RT9245PC datasheet(17/22 Pages) RICHTEK

17 / 22 page

RT9245

DS9245-06

March 2007

www.richtek.com

Design Procedure Suggestion

a.Output filter pole and zero (Inductor, output capacitor

value & ESR).

b.Error amplifier compensation & sawtooth wave amp-

litude (compensation network).

c.Kelvin sense for VCORE.

Current Loop Setting

a.GM amplifier S/H current (current sense component

DCR, CSN pin external resistor value).

b.Over-current protection trip point (RIMAX resistor).

VRM Load Line Setting

a.Droop amplitude (ADJ pin resistor).

b.No load offset (RCSN2)

c.DAC offset voltage setting (VOSS pin & compen- sation

network resistor RB1).

Power Sequence & SS

DVD pin external resistor and SS pin capacitor.

PCB Layout

a.Kelvin sense for current sense GM amplifier input.

b.Refer to layout guide for other items.

Voltage Loop Setting

Design Example

Given:

Apply for four phase converter

VIN = 12V

VCORE = 1.5V

ILOAD (MAX) = 100A

VDROOP = 100mV at full load (1m

Ω Load Line)

OCP trip point set at 40A for each channel (S/H)

DCR = 1m

Ω of inductor at 25°C

L = 1.5

μH

COUT = 8000

μF with 5mΩ equivalent ESR.

1. Compensation Setting

a. Modulator Gain, Pole and Zero:

From the following formula:

Modulator Gain =VIN/VRAMP =12/2.4=5 (i.e 14dB)

where VRAMP : ramp amplitude of saw-tooth wave

LC Filter Pole = = 1.45kHz and

ESR Zero =3.98kHz

b. EA Compensation Network:

Select R1 = 4.7k, R2 = 15k, C1 = 12nF, C2 = 68pF and

use the Type 2 compensation scheme shown in Figure

25. By calculation, the FZ = 0.88kHz, FP = 322kHz and

Middle Band Gain is 3.19 (i.e 10.07dB).

Figure 25. Type 2 compensation network of EA

The bode plot of EA compensation is shown as Figure 26.

The bode plot of power stage is shown as Figure 27. The

total loop gain is in Figure 28.

3. Over-Current Protection Setting

Consider the temperature coefficient of copper

3900ppm/

°C,

Let RIMAX = 14k

4. Soft-Start Capacitor Selection

For most application cases, 0.1

μF is a good engineering

value.

RB2

RB1

15k

12nF

C2 68pF

4.7k

⇔

330

1.39m

40A

1.690V

DCR

IMAX

COMMON

IMAX

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