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LM3402 Datasheet(PDF) 11 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part # LM3402
Description  LM3402/LM3404 Fast Dimming and True Constant LED Current Evaluation Board
PDF  18 Pages
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM3402 Datasheet(HTML) 11 Page - National Semiconductor (TI)

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Modified Application Circuit Design
Example 4
•
V
IN = 48V (±20%)
•
Driving 3, 4, or 5 HB LEDs with V
F = 3.4V
•
I
F = 500 mA (typical application)
•
Estimated efficiency = 82%
•
f
SW = 500 kHz (typ app)
The inductor, R
ON resistor, and the RSNS resistor are calcu-
lated for a typical or average design.
• V
OUT = 3 x 3.4V + 200 mV = 10.4V
• V
OUT = 4 x 3.4V + 200 mV = 13.8V
• V
OUT = 5 x 3.4V + 200 mV = 17.2V
Reduce switching frequency for the typical application to
about 500 kHz to increase efficiency.
Calculate t
ON, tOFF & RON
•
V
OUT = 13.8V
•
V
IN = 48V
•
I
F = 500 mA
•
t
D = 220 ns
•
η = 0.85
•
f
SW = 500 kHz
t
ON ≊ 705 ns
R
ON ≊ 179 kΩ (use standard value of 182 kΩ)
Calculate Inductor Value
•
I
F = 500 mA
•
Δi
L = 250 mA (target)
•
R
ON = 182 kΩ
•
L = 100 µH
Calculate
Δi
L with L = 100 µH (VIN = 48V, VOUT = 13.8V)
Δi
L = 241 mA (all combinations)
Calculate Switching Frequency
Example 4 Switching Frequency
V
IN (V)
V
OUT (V)
f
SW (kHz)
Three Series LEDs
36
10.4
374
48
10.4
412
V
IN (V)
V
OUT (V)
f
SW (kHz)
Three Series LEDs
60
10.4
435
Four Series LEDs
36
13.8
430
48
13.8
497
60
13.8
537
Five Series LEDs
36
17.2
454
48
17.2
558
60
17.2
620
Calculate R
SNS
•
V
OUT = 13.8V
•
V
IN = 48V
•
I
F = 500 mA
•
t
D = 220 ns
•
η = 0.85
•
L = 100 µH
R
SNS = 488 mΩ
Calculate Average Current through LED
Example 4 Average LED Current
V
IN (V)
V
OUT (V)
I
F (A)
Three Series LEDs
36
10.4
0.507
48
10.4
0.507
60
10.4
0.507
Four Series LEDs
36
13.8
0.500
48
13.8
0.500
60
13.8
0.500
Five Series LEDs
36
17.2
0.493
48
17.2
0.493
60
17.2
0.493
In the reduced frequency application you can see that there
is a difference of 14 mA between the low and high of the av-
erage current.
If the original t
ON circuit was used (no PNP transistor) with the
switching frequency centered around 500 kHz the difference
between the high and low values would be about 67 mA.
11
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