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LM3402 датащи(PDF) 5 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
номер детали LM3402
подробное описание детали  LM3402/LM3404 Fast Dimming and True Constant LED Current Evaluation Board
PDF  18 Pages
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производитель  NSC [National Semiconductor (TI)]
домашняя страница  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM3402 датащи(HTML) 5 Page - National Semiconductor (TI)

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Rearranging the above equation and solving for t
OFF with
t
ON set to 510 ns
t
OFF = 938 ns (satisfied)
Example 1 ON & OFF Times
V
IN (V)
V
OUT (V)
t
ON
t
OFF
36
10.4
5.10E-07
9.38E-07
48
10.4
3.82E-07
1.06E-06
60
10.4
3.06E-07
1.14E-06
Calculate Switching Frequency
V
IN = 36V, 48 and 60V.
Substituting equations:
f
SW = 691kHz (VIN = 36V, 48V, & 60V)
Calculate Inductor Value
With 50% ripple at V
IN = 48V
• I
F = 500 mA
• Δi
L = 250 mA (target)
• L = 57 µH (68 µH standard value)
Calculate
Δi for V
IN = 36V, 48V, and 60V with L = 68 µH
Example 1 Ripple Current
V
IN (V)
V
OUT (V)
Δi
L (A)
36
10.4
0.192
48
10.4
0.211
60
10.4
0.223
Calculate R
SNS
Calculate R
SNS at VIN typical (48V), and average LED current
(I
F) set to 500 mA.
30061623
FIGURE 8. Inductor Current Waveform
•
I
F = 500 mA
•
V
IN = 48V
•
V
OUT = 10.4V
•
L = 68 µH
•
t
D = 220 ns
•
t
ON = 382 ns
Using equations from the COT Overview section, calculate
R
SNS.
Therefore: R
SNS = 467 mΩ
Calculate Average LED current (I
F)
Calculate average current through the LEDs for V
IN = 36V and
60V.
Example 1 Average LED Current
V
IN (V)
V
OUT (V)
I
F (A)
36
10.4
0.490
48
10.4
0.500
60
10.4
0.506
5
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