LM3424 Datasheet, PDF (36/69 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)
¨¨©§^1- _Z^s_Z1¸¸¹·
T = T_U0 x _{¨¨©§1+ ZsP1¸¸¹· x ¨¨©§1+ ZsP2¸¸¹· x ¨¨©§1+ ZsP3¸¸¹·}
(71)
8.2.1.2.10 Input Capacitance
Set the nominal input voltage ripple (Δv_IN-PP) by solving for the required capacitance (C_IN):
Buck
I_LED x (1 - D) x D
C_IN = _{VIN-PP x fSW}
(72)
Boost
i_L-PP
C_IN = _{8 x VIN-PP x fSW}
(73)
Buck-Boost
I_LED x D
C_IN = _{VIN-PP x fSW}
(74)
Use D_MAX to set the worst case input voltage ripple, when solving for C_IN in a buck-boost regulator and D_MID = 0.5
when solving for C_IN in a buck regulator.
The minimum allowable RMS input current rating (I_CIN-RMS) can be approximated:
Buck
I_{CIN- RMS} = I_LED x D_MID x (1-D_MID)
(75)
Boost
i_L-PP
I_CIN-RMS =
12
(76)
Buck-Boost
D_MAX
I_CIN-RMS = I_LED x _1-DMAX
(77)
8.2.1.2.11 NFET
The NFET voltage rating should be at least 15% higher than the maximum NFET drain-to-source voltage (V_T-
MAX^):
Buck
V_{T - MAX} = V_{IN - MAX}
(78)
Boost
V_{T - MAX} = V_O
(79)
Buck-Boost
V_{T- MAX} = V_{IN- MAX} + V_O
(80)
The current rating should be at least 10% higher than the maximum average NFET current (I_T-MAX):
Buck
I_T-MAX = D_MAX x I_LED
(81)
Boost and Buck-Boost
36
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