LM3445MSLASHNOPB datasheet(30/42 Pages) TI1

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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

SNVS570M – JANUARY 2009 – REVISED NOVEMBER 2015

www.ti.com

Typical Application (continued)

8.2.1 Design Requirements

Known:

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

2. Number of LEDs in series = 7

3. Forward voltage drop of a single LED = 3.6 V

4. LED stack voltage = (7 × 3.6V) = 25.2 V

Choose:

1. Nominal switching frequency, fSW-TARGET = 350 kHz

2. ILED(AVE) = 400 mA

Δi (usually 15% - 30% of ILED(AVE)) = (0.30 × 400 mA) = 120 mA

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

5. Assumed minimum efficiency = 80%

8.2.2 Detailed Design Procedure

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

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)

9. Use standard value of 120 pF

10. Calculate ripple current: 400 mA × 0.30 = 120 mA

11. Calculate inductor value at tOFF = 3 µs:

(42)

12. Choose C10: 1 µF 200 V

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