LM3445-120VFLBK/NOPB National Semiconductor, LM3445-120VFLBK/NOPB Datasheet - Page 22

• Manufacturer Part Number: LM3445-120VFLBK/NOPB
• Description: PROGRAMMERS, DEVELOPMENT SYSTEMS
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3445-120VFLBK/NOPB

• Current - Output / Channel: 365mA
• Outputs And Type: 1, Isolated
• Voltage - Output: 20V
• Features: Dimmable, TRIAC Dimmable
• Voltage - Input: 90 ~ 135VAC
• Utilized Ic / Part: LM3445
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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Design Example
The following design example illustrates the process of cal-
culating external component values.
Known:
1.
2.
3.
4.
Choose:
1.
2.
3.
4.
5.
Calculate:
1.
2.
3.
4.
Input voltage range (90V
Number of LEDs in series = 7
Forward voltage drop of a single LED = 3.6V
LED stack voltage = (7 x 3.6V) = 25.2V
Nominal switching frequency, f
I
Δi (usually 15% - 30% of
120 mA
Valley fill stages (1,2, or 3) = 2
Assumed minimum efficiency = 80%
Calculate minimum voltage V
Calculate maximum voltage V
Calculate t
Calculate t
t
LED(AVE)
ON(MIN)
> 200 ns:
= 400 mA
OFF
ON(MIN)
at V
at high line to ensure that
BUCK
nominal line voltage:
AC
ILED(AVE)
– 135V
BUCK
BUCK
SW-TARGET
) = (0.30 x 400 mA) =
equals:
AC
equals:
)
= 250 kHz
22
5.
6.
7.
8.
9.
10. Use standard value of 120 pF
11. Calculate ripple current: 400 mA X 0.30 = 120 mA
12. Calculate inductor value at t
13. Choose C10: 1.0 µF 200V
14. Calculate valley-fill capacitor values: V
Calculate C11 and R4:
Choose current through R4: (between 50 µA and 100 µA)
70 µA
Use a standard value of 365 kΩ
Calculate C11:
90V
maximum LED current). Set droop for 20V maximum at
full load and low line.
i) equals P
2.77 ms, and then C
C9 = 22 µF
AC
, V
BUCK
OUT
/V
minimum equals 60V (no triac dimming at
BUCK
TOTAL
(270 mA), dV equals 20V, dt equals
equals 37 µF. Therefore C7 =
OFF
= 3 µs:
AC
low line =