The minimum allowable inductor RMS current rating (I
L-RMS
)
can be calculated as:
Buck
Boost and Buck-boost
6. LED RIPPLE CURRENT
Set the nominal LED ripple current (
Δ
i
LED-PP
), by solving for
the output capacitance (C
O
):
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
Boost and Buck-boost
7. PEAK CURRENT LIMIT
Set the peak current limit (I
LIM
) by solving for the transistor
path sense resistor (R
LIM
):
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
:
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
Where the pole (
ω
P1
) is approximated:
Buck
Boost
Buck-boost
And the RHP zero (
ω
Z1
) is approximated:
Boost
Buck-boost
25
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LM3424
