RT8801 datasheet(13/23 Pages) RICHTEK

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RT8801

Preliminary

DS8801-04

August 2007

www.richtek.com

All brandname or trademark belong to their owner respectively

Application Information

RT8801 is a multi-phase DC/DC controller that precisely

regulates CPU core voltage and balances the current of

different power channels. The converter consisting of

RT8801 and its companion MOSFET driver RT9607/

RT9607A provides high quality CPU power and all

protection functions to meet the requirement of modern

VRM.

Voltage Control

RT8801 senses the CPU VCORE by SGND pin to sense

the return of CPU to minimize the voltage drop on PCB

trace at heavy load. OVP is sensed at FB pin. The internal

high accuracy VIDDAC provides the reference voltage for

VRD10.X compliance. Control loop consists of error

amplifier, multi-phase pulse width modulator, driver and

power components. As conventional voltage mode PWM

controller, the output voltage is locked at the VREF of error

amplifier and the error signal is used as the control signal

of pulse width modulator. The PWM signals of different

channels are generated by comparison of EA output and

split-phase sawtooth wave. Power stage transforms VIN

to output by PWM signal on-time ratio.

Current Balance

RT8801 senses the inductor current via inductor's DCR

for channel current balance and droop tuning. 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 balance

circuit.

The current balance circuit sums and averages the current

signals and then produces the balancing signals injected

to pulse width modulator. If the current of some power

channel is larger than average, the balancing signal

reduces that channels pulse width to keep current balance.

The use of single GM amplifier via time sharing technique

to sense all inductor currents can reduce the offset errors

and linearity variation between GMs. Thus it can greatly

improve signal processing especially when dealing with

such small signal as voltage drop across DCR.

Load Droop

The sensed power channel current signals regulate the

reference of DAC to form an output voltage droop

proportional to the load current. The droop or so call

“active

voltage positioning

” can reduce the output voltage ripple

at load transient and the LC filter size.

Fault Detection

The chip detects FB for over voltage. The

“hiccup mode”

operation of over current protection is adopted to reduce

the short circuit current. The inrush current at the start up

is suppressed by the soft start circuit through clamping

the pulse width and output voltage.

Phase Setting and Converter Start Up

RT8801 interfaces with companion MOSFET drivers (like

RT9619, RT9607 series) for correct converter initialization.

The tri-state PWM output (high, low and high impedance)

senses its interface voltage when IC POR acts (both VDD

and DVD trip). The channel is enabled if the pin voltage is

1.2V less than VDD. Tie the PWM to VDD and the

corresponding current sense pins to GND or left float if

the channel is unused. For example, for 3-Channel

application, connect PWM4 high.

Current Sensing Setting

RT8801 senses the current flowing through inductor via

its DCR for channel current balance and droop tuning. 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 1).

Figure 1. Current Sense Circuit

Figure 2 is the test circuit for GM. We apply test signal at

GM inputs and observe its signal process output at ADJ

pin. Figure 3 shows the variation of signal processing of

all channels. We observe zero offsets and good linearity

between phases.

CSN

DCR

RCSN

GMx

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