rt9246a Richtek Technology Corporation, rt9246a Datasheet - Page 9

• Manufacturer Part Number: rt9246a
• Description: Multi-phase Controller Core Power Supply
• Manufacturer: Richtek Technology Corporation

Applications Information
RT9246A is a multi-phase DC/DC controller specifically
designed to deliver high quality power for next generation
CPU. Phase currents are sensed by innovative time-
sharing DCR current sensing technique for channel current
balance, droop tuning, and over current protection. Using
one common GM amplifier for current sensing eliminates
offset errors and linearity variation between GMs. As sub-
milli-ohm-grade inductors are widely used in modern
mother boards, slight mismatch of GM amplifiers offset
and linearity results in considerable current shift between
phases. The time-sharing DCR current sensing technique
is extremely important to guarantee phase current balance
at mass production.
Converter Initialization, Phase Selection, and
Power Good Function
The RT9246A initiates only after two pins are ready: VCC
pin power on reset
VCC POR is to make sure RT9246A is powered by a
voltage for normal work. The rising threshold voltage of
VCC POR is 4.2V typically. At VCC POR, RT9246A checks
PWM3 status to determine phase number of operation.
Pull high PWM3 for two-phase operation. The unused
current sense pin should be connected to GND or left
floating.
DVD is to make sure that ATX12V is ready for companion
MOSFET drivers(RT960X series) to work normally.
Connect a voltage divider from ATX12V to DVD pin as
shown in the Typical Application Circuit. Make sure that
DVD pin voltage is below its threshold voltage before drivers
are ready and above its threshold voltage for minimum
ATX12V during normal operation.
If either one of VCC and DVD is not ready, RT9246A keeps
its PWM outputs high impedance and the companion
drivers turn off both upper and lower MOSFETs. After VCC
and DVD are ready,
as shown in Figure 1. A time-variant internal current source
charges the capacitor connected to SS pin. SS voltage
ramps up piecewise linearly and
with a specified voltage drop. Consequently, V
up according to VID_DAC output. PGOOD output is
tripped to high impedance when V
DS9246A-04 March 2007
(POR) and DVD pin is higher than 1V.
RT9246A initiates its soft start cycle
CORE
locks VID_DAC output
reaches VID_DAC
CORE
is built
output with 1~2ms delay. An SS capacitor about 47nF is
recommend for typical
Voltage Control
CPU V
pins and precisely regulated to VID_DAC output by internal
high gain Error Amplifier (EA). The sensed signal is also
used for power good and over voltage function. The typical
OVP trip point is 400mV above VID_DAC output. RT9246A
pulls PWM outputs low and latches up upon OVP trip to
prevent CPU from damaging. It can only restart by resetting
either VCC or DVD pin.
The VID pins are internally pulled high to internal 2.2V
with 13kΩ resistors and are easily interfaced with CPU
VID outputs. The change of VID_DAC output at VID Jump
is also smoothed by capacitor connected to SS pin.
Consequently, V
DCR Current Sensing
RT9246A adopts an innovative time-sharing DCR current
sensing technique to sense the phase currents for phase
current balance (phase thermal balance) and load line
regulation as shown
GM samples and holds voltages V
sensing capacitor C
According to the Basic Circuit Theory, if
Consequently, the sensing current I
inductor current I
I =
Figure 1. Timming Diagram During Soft Start Interval
R
X
L
LX
VCC POR and DVD ready
X
1~2ms
I x R
CORE
= R x C then V = I x R
LX
R
COMM
X
voltage is Kelvin sensed by FB and FBRTN
LX
X
1~2ms
CORE
LX
and is expressed as
in Figure 2.
X
shifts to
application.
by turns in a switching cycle.
X
1~2ms
LX
its new position smoothly.
Current sensing amplifier
LX
PGOOD
V
X
RT9246A
SS
CORE
X
across the current
is proportional to
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VID Jump
9