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| LM3429_14 Datasheet, PDF (28/50 Pages) Texas Instruments – LM3429Q1 N-Channel Controller for Constant Current LED Drivers | |||
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LM3429
SNVS616F â APRIL 2009 â REVISED JANUARY 2010
Buck-boost
VT- MAX = VIN- MAX + VO
The current rating should be at least 10% higher than the maximum average NFET current (IT-MAX):
Buck
IT-MAX = DMAX x ILED
Boost and Buck-boost
DMAX
IT-MAX = 1 - DMAX x ILED
Approximate the nominal RMS transistor current (IT-RMS) :
Buck
IT- RMS = ILED x D
Boost and Buck-boost
www.ti.com
(70)
(71)
(72)
(73)
IT -
RMS
=
ILED
Dc
x
D
(74)
Given an NFET with on-resistance (RDS-ON), solve for the nominal power dissipation (PT):
PT
=
IT -
2
RMS
x
RDSON
(75)
10. DIODE
The Schottky diode voltage rating should be at least 15% higher than the maximum blocking voltage (VRD-MAX):
Buck
VRD-MAX = VIN-MAX
(76)
Boost
VRD-MAX = VO
(77)
Buck-boost
VRD-MAX = VIN-MAX + VO
(78)
The current rating should be at least 10% higher than the maximum average diode current (ID-MAX):
Buck
ID-MAX = (1 - DMIN) x ILED
(79)
Boost and Buck-boost
ID-MAX = ILED
(80)
Replace DMAX with D in the ID-MAX equation to solve for the average diode current (ID). Given a diode with forward
voltage (VFD), solve for the nominal power dissipation (PD):
PD = ID x VFD
(81)
28
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