LM3409 Datasheet, PDF (16/30 Pages) National Semiconductor (TI) – PFET Buck Controller for High Power LED Drives

Design Guide
TYPICAL APPLICATION
SPECIFICATIONS
Nominal input voltage: V_IN
Maximum input voltage: V_IN-MAX
Nominal output voltage (# of LEDs x forward voltage): V_O
LED string dynamic resistance: r_D
Switching frequency (at nominal V_IN, V_O): f_SW
Average LED current: I_LED
Inductor current ripple: Δi_L-PP
LED current ripple: Δi_LED-PP
Input voltage ripple: Δv_IN-PP
UVLO characteristics: V_TURN-ON and V_HYS
Expected efficiency: η
1. NOMINAL SWITCHING FREQUENCY
Calculate switching frequency (f_SW) at the nominal operating
point (V_IN and V_O). Assume a C_OFF value (between 470pF and
1nF) and a system efficiency (η). Solve for R_OFF:
2. INDUCTOR RIPPLE CURRENT
Set the inductor ripple current (Δi_L-PP) by solving for the ap-
propriate inductor (L1):
3. AVERAGE LED CURRENT
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Set the average LED current (I_LED) by first solving for the peak
inductor current (I_L-MAX):
Peak inductor current is detected across the sense resistor
(R_SNS). In most cases, assume the maximum value (V_ADJ =
1.24V) at the IADJ pin and solve for R_SNS:
If the calculated R_SNS is far from a standard value, the begin-
ning of the process can be iterated to choose a new R_OFF, L1,
and R_SNS value that is a closer fit. The easiest way to ap-
proach the iterative process is to change the nominal f_SW
target knowing that the switching frequency varies with oper-
ating conditions anyways.
Another method for finding a standard R_SNS value is to change
the V_ADJ value. However, this would require an external volt-
age source or a resistor from the IADJ pin to GND as ex-
plained in the Theory of Operation section of this datasheet.
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