FAN5091MTC Fairchild Semiconductor, FAN5091MTC Datasheet - Page 16

• Manufacturer Part Number: FAN5091MTC
• Description: IC CTRLR DC-DC SYNC 2PH 24TSSOP
• Manufacturer: Fairchild Semiconductor

Specifications of FAN5091MTC

• Applications: Controller, Intel Pentium® IV
• Voltage - Input: 5V
• Number Of Outputs: 1
• Voltage - Output: 1.1 ~ 1.85 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 24-TSSOP
• Output Voltage: 13.2 V
• Mounting Style: SMD/SMT
• Operating Temperature Range: 0 C to + 70 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

FAN5091
BYPASS Reference
The internal logic of the FAN5091 runs on 5V. To permit the
IC to run with 12V only, it produces 5V internally with a
linear regulator, whose output is present on the BYPASS pin.
This pin should be bypassed with a 1µF capacitor for noise
suppression. The BYPASS pin should not have any external
load attached to it.
Dynamic Voltage Adjustment
The FAN5091 has internal pullups on its VID lines. External
pullups should not be used. The FAN5091 can have its output
voltage dynamically adjusted to accommodate low power
modes. The designer must ensure that the transitions on the
VID lines all occur simultaneously (within less than
to avoid false codes generating undesired output voltages.
The Power Good flag tracks the VID codes, but has a
500µsec delay transitioning from high to low; this is long
enough to ensure that there will not be any glitches during
dynamic voltage adjustment.
Power Good (PWRGD)
The FAN5091 Power Good function is designed in accor-
dance with the Pentium IV DC-DC converter specifications
and provides a continuous voltage monitor on the VFB pin.
The circuit compares the VFB signal to the VREF voltage
and outputs an active-low interrupt signal to the CPU should
the power supply voltage deviate more than +14%/-9% of its
nominal setpoint. The output is guaranteed open-collector
high when the power supply voltage is within +6%/-11% of
its nominal setpoint. The Power Good flag provides no
control functions to the FAN5091.
Output Enable/Soft Start (ENABLE/SS)
The FAN5091 will accept an open collector/TTL signal for
controlling the output voltage. The low state disables the
output voltage. When disabled, the PWRGD output is in the
low state.
Even if an enable is not required in the circuit, this pin
should have attached a capacitor (typically 100nF) to soft-
start the switching. A softstart capacitor may be approxi-
mately chosen by the formula:
However, C must be 10nF.
Oscillator
The FAN5091 oscillator section runs at a frequency deter-
mined by a resistor from the RT pin to ground according to
the formula
16
RT
C
=
t 10 A
--------------------- -
=
1
+
25 10
--------------------- -
V
f Hz
out
9
nsec)
The oscillator generates two internal sawtooth ramps, each at
one-half the oscillator frequency, and running 180 out of
phase with each other. These ramps cause the turn-on time of
the two slices to be phased apart. The oscillator frequency of
the FAN5091 can be programmed from 200KHz to 2MHz
with each slice running at 100KHz to 1MHz, respectively.
Selection of a frequency will depend on various system
performance criteria, with higher frequency resulting in
smaller components but lower efficiency.
Programmable Active Droop™
The FAN5091 features Programmable Active Droop™: as
the output current increases, the output voltage drops propor-
tionately an amount that can be programmed with an exter-
nal resistor. This feature is offered in order to allow
maximum headroom for transient response of the converter.
The current is sensed losslessly by measuring the voltage
across the low-side MOSFET during its on time. Consult the
section on current sensing for details. Note that this method
makes the droop dependent on the temperature and initial
tolerance of the MOSFET, and the droop must be calculated
taking account of these tolerances. Given a maximum output
current, the amount of droop can be programmed with a
resistor to ground on the droop pin, according to the formula
with V
resistor, I
and R
MOSFET.
Typical response time of the FAN5091 to an output voltage
change is 100nsec.
Important Note! The oscillator frequency must be selected
before selecting the droop resistor, because the value of RT
is used in the calculation of R
Higher Current Converters
Active droop makes it possible to parallel multiple
FAN5091s for even higher output current requirements.
Synchronized parallelization may be obtained with the
similar FAN5094. Please refer to Application Bulletin
AB-XX for details.
Over-Voltage Protection
The FAN5091 constantly monitors the output voltage for
protection against over-voltage conditions. If the voltage at
the VFB pin exceeds 2.2V, an over-voltage condition is
assumed and the FAN5091 latches on the external low-side
MOSFET and latches off the high-side MOSFET. The
DC-DC converter returns to normal operation only after V
has been recycled.
DS, on
Droop
max
the on-state resistance of one slice’s low-side
the desired droop voltage, RT the oscillator
the output current at which the droop is desired,
R
Droop
=
Droop
------------------------------------ -
I
V
max
Droop
PRODUCT SPECIFICATION
.
R
DS on
RT
REV. 1.0.0 5/10/01
CC