RT9245 Datasheet, PDF (11/22 Pages) Richtek Technology Corporation – Multi-Phase PWM Controller for CPU Core Power Supply

Application Information
RT9245 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
RT9245 and its companion MOSFET driver RT9603/
RT9603A provides high quality CPUpower and all
protection functions to meet the requirement of modern
VRM.
Voltage Control
RT9245 senses the CPU VCORE by SGND pin to sense
the return of CPU to minimize the voltage drop on PCB
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 VREF of
error 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
RT9245 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.
DS9245-06 March 2007
RT9245
Load Droop
The sensed power channel current signals regulate the
reference of DAC to form an output voltage droop
proportional to the load current. The droop or so call “active
voltage positioning” can reduce the output voltage ripple
at load transient and the LC filter size.
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 Up
RT9245 interfaces with companion MOSFET drivers (like
RT9603, RT9602 series) for correct converter initialization.
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
RT9245 senses the current flowing through inductor via
its 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 circuit
(see Figure 1).
L _=R×C
DCR
V_C = DCR×I_L
I_X
=
V_C
R_CSN
L
DCR
+
-
GMx
R
C
R_CSN
I_x
Figure 1. Current Sense Circuit
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