RT9245BPC datasheet(16/23 Pages) RICHTEK

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RT9245B

DS9245B-02

March 2007

www.richtek.com

PGOOD Function

During start-up, RT9245B will detect 5VCC and 12VIN

(through DVD pin). In Figure 21, 5VCC or 12VIN is not ready

during T1. V(SS) (in Figure 20) is pulled to GND by FAULT.

V(EAP) is also equal to GND. V(FB) and VOUT will try to

follow V(EAP) thus both V(FB) and VOUT are equal to GND

during T1. During T2, both 5VCC and 12VIN are ready,

FAULT = low, OPSS starts charging up CSS. In the design

of RT9245B, ISS (the maximal current sink and source

capability of OPSS) is limited and time-variant. During T2

(V1 = 0.4V > V(SS) > 0), ISS(T2) is equal to about 10uA.

After V(SS) > V1, ISS changes to about 20uA. The rising

speed of V(SS) becomes about 2 times faster than in T1.

In Figure 20, MOSFET N1 will turn on only if V(SS) > VTH_N1

(threshold voltage of N1)

≅ 0.7V = V2. Before N1 turns

on, V(EAP) is still 0V.

After V(SS) > V2, MOSFET N1 turns on, V(EAP) starts rising.

ISS(T4) is still equal to about 20uA. V(SS,EAP) is equal to

VTH_N1. Due to the body effect of MOSFET N1, VTH_N1

increases with higher V(EAP). For example, if VOUT target

is 1.4V, V(SS,EAP) will be equal to about 0.7V at the

beginning of T4 and equal to about 1.1V at the end of T4.

At the end of T4, VOUT is very close to the target (within

the range of

±40mV). An internal 1ms timer starts. After

about 1ms(T5), The open-drain output PGOOD releases.

After PGOOD releases, ISS(T6) becomes about 320uA to

accelerate OPSS. RT9245B enters normal operation mode

and is capable to follow VID on the fly.

When any of the fault conditions happens, V(SS) and

PGOODwill be pulled low immediately. If the fault condition

is one of 5VCC low, DVD low, OC or VID_OFF, RT9245B

will try to turn off both high side MOSFET and low side

MOSFET. VOUT will fall slowly to avoid negative VOUT. The

typical waveform is shown in Figure 22.

SS(T2)

4x10

≅

SS(T3)

1.5x10

V1)

(V2

≅

SS(T4)

9x10

V2)

(V4

≅

COMP

OUT

DAC

FAULT

EAP

OPSS

Figure 20. Soft Start Circuit

Figure 22. Waveform for 5VCC_Low, DVD_Low, OC or

VID_OFF

PGOOD

(SS)

OUT

_Low + DVD_Low + OC + VID_OFF

_ready

and DVD_ready

PGOOD

(SS)

OUT

Figure 21. Soft Start Waveform

If the fault condition is OV, V(SS) and PGOOD will be pulled

low immediately also. RT9245B will try to turn on low

side MOSFET and turn off high side MOSFET. VOUT will

fall quickly to protect CPU from high voltage. The typical

waveform is shown in Figure 23.

RT9245BPC Datasheet(PDF) 16 Page - Richtek Technology Corporation

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