RT9245CPC datasheet(21/25 Pages) RICHTEK

21 / 25 page

RT9245C

DS9245C-02

March 2007

www.richtek.com

Figure 29. The Frequency Response with No Load

1. Compensation Setting

a. Modulator Gain, Pole and Zero :

From the following formula :

Modulator Gain = VIN/VRAMP = 12/1.9 = 6.3 (i.e 16dB)

where VRAMP : Ramp amplitude of saw-tooth wave

LC Filter Pole = 3.88kHz and

ESR Zero = 28kHz

b. EA Compensation Network :

Select RB1 = 1.5k, RB2 = 15k, C1 = 2.7nF, C2 = 5.6pF,

C3 = 680pF and use the Type 3 compensation scheme

shown in Figure 28. By calculation.

Figure 28. Type 3 compensation network of EA

RB2

RB1

15k

2.7nF

C2 5.6pF

1.5k

C3 680pF

⇒

The over all loop gain with load is shown in Figure 29 to

Figure 31.

2. Over-Current Protection Setting

Consider the tem perature coeff icient of copper

3900ppm/

°C,

RIMAX = 8.9k

Ω

3. Soft-Start Capacitor Selection

For most application cases, 0.1

µF is a good engineering

value.

Ω

⇔

330

1.39m

40A

DCR

IMAX

CSN

IMAX

20dB)

(i.e.

Gain

Band

Middle

5.8kHz

(C2//C1)

RB2

3.9kHz

RB2

156kHz

RB1

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