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| LM3424 Datasheet, PDF (40/69 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)
ILED = 1A
ÎiL-PP = 700 mA
ÎiLED-PP = 12 mA
ÎvIN-PP = 100 mV
ILIM = 6A
VTURN-ON = 10V
VHYS = 3V
VTURN-OFF = 40V
VHYSO = 10V
TBK = 70°C
TEND= 120°C
tTSU = 30 ms
8.2.2.2 Detailed Design Procedure
8.2.2.2.1 Operating Point
Solve for VO and rD:
VO = N x VLED = 6 x 3.5V = 21V
rD = N x rLED = 6 x 325 m: = 1.95:
Solve for D, D', DMAX, and DMIN:
D
=
VO
VO + VIN
=
21V
21V + 24V
=
0.467
D' = 1- D = 1- 0.467= 0.533
DMIN =
VO
+
VO
VIN-MAX
=
21V
21V + 70V
= 0.231
DMAX =
VO
VO + VIN-MIN
=
21V
21V +10V
= 0.677
8.2.2.2.2 Switching Frequency
Solve for RT:
R
T
=
1+1.95
1.4 x
x 10-8
10-10 x
x fSW
f
SW
1+1.95 x 10-8 x 500 kHz
= 1.4 x 10-10 x 500 kHz
=
14.4
kΩ
The closest standard resistor is 14.3 k⦠therefore fSW is:
1
fSW = 1.4 x 10-10x RT - 1.95 x 10-8
1
f
SW
=
1.4 x 10-10x14.3 kΩ - 1.95 x 10-8 = 504 kHz
The chosen component from step 2 is:
RT = 14.3 k:
8.2.2.2.3 Average LED Current
Solve for RSNS:
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