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LM3402 Datasheet(PDF) 7 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) 7 Page - National Semiconductor (TI)

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Example 2 Ripple Current
V
IN (V)
V
OUT (V)
Δi
L (A)
Three Series LEDs
36
10.4
0.192
48
10.4
0.211
60
10.4
0.223
Four Series LEDs
36
13.8
0.166
48
13.8
0.192
60
13.8
0.208
Four Series LEDs
36
17.2
0.141
48
17.2
0.173
60
17.2
0.193
Calculate R
SNS
Calculate R
SNS at VIN typical (48V), with four series LEDs
(13.8V = V
OUT), and average LED current (IF) set to 500 mA.
•
I
F = 500 mA
•
V
IN = 48V
•
V
OUT = 13.8V
•
L = 68 µH
•
t
D = 220 ns
•
t
ON = 382 ns
R
SNS = 446 mΩ
Calculate Average Current through LED
All combinations of V
IN, VOUT with RSNS = 446 mΩ
Example 2 Average LED Current
V
IN (V)
V
OUT (V)
I
F (A)
Three Series LEDs
36
10.4
0.511
48
10.4
0.521
60
10.4
0.526
Four Series LEDs
36
13.8
0.487
48
13.8
0.500
60
13.8
0.508
Five Series LEDs
36
17.2
0.463
48
17.2
0.479
60
17.2
0.489
In this application you can see that there is a difference of 63
mA between the low and high of the average LED current.
Modified COT Application Circuit
With the addition of one pnp transistor and one resistor (Q1
and R3) the average current through the LEDs can be made
to be more constant over input and output voltage variations.
Refer to page one figure 1. Resistor R
ON (R2) and Q1 turn the
t
ON equation into:
Ignore the PNP transistor’s V
BE voltage drop.
Design to the same criteria as the previous example with the
improved application and compare results.
7
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