LM3445MM datasheet(22/26 Pages) NSC

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Design Example

The following design example illustrates the process of cal-

culating external component values.

Known:

Input voltage range (90V

AC – 135VAC)

Number of LEDs in series = 7

Forward voltage drop of a single LED = 3.6V

LED stack voltage = (7 x 3.6V) = 25.2V

Choose:

Nominal switching frequency, f

SW-TARGET = 250 kHz

LED(AVE) = 400 mA

Δi (usually 15% - 30% of

ILED(AVE)) = (0.30 x 400 mA) =

120 mA

Valley fill stages (1,2, or 3) = 2

Assumed minimum efficiency = 80%

Calculate:

Calculate minimum voltage V

BUCK equals:

Calculate maximum voltage V

BUCK equals:

Calculate t

OFF at VBUCK nominal line voltage:

Calculate t

ON(MIN) at high line to ensure that

ON(MIN) > 200 ns:

Calculate C11 and R4:

Choose current through R4: (between 50 µA and 100 µA)

70 µA

Use a standard value of 365 k

Ω

Calculate C11:

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:

13. Choose C10: 1.0 µF 200V

14. Calculate valley-fill capacitor values: V

AC low line =

90V

AC, VBUCK minimum equals 60V (no triac dimming at

maximum LED current). Set droop for 20V maximum at

full load and low line.

i) equals P

OUT/VBUCK (270 mA), dV equals 20V, dt equals

2.77 ms, and then C

TOTAL equals 37 µF. Therefore C7 =

C9 = 22 µF

LM3445MM Datasheet(PDF) 22 Page - National Semiconductor (TI)

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