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| LM3424 Datasheet, PDF (37/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs | |||
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LM3424, LM3424-Q1
SNVS603C â AUGUST 2009 â REVISED AUGUST 2015
Typical Applications (continued)
DMAX
IT-MAX = 1 - DMAX x ILED
(82)
Approximate the nominal RMS transistor current (IT-RMS) :
Buck
IT- RMS = ILED x D
(83)
Boost and Buck-Boost
IT - RMS
=
ILED
D?
x
D
(84)
Given an NFET with on-resistance (RDS-ON), solve for the nominal power dissipation (PT):
PT
=
IT -
2
RMS
x
RDSON
(85)
8.2.1.2.12 Diode
The Schottky diode voltage rating should be at least 15% higher than the maximum blocking voltage (VRD-MAX):
Buck
VRD-MAX = VIN-MAX
(86)
Boost
VRD-MAX = VO
(87)
Buck-Boost
VRD-MAX = VIN-MAX + VO
(88)
The current rating should be at least 10% higher than the maximum average diode current (ID-MAX):
Buck
ID-MAX = (1 - DMIN) x ILED
(89)
Boost and Buck-Boost
ID-MAX = ILED
(90)
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
(91)
8.2.1.2.13 Output OVLO
For boost and buck-boost regulators, output OVLO is programmed with the turn-off threshold voltage (VTURN-OFF)
and the desired hysteresis (VHYSO). To set VHYSO, solve for ROV2:
ROV2
=
VHYSO
20 2A
(92)
To set VTURN-OFF, solve for ROV1:
Boost
ROV1
=
1.24V x ROV2
VTURN- OFF - 1.24V
(93)
Buck-Boost
ROV1 =
1.24V x ROV2
VTURN- OFF - 620 mV
(94)
Copyright © 2009â2015, Texas Instruments Incorporated
Product Folder Links: LM3424 LM3424-Q1
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