RT9243GS datasheet(12/18 Pages) RICHTEK

12 / 18 page

RT9243

DS9243-07 March 2007

www.richtek.com

Application Information

RT9243 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

RT9243 and its companion MOSFET driver provides high

quality CPU power and all protection functions to meet

the requirement of modern VRM.

Voltage Control

RT9243 senses the CPU VCORE by an precise instrumental

amplifier to minimize the voltage drop on PCB trace at

heavy load. VSEN & SGND are the differential inputs. VDIF

is the output node of the differential voltage & the input for

PGOOD & OVP sense. The internal high accuracy VID

DAC provides the reference voltage for VRD10.x com-

pliance. Control loop consists of error amplifier, multi-phase

pulse width modulator, driver and power components. Like

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 VC of pulse width

modulator. The PWM signals of different channels are

generated by comparison of EA output and split-phase

saw-tooth wave. Power stage transforms VIN to output by

PWM signal on-time ratio.

Current Balance

RT9243 senses the current of low side MOSFET in each

synchronous rectifier when it is conducting 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 RDS(ON) of the low side

MOSFET) 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 greater than average, the balancing signal

reduces the output pulse width to keep the balance.

Load Droop

The sensed power channel current signals regulate the

reference of DAC to form a output voltage droop

proportional to the load current. The droop or so-called

“active voltage positioning” can reduce the output voltage

ripple at load transient and the LC filter size.

Fault Detection

The chip detects VCORE for over voltage and power good

detection. 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

RT9243 interfaces with companion MOSFET drivers (like

RT9600, RT9602 series) for correct converter initialization.

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

pins sense the interface voltage at IC POR period (both

VCC and DVDtrip). The channel is enabled if the pin voltage

is 1.2V less than VCC. Please tie the PWM output to

VCC and the current sense pins to GND or left floating if

the channel is unused. For 3-Channel application, connect

PWM4 high.

Current Sensing Setting

RT9243 senses the current of low side MOSFET in each

synchronous rectifier when it is conducting 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 RDS(ON) of the low side

MOSFET) to current signal into internal circuit (see

Figure 1).

Figure 1. Current Sense Circuit

ISPx

ISNx

SP1

SN1

Sample & Hold

Current

Balance

Droop Tune

Over-Current

Detection

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