LM3445M datasheet(28/39 Pages) TI

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25.2V

115

(350 kHz x 0.1A)

L2 =

= 580 PH

25.2V 1

0.8

C11 =

tOFF

1.276

VLED

= 175 pF

VLED

R4 =

= 360 k:

ICOLL

25.2V

135

u 3.23 Ps = 638 ns

0.8

tON (MIN) =

25.2V

135

0.8

25.2V

115

(250 kHz)

= 3.23 Ps

0.8

tOFF =

VBUCK(MAX) = 135 2 = 190V

VBUCK(MIN) =

x SIN(135

= 45V

LM3445

SNVS570L – JANUARY 2009 – REVISED MAY 2013

www.ti.com

DESIGN EXAMPLE

The following design example illustrates the process of calculating external component values.

Known:

1. Input voltage range (90VAC – 135VAC)

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, fSW-TARGET = 250 kHz

2. ILED(AVE) = 400 mA

Δi (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 VBUCK equals:

(36)

2. Calculate maximum voltage VBUCK equals:

(37)

3. Calculate tOFF at VBUCK nominal line voltage:

(38)

4. Calculate tON(MIN) at high line to ensure that tON(MIN) > 200 ns:

(39)

5. Calculate C11 and R4:

6. Choose current through R4: (between 50 µA and 100 µA) 70 µA

(40)

7. Use a standard value of 365 k

Ω

8. Calculate C11:

(41)

10. Use standard value of 120 pF

11. Calculate ripple current: 400 mA X 0.30 = 120 mA

12. Calculate inductor value at tOFF = 3 µs:

(42)

13. Choose C10: 1.0 µF 200V

14. Calculate valley-fill capacitor values: VAC low line = 90VAC, VBUCK minimum equals 60V (no triac dimming at

maximum LED current). Set droop for 20V maximum at full load and low line.

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