LM3429_14 Datasheet, PDF (25/50 Pages) Texas Instruments – LM3429Q1 N-Channel Controller for Constant Current LED Drivers

LM3429
www.ti.com
SNVS616F – APRIL 2009 – REVISED JANUARY 2010
5. LED RIPPLE CURRENT
Set the nominal LED ripple current (Δi_LED-PP), by solving for the output capacitance (C_O):
Buck
C_O
=
8 x f_SW
'i_{L - PP}
x r_D x 'i_{LED- PP}
(41)
Boost and Buck-boost
C_O
=
r_D
ü^{ILED x D}
x _{i LED-PP} x
f_SW
(42)
To set the worst case LED ripple current, use D_MAX when solving for C_O.
The minimum allowable RMS output capacitor current rating (I_CO-RMS) can be approximated:
Buck
300 _ICO
-
RMS
=
üi_{LED -}
12
PP
(43)
Boost and Buck-boost
D_MAX
I_CO-RMS = I_LED x _1-DMAX
(44)
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
(45)
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
T_U
=
T_U0
x
1
¨¨©§¹⁺
s
Z
P1
¸¸¹·
(46)
Boost and Buck-boost
¨¨©§^{1 -}
s
Z
Z1
¸¸¹·
T_U
=
T_U0
x
¨¨©§¹⁺
s
Z
P1
¸¸¹·
(47)
Where the pole (ω_P1) is approximated:
Buck ₃
1
Z_P1= _{rD x CO}
(48)
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