RT9241A Datasheet, PDF (11/17 Pages) Richtek Technology Corporation – Two-Phase DC/DC Controller for CPU Core Power Supply

RT9241A/B
Application Information
RT9241A/B is a two-phase DC/DC controller that precisely
regulates CPU core voltage and balances the current of
different power channels. The converter consists of
RT9241A/B and its companion MOSFET driver provide high
quality CPU power and all protection function to meet the
requirement of modern VRM.
Voltage control
The reference of V_CORE is provided by a 5-bit DAC of
VRM9.0 specification. Control loop consists of error
amplifier, two-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 saw-tooth wave. Power stage
transforms V_IN to output by PWM signal on-time ratio.
Current balance
RT9241A/B 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 call
“ctive voltage positioning” can reduce the output voltage
ripple at load transient and the LC filter size.
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 in-rush current at the start up is suppressed by the
soft start circuit through clamping the pulse width and
output voltage.
MOSFET driver detection and converter start up
RT9241A/B interface with companion MOSFET driver (like
RT9600 or HIP660X series) for correct converter
initialization. The tri-phase PWM output (high, low, high
impedance) pins sense the interface voltage at IC POR
acts (both V_DD and V_DVD trip). The channel is enabled if
the pin voltage is 1.2V less than V_DD. Please tie the both
PWM output to driver input for correct converter start-
up.
Current sensing setting
RT9241A/B 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).
I_X
-
I_BP ISP_X ^RSP
Sample & Hold
To Current Balance
To Droop Tune ^{2/3 IX}
GM₊
I_{BN ISNX RSN}
R_S
I_L
2/3 I_X
To Over Current Detection
2/3 I_X
Figure 1. Current Sense Circuit
DS9241AB-05 March 2007
www.richtek.com
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