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RT9238

VRM8.5 PWM and Triple Linear Power System Controller

Richtek Technology 

Preliminary

RT9238

provide closed loop transfer function with 0 dB

crossing frequency and adequate phase margin. For

some suggestion LC combinations are L/C =

4µH/6000µF, 3uH/8000uF or 2µH/12000µF. Fig.9

shows an asymptotic plot of the DC-DC converter’s

gain vs. frequency.

Volt

1.70

VCORE Maximum

VCORE Minimum

1.60

VCORE Typical

EA

Active Cap

Fig.8 Error Amplifier Compensation

100

80

60

40

COMPENSATION

20

FZ1

FP1

0

20log

VIN

VP-P

FLC

FESR

-20

MODULATOR

-40

GAIN

CLOSED

LOOP

GAIN

-60

10

100

1K

10K 100K 1M

Fig.9 PWM Bode Plot

Transient Response

Modern micro-processsor’s power supply request a

voltage droop at load transient response and a

loadlines at static state load changing as shown in

Fig.10, and Fig.11.

∆ IO

VCORE Ripple

Fig.10 Transient Response

DS9238-01 July 2001

10

20

30 A

Fig.11 VCORE Load Line

Two things have to be completed at load transient

response, first is to sense the instantaneous load

current, second is real time to drop the VCORE voltage

based on the loadlines. A RT9238 internal trans-

conductance (Gm) amplifier sample the on-state drop

across the lower MOSFET per clock cycle, the

voltage drop is simply RDS-ON X inductor current IL. In

step-down DC-DC converter, the IL is relative to load

current. The transferred rate of the Gm amplifier is

1/3k, this mean the current output of Gm amplifier is

ISENSE = RDS(ON) x IL/3k.

Fig.12 shown the voltage droop tuning circuit, a

DACOUT buffer amplifier connect a RF resistor to the

non-inverting input of the PWM error amplifier, when

load transient happened, the feedback ISENSE current

fitted to the non-inverting input of the PWM error

amplifier to drop the VCORE voltage. The drop

voltage is equal to

VDROOP = ISENSE x RF

Control

Logic

Sample Hold

RS

UG

PHASE

LG

V5V

IM1

IL

IM2

VCORE

+

Gm_

RS

Low Side Current Sense

Error Amp

COMP _

Rds-on

VNI

RF

Isense

REF

DAC Buffer

REFOUT

Reference Output

Fig.12 Voltage Droop Tuning

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