LM3424 Datasheet, PDF (25/50 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

The minimum allowable inductor RMS current rating (I_L-RMS)
can be calculated as:
Buck
8. SLOPE COMPENSATION
For all topologies, the preferred method to set slope compen-
sation is to ensure any duty cycle is attainable for the nominal
V_O and chosen L by solving for R_SLP:
Boost and Buck-boost
6. LED RIPPLE CURRENT
Set the nominal LED ripple current (Δi_LED-PP), by solving for
the output capacitance (C_O):
Buck
Boost and Buck-boost
9. LOOP COMPENSATION
Using a simple first order peak current mode control model,
neglecting any output capacitor ESR dynamics, the neces-
sary loop compensation can be determined.
First, the uncompensated loop gain (T_U) of the regulator can
be approximated:
Buck
Boost and Buck-boost
To set the worst case LED ripple current, use D_MAX when
solving for C_O. Remember, when PWM dimming it is recom-
mended to use a minimum of 40 µF of output capacitance to
improve performance.
The minimum allowable RMS output capacitor current rating
(I_CO-RMS) can be approximated:
Buck
Where the pole (ω_P1) is approximated:
Buck
Boost
Boost and Buck-boost
Buck-boost
7. PEAK CURRENT LIMIT
Set the peak current limit (I_LIM) by solving for the transistor
path sense resistor (R_LIM):
And the RHP zero (ω_Z1) is approximated:
Boost
Buck-boost
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