LM3424 Datasheet, PDF (34/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

LM3424, LM3424-Q1
SNVS603C – AUGUST 2009 – REVISED AUGUST 2015
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Typical Applications (continued)
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
=
üi_{LED - PP}
12
(52)
Boost and Buck-Boost
D_MAX
I_CO-RMS = I_LED x _1-DMAX
(53)
8.2.1.2.7 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
(54)
8.2.1.2.8 Slope Compensation
For all topologies, the preferred method to set slope compensation is to ensure any duty cycle is attainable for
the nominal V_O and chosen L by solving for R_SLP:
1.5x10¹³ x L1
R_{SLP VOxRTxRLIM}
(55)
8.2.1.2.9 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
T_U
=
T_U0
x
1
¨¨©§¹⁺
s
Z
P1
¸¸¹·
(56)
Boost and Buck-Boost
¨¨©§^{1 -}
s
Z
Z1
¸¸¹·
T_U
=
T_U0
x
¨¨©§¹⁺
s
Z
P1
¸¸¹·
(57)
Where the pole (ω_P1) is approximated:
Buck ₃
1
Z_P1= _{rD x CO}
(58)
Boost ₃
Z_P1 =
r_D
2
x C_O
(59)
Buck-Boost
34
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