RT9248 Datasheet, PDF (8/14 Pages) Richtek Technology Corporation – Multi-Phase PWM Controller for CPU Core Power Supply

RT9248
Preliminary
Application Information
RT9248 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
RT9248 and its companion MOSFET driver provides high
quality CPU power and all protection functions to meet
the requirement of modern VRM.
Voltage Control
RT9248 senses the CPU V_CORE 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 allows selection of either VRM9 or VRD10 compliance
via VID125 pin setting. 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 V_REF of error
amplifier and the error signal is used as the control signal
V_C 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 V_IN to
output by PWM signal on-time ratio.
Current Balance
RT9248 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 R_DS(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.
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Fault Detection
The chip detects V_CORE 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
RT9248 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
RT9248 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 R_DS(ON) of the low side
MOSFET) to current signal into internal circuit (see
Figure 1). Be careful to choose GND sense input ISN23 of
GM amplifier of channel 2 & 3 (tied together by internal
wiring) for effective channel current balance.
Current
< ^Balance
I_X
< ^{Droop Tune 2IX}
<
I_X
Over-Current
Detection
Sample
&
Hold
Current
< ^Balance
I_X
< ^{Droop Tune 2IX}
<
I_X
Over-Current
Detection
Current
< ^Balance
I_X
< ^{Droop Tune 2IX}
<
I_X
Over-Current
Detection
Sample
&
Hold
Sample
&
Hold
I_X1
I_BP
-
GM
+
I_BN
I_X2
I_BP
-
GM
+
I_BN
I_BN
+
GM
-
I_BP
I_X3
ISP1 ^RSP1
ISN1
R_{S IL}
R_SN1
ISP2 ^RSP2
R_{S IL}
ISN23 ^RSN23
Channel 2 & 3
GND return
ISP3
R_SP3
R_{S IL}
Figure 1. Current Sense Circuit
DS9248-05 March 2007