RT9245BPC datasheet(12/23 Pages) RICHTEK

12 / 23 page

RT9245B

DS9245B-02

March 2007

www.richtek.com

Figure 3 is the test circuit for GM. We apply test signal at

GM inputs and observe its signal process output at ADJ

pin. Figure 4 shows the variation of signal processing of

all channels. We observe zero offsets and good linearity

between phases.

Figure 4. The Linearity of GMx

Figure 5 shows the time sharing technique of GM amplifier.

We apply test signal at phase 4 and observe the waveforms

at both pins of GM amplifier. The waveforms show time

sharing mechanism and the perfomance of GM to hold

both input pins equal when the shared time is on.

Over Current Protection

RT9245B uses an external resistor RIMAX to set a

programmable over current trip point. OCP comparator

compares each inductor current with this reference current.

RT9245B uses hiccup mode to eliminate fault detection

of OCP or reduce output current when output is shorted

to ground.

Figure 6. Over Current Comparator

1/3 IX

1/2 IIMAX

OCP Comparator

Figure 3. The Test Circuit of GM

Figure 5

Time Sharing of GM

Time (1

µs/Div)

PWM3

VCSP4

and

VCSN

CH1:(2V/Div)

CH2:(50mV/Div)

CH3:(50mV/Div)

VCSP4

VCSN

The differential sensing GM amplifier converts the voltage

on the sense component (can be a sense resistor or the

DCR of the inductor) to current signal into internal circuit

(see Figure 2).

Figure 2. Current Sense Circuit

CSN

DCR

RCSN

GMx

+VC-

RCSN

CSPX

CSN

MUX

SUM/M

RADJ

ADJ

VADJ

OCP

Setting

VIMX

RIMX

100

150

200

250

300

100

125

150

VC (mV)

CSN

IMAX

DCR

⇔

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