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LM3421, LM3421-Q1
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Boost and Buck-boost
SNVS574E – JULY 2008 – REVISED MAY 2013
§ ' I L-PP x D' ·
? 1
I L-RMS =
I LED
D'
x 1+
1
12
I LED
x ¨ ?
2
(47)
5. LED RIPPLE CURRENT
Set the nominal LED ripple current ( Δ i LED-PP ), by solving for the output capacitance (C O ):
Buck
C O =
' i L - PP
8 x f SW x r D x ' i LED - PP
(48)
Boost and Buck-boost
I LED x D
C O =
r D x
ü
i LED - PP
x f SW
(49)
To set the worst case LED ripple current, use D MAX when solving for C O . Remember, when PWM dimming it is
recommended 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
I CO - RMS =
ü i LED - PP
12
(50)
Boost and Buck-boost
I CO-RMS = I LED x
D MAX
1-D MAX
(51)
6. PEAK CURRENT LIMIT
Set the peak current limit (I LIM ) by solving for the transistor path sense resistor (R LIM ):
R LIM =
245 mV
I LIM
(52)
7. LOOP COMPENSATION
Using a simple first order peak current mode control model, neglecting any output capacitor ESR dynamics, the
necessary loop compensation can be determined.
First, the uncompensated loop gain (T U ) of the regulator can be approximated:
Buck
¨ 1 +
s ·
Z P 1 ?
T U = T U 0 x
§
¨
?
1
?
1
(53)
Copyright ? 2008–2013, Texas Instruments Incorporated
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