rt9245c Richtek Technology Corporation, rt9245c Datasheet - Page 13

• Manufacturer Part Number: rt9245c
• Description: Multi-phase Controller Core Power Supply
• Manufacturer: Richtek Technology Corporation

Application Information
RT9245C 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
RT9245C and its companion MOSFET driver RT9619
provides high quality CPU power and all protection
functions to meet the requirement of modern VRM.
Voltage Control
RT9245C senses
the return of CPU to minimize the
trace at heavy load. OVP is sensed at FB pin. The internal
high accuracy VID DAC 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 V
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
RT9245C 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.
Droop & Load Line Setting
RT9245C injects averaged current I
connected to ADJ pin to generate a load-current-
dependent voltage R
DS9245C-02 March 2007
the CPU V
ADJ
for droop setting :
CORE
by SGND pin to sense
X
voltage drop on PCB
into the resistor R
REF
of error
ADJ
V
reference voltage at non-inverting input of the error amplifier
as shown if Figure 1. Consequently, load line slope is
calculated as :
where N is the phase number of operation.
Fault Detection
The chip detects FB for over voltage and power good
detection. The “hiccup mode” operation of over current
protection is adopted to reduce the short circuit current.
The in-rush 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
RT9245C interfaces with companion MOSFET drivers (like
RT9619, RT9607 series)
The tri-state PWM output (high, low and high impedance)
senses its interface voltage when IC POR acts (both VCC
and DVD trip). The channel is enabled if the pin voltage is
1.2V less than VCC. Tie the PWM to VCC 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
RT9245C senses the current flowing through inductor via
its DCR for channel current balance and droop tuning.
Load
ADJ
V
ADJ
is then subtracted from VID_DAC output as the real
Line
=
Figure 1. Load Line and Offset Function
8
=
I x
V
CORE
ΔV
X
ΔI
x
CORE
CORE
R
DAC
ADJ
R
=
CSN
8
for correct converter initialization.
I
x
VOSS
N
R
ADJ
x
V
/4
ADJ
R
-
+
EA
CSN
x
R
DCR
ADJ
RT9245C
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