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| LM3424 Datasheet, PDF (43/69 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)
ÏZ1 is approximated:
ZZ1
=
rD x
Dx
Dc2
L1
=
1.95:
0.467
x
x
0.5332
33 PH
=
36k
rad
sec
TU0 is approximated:
TU0
=
Dc x 620V
(1+D) x ILED x RLIM
=
0.533 x
1.467x1A
620V
x 0.04:
=
5630
To ensure stability, calculate ÏP2:
ZP2 =
min(ZP1,ZZ1)
5 x TU0
=
5
ZP1
x 5630
=
19k
rad
sec
5 x 5630
=
0.675
rad
sec
Solve for CCMP:
1
1
CCMP = ÏP2 x 5 x 106Ω = 0.675 rad x 5 x 106Ω = 0.30μF
sec
To attenuate switching noise, calculate ÏP3:
ZP3 = (maxZP1, ZZ1) x10 = ZZ1 x 10
ZP3
=
36k
rad
sec
x10
=
360k
rad
sec
Assume RFS = 10⦠and solve for CFS:
1
1
CFS = 10: x ZP3 = 10: x 360krad = 0.28 PF
sec
The chosen components from step 9 are:
CCMP = 0.33 PF
RFS = 10:
CFS = 0.27PF
(130)
(131)
(132)
(133)
(134)
(135)
(136)
8.2.2.2.10 Input Capacitance
Solve for the minimum CIN:
CIN =
ILED x D
vIN-PP x fSW
=
1A x
100 mV
0.467
x 504 kHz
=
9.27 2F
(137)
To minimize power supply interaction a 200% larger capacitance of approximately 20 µF is used, therefore the
actual ÎvIN-PP is much lower. Since high voltage ceramic capacitor selection is limited, four 4.7 µF X7R capacitors
are chosen.
Determine minimum allowable RMS current rating:
IIN-RMS = ILED x
DMAX
1- DMAX
= 1A x
0.677
1- 0.677
=
1.45A
(138)
The chosen components from step 10 are:
CIN = 4 x 4.7 2F
(139)
8.2.2.2.11 NFET
Determine minimum Q1 voltage rating and current rating:
VT-MAX = VIN- MAX + VO = 70V + 21V = 91V
(140)
Copyright © 2009â2015, Texas Instruments Incorporated
Product Folder Links: LM3424 LM3424-Q1
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