.2 1.45 1.45 Coiltronics www.cooperet.com LQH3C4R7M24 LQH3C100M24 4.7 10 0.260 0.300 2.2 2.2 Murata www.murada.com LB2016B4R7 LB2016B100 4.7 6.8 0.250 0.350 1.6 1.6 Taiyo Yuden www.t-yuden.com CMD4D06-4R7 CLQ4D10-4R7 CLQ4D10-6R8 4.7 4.7 6.8 0.216 0.162 0.195 0.8 1.2 1.2 Sumida www.sumida.com IHLP2525CZ1R5 1.5 3.3 4.7 0.014 0.028 0.037 3.0 Vishay www.vishay.com IHLP2525CZ3R3 IHLP2525CZ4R7 3.0 3.0 |
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