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| LM3409 Datasheet, PDF (21/30 Pages) National Semiconductor (TI) – PFET Buck Controller for High Power LED Drives | |||
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Design Example #2
ANALOG DIMMING APPLICATION FOR 4 LEDS
SPECIFICATIONS
fSW = 500kHz
VIN = 24V; VIN-MAX = 42V
VO = 14V
ILED = 1A
ÎiL-PP = 450mA; ÎiLED-PP = 50mA
ÎvIN-PP = 1V
VTURN-ON = 10V; VHYS = 1.1V
η = 0.90
1. NOMINAL SWITCHING FREQUENCY
Assume COFF = 470pF and η = 0.90. Solve for ROFF:
30085619
The chosen components from step 1 are:
2. INDUCTOR RIPPLE CURRENT
Solve for L1:
The closest standard inductor value is 22 µH therefore the
actual ÎiL-PP is:
The closest 1% tolerance resistor is 15.4 k⦠therefore the
actual tOFF and target fSW are:
The chosen component from step 2 is:
21
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