Preliminary
Application Information
RT9247 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
RT9247 and its companion MOSFET driver provides high
quality CPU power and all protection functions to meet
the requirement of modern VRM.
Voltage Control
RT9247 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 provides the reference voltage for VRD10 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 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
RT9247 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.
Fault Detection
RT9247
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
RT9247 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
RT9247 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.
I
X1
<
Current
Balance
I
X
Sample
&
Hold
-
GM
+
I
BP
Droop Tune 2I
X
<
I
X
<
Over-Current
Detection
ISP1
ISN1
R
SP1
R
S
R
SN1
I
L
I
BN
I
X2
<
<
<
Current
Balance
I
X
I
X
Sample
&
Hold
-
GM
+
I
BP
Droop Tune 2I
X
Over-Current
Detection
Current
Balance
I
X
I
X
Over-Current
Detection
ISP2
R
SP2
R
S
I
L
I
BN
ISN23
I
BN
R
SN23
<
<
<
Droop Tune 2I
X
Sample
&
Hold
I
X3
+
GM
-
I
BP
Channel 2 & 3
GND return
ISP3
R
SP3
R
S
I
L
Figure 1. Current Sense Circuit
DS9247-05 March 2007
www.richtek.com
9
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
