6
FN9015.3
April 18, 2005
Functional Pin Descriptions
VCC (Pin 28)
Provide a 12V bias supply for the IC to this pin. This pin also
provides the gate bias charge for all the MOSFETs
controlled by the IC. The voltage at this pin is monitored for
Power-On Reset (POR) purposes.
GND (Pin 17)
Signal ground for the IC. All voltage levels are measured
with respect to this pin.
PGND (Pin 24)
This is the power ground connection. Tie the synchronous
PWM converter’s lower MOSFET source to this pin.
VAUX (Pin 16)
Connect this pin to the ATX 3.3V output. The voltage present
at this pin is monitored for sequencing purposes. This pin
provides the necessary base bias for the NPN pass
transistors, as well as the current sunk through the 5kW VID
pull-up resistors.
SS13 (Pin 13)
Connect a capacitor from this pin to ground. This capacitor,
along with an internal 28mA current source, sets the soft-start
interval of the synchronous switching converter (V
OUT1
) and
the AGP regulator (V
OUT3
). A VTTPG high signal is also
delayed by the time interval required by the charging of this
capacitor from 0V to 1.25V (see Soft-Start details).
SS24 (Pin 12)
Connect a capacitor from this pin to ground. This capacitor,
along with an internal 28mA current source, sets the
soft-start interval of the V
OUT2
regulator. Pulling this pin
below 0.8V induces a chip reset (POR) and shutdown.
VTTPG (Pin 9)
VTTPG is an open collector output used to indicate the
status of the V
OUT2
regulator output voltage. This pin is
pulled low when the V
OUT2
output is below the undervoltage
threshold or when the SS13 pin is below 1.25V.
PGOOD (Pin 8)
PGOOD is an open collector output used to indicate the
status of the output voltages. This pin is pulled low when the
synchronous regulator output is not within
±10% of the
DACOUT reference voltage or when any of the other outputs
is below its undervoltage threshold.
VID3, VID2, VID1, VID0, VID25 (Pins 3-7)
VID3-25 are the TTL-compatible input pins to the 5-bit DAC.
The logic states of these five pins program the internal
voltage reference (DACOUT). The level of DACOUT sets the
microprocessor core converter output voltage (V
OUT1
), as
well as the corresponding PGOOD and OVP thresholds.
Each VID pin is connected to the VAUX pin through a 5kW
pull-up resistor.
OCSET (Pin 23)
Connect a resistor (R
OCSET
) from this pin to the drain of the
upper MOSFET. R
OCSET
, an internal 200mA current source
(I
OCSET
), and the upper MOSFET’s on-resistance (r
DS(ON)
)
set the converter overcurrent (OC) trip point according to the
following equation:
An overcurrent trip cycles the soft-start function.
The voltage at OCSET pin is monitored for power-on reset
(POR) purposes.
PHASE (Pin 26)
Connect the PHASE pin to the PWM converter’s upper
MOSFET source. This pin represents the gate drive return
current path and is used to monitor the voltage drop across
the upper MOSFET for overcurrent protection.
UGATE (Pin 27)
Connect UGATE pin to the PWM converter’s upper
MOSFET gate. This pin provides the gate drive for the upper
MOSFET.
LGATE (Pin 25)
Connect LGATE to the synchronous PWM converter’s lower
MOSFET gate. This pin provides the gate drive for the lower
MOSFET.
COMP and FB (Pins 20, 21)
COMP and FB are the available external pins of the
synchronous PWM regulator error amplifier. The FB pin is
the inverting input of the error amplifier. Similarly, the COMP
pin is the error amplifier output. These pins are used to
compensate the voltage-mode control feedback loop of the
synchronous PWM converter.
VSEN1 (Pin 22)
This pin is connected to the synchronous PWM converters’
output voltage. The PGOOD and OVP comparator circuits
use this signal to report output voltage status.
DRIVE2 (Pin 1)
Connect this pin to the gate/base of a N-type external pass
transistor (MOSFET or bipolar). This pin provides the drive
for the 1.2V regulator’s pass transistor.
VSEN2 (Pin 11)
Connect this pin to the output of the standard buck PWM
regulator. The voltage at this pin is regulated to a 1.2V level.
This pin is also monitored for undervoltage events.
FIX (Pin 2)
Grounding this pin bypasses the internal resistor dividers that
set the output voltage of the 1.5V and 1.8V linear regulators.
This way, the output voltage of the two regulators can be
adjusted from 1.26V up to the input voltage (+3.3V or +5V;
I
PEAK
I
OCSET
R
OCSET
×
r
DS ON()
----------------------------------------------------
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