LM3429_14 Datasheet, PDF (24/50 Pages) Texas Instruments – LM3429Q1 N-Channel Controller for Constant Current LED Drivers

LM3429
SNVS616F – APRIL 2009 – REVISED JANUARY 2010
Buck (Constant Ripple vs. V_O)
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R_T
=
25 x (V_IN x V_O - V_O2)
f_SW x C_T x V_IN2
(33)
Boost and Buck-boost
R_T
=
25
f_SW x C_T
(34)
3. AVERAGE LED CURRENT
For all topologies, set the average LED current (I_LED) knowing the desired current sense voltage (V_SNS) and
solving for R_SNS:
R_SNS =
V_SNS
I_LED
(35)
If the calculated R_SNS is too far from a desired standard value, then V_SNS will have to be adjusted to obtain a
standard value.
Setup the suggested signal current of 100 µA by assuming R_CSH = 12.4 kΩ and solving for R_HSP:
R_HSP =
I_LED
x R_CSH x R_SNS
1.24V
(36)
If the calculated R_HSP is too far from a desired standard value, then R_CSH can be adjusted to obtain a standard
value.
4. INDUCTOR RIPPLE CURRENT
Set the nominal inductor ripple current (Δi_L-PP) by solving for the appropriate inductor (L1):
Buck
L1=
(_üV_IN - V_O) x D
i ^{L -PP x fSW}
(37)
Boost and₃₀Buck-boost
L1
=
V_IN x D
üiL- PP ^{x f}SW
(38)
To set the worst case inductor ripple current, use V_IN-MAX and D_MIN when solving for L1.
The minimum allowable inductor RMS current rating (I_L-RMS) can be calculated as:
Buck
¨©§ ¸¹· ^IL-RMS ^{= I}LED ^x
1 + ¹ x ^{'IL-PP 2}
12 ILED
(39)
Boost and Buck-boost
§ · ^ILED
¨© ¸¹ ^IL-RMS ⁼ _D' ^x
1+ ¹ x
12
'I_L-PP x D'
I_LED
2
(40)
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