RT9246PC datasheet(11/17 Pages) RICHTEK

11 / 17 page

RT9246

DS9246-06

March 2007

www.richtek.com

Application Information

RT9246 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

RT9246 and its companion MOSFET driver provides high

quality CPU power and all protection functions to meet

the requirement of modern VRM.

Voltage Control

RT9246 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 K8 compliance.

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

sawtooth wave. Power stage transforms VIN to output by

PWM signal on-time ratio.

Current Balance

RT9246 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 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

RT9246 interfaces with companion MOSFET drivers (like

RT9600, RT9602 or RT9603 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 DVD trip). 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 2-Channel application,

connect PWM3 high.

Current Sensing Setting

RT9246 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). Be careful to choose GND sense input, ISN, of

the GM amplifier for effective channel current balance.

Figure 1. Current Sense Circuit

ISP1

ISN1

SP1

SN1

ISP2

ISN23

SP2

SN23

ISP3

SP3

Channel 2 & 3

GND return

Sample

Hold

Current

Balance

Droop Tune

Over-Current

Detection

Sample

Hold

Current

Balance

Droop Tune

Over-Current

Detection

Sample

Hold

Current

Balance

Droop Tune

Over-Current

Detection

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