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LM3445
SNVS570L – JANUARY 2009 – REVISED MAY 2013
DESIGN EXAMPLE
The following design example illustrates the process of calculating external component values.
Known:
1. Input voltage range (90V AC – 135V AC )
2. Number of LEDs in series = 7
3. Forward voltage drop of a single LED = 3.6V
4. LED stack voltage = (7 x 3.6V) = 25.2V
Choose:
1. Nominal switching frequency, f SW-TARGET = 250 kHz
2. I LED(AVE) = 400 mA
3. Δ i (usually 15% - 30% of ILED(AVE) ) = (0.30 x 400 mA) = 120 mA
4. Valley fill stages (1,2, or 3) = 2
5. Assumed minimum efficiency = 80%
Calculate:
1. Calculate minimum voltage V BUCK equals:
www.ti.com
90 2 x SIN(135 )
V BUCK(MIN) =
2
o
= 45V
(36)
2. Calculate maximum voltage V BUCK equals:
V BUCK(MAX) = 135 2 = 190V
3. Calculate t OFF at V BUCK nominal line voltage:
(37)
u 25.2V
t OFF =
1
1
0.8 115 2
(250 kHz)
= 3.23 P s
(38)
4. Calculate t ON(MIN) at high line to ensure that t ON(MIN) > 200 ns:
u 25.2V
u 25.2V
t ON (MIN) =
1
1
0.8 135 2
1
0.8 135 2
u 3.23 P s = 638 ns
(39)
5. Calculate C11 and R4:
6. Choose current through R4: (between 50 μA and 100 μA) 70 μA
V LED
R4 =
I COLL
= 360 k :
(40)
7. Use a standard value of 365 k ?
8. Calculate C11:
9.
C11 =
V LED
R4
t OFF
1.276
= 175 pF
(41)
10. Use standard value of 120 pF
11. Calculate ripple current: 400 mA X 0.30 = 120 mA
12. Calculate inductor value at t OFF = 3 μs:
u 25.2V
25.2V 1
L2 =
1
0.8 115 2
(350 kHz x 0.1A)
= 580 P H
(42)
13. Choose C10: 1.0 μF 200V
14. Calculate valley-fill capacitor values: V AC low line = 90V AC , V BUCK minimum equals 60V (no triac dimming at
maximum LED current). Set droop for 20V maximum at full load and low line.
28
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