LM3424 Datasheet, PDF (27/50 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

The current rating should be at least 10% higher than the
maximum average NFET current (I_T-MAX):
Buck
13. OUTPUT OVLO
For boost and buck-boost regulators, output OVLO is pro-
grammed with the turn-off threshold voltage (V_TURN-OFF) and
the desired hysteresis (V_HYSO). To set V_HYSO, solve for R_OV2:
Boost and Buck-boost
Approximate the nominal RMS transistor current (I_T-RMS) :
Buck
Boost and Buck-boost
To set V_TURN-OFF, solve for R_OV1:
Boost
Buck-boost
Given an NFET with on-resistance (R_DS-ON), solve for the
nominal power dissipation (P_T):
12. DIODE
The Schottky diode voltage rating should be at least 15%
higher than the maximum blocking voltage (V_RD-MAX):
Buck
Boost
A small filter capacitor (C_OVP = 47 nF) should be added from
the OVP pin to ground to reduce coupled switching noise.
14. INPUT UVLO
For all topologies, input UVLO is programmed with the turn-
on threshold voltage (V_TURN-ON) and the desired hysteresis
(V_HYS).
Method #1: If no PWM dimming is required, a two resistor
network can be used. To set V_HYS, solve for R_UV2:
To set V_TURN-ON, solve for R_UV1:
Buck-boost
The current rating should be at least 10% higher than the
maximum average diode current (I_D-MAX):
Buck
Method #2: If PWM dimming is required, a three resistor net-
work is suggested. To set V_TURN-ON, assume R_UV2 = 10 kΩ
and solve for R_UV1 as in Method #1. To set V_HYS, solve for
R_UVH:
Boost and Buck-boost
Replace D_MAX with D in the I_D-MAX equation to solve for the
average diode current (I_D). Given a diode with forward voltage
(V_FD), solve for the nominal power dissipation (P_D):
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