FAN5092MTC Datasheet, PDF (15/20 Pages) Fairchild Semiconductor – High Current System Voltage Buck Converter

PRODUCT SPECIFICATION
FAN5092
Oscillator
The FAN5092 oscillator section runs at a frequency deter-
mined by a resistor from the RT pin to ground according to
the formula
RT(Ω) = ⁵----^0f--₍---^•_H----^1z---⁰-₎---⁹-
The oscillator generates two square waves, 180° out of phase
with each other. One is used internally, the other is sent to a
second FAN5092 on the CLK pin.
The square wave generates two internal sawtooth ramps,
each at one-half the square wave 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 and the four
phases to be 90° apart each. The oscillator frequency of the
FAN5092 can be programmed from 400KHz to 4MHz with
each phase running at 100KHz to 1MHz, respectively. Selec-
tion of a frequency will depend on various system
performance criteria, with higher frequency resulting in
smaller components but lower efficiency.
Programmable Active Droop™
The FAN5092 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 load
current, the amount of droop can be programmed with a
resistor to ground on the droop pin, according to the formula
R_Droop(Ω) = ²-----^•--_I--_m-ⁿ----_a^•---_x--^V--_•--^D--_R--^r--^o-_D--^o-_S--^p--_,--_o^•---_n-^R-----^T--
with VDroop the desired droop voltage, RT the oscillator
resistor, Imax the load current at which the droop is desired,
and RDS, on the on-state resistance of one phase low-side
MOSFET.
Typical response time of the FAN5092 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 RDroop.
Over-Voltage Protection
The FAN5092 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 FAN5092 latches on the external low-side
MOSFET and latches off the high-side MOSFET. The
DC-DC converter returns to normal operation only after VCC
has been recycled.
Thermal Design Considerations
Because of the very large gate capacitances that the
FAN5092 may be driving, the IC may dissipate substantial
power. It is important to provide a path for the IC’s heat to be
removed, to avoid overheating. In practice, this means that
each of the pins should be connected to as large a trace as
possible. Use of the heavier weights of copper on the PCB is
also desirable. Since the MOSFETs also generate a lot of
heat, efforts should be made to thermally isolate them from
the IC.
Over Temperature Protection
If the FAN5092 die temperature exceeds approximately
150°C, the IC shuts itself off. It remains off until the temper-
ature has dropped approximately 25°C, at which time it
resumes normal operation.
Component Selection
MOSFET Selection
This application requires N-channel Enhancement Mode Field
Effect Transistors. Desired characteristics are as follows:
• Low Drain-Source On-Resistance,
• RDS,ON < 10mΩ (lower is better);
• Power package with low Thermal Resistance;
• Drain-Source voltage rating > 15V;
• Low gate charge, especially for higher frequency
operation.
For the low-side MOSFET, the on-resistance (RDS,ON) is the
primary parameter for selection. Because of the small duty
cycle of the high-side, the on-resistance determines the
power dissipation in the low-side MOSFET and therefore
significantly affects the efficiency of the DC-DC converter.
For high current applications, it may be necessary to use two
MOSFETs in parallel for the low-side for each slice.
For the high-side MOSFET, the gate charge is as important
as the on-resistance, especially with a 12V input and with
higher switching frequencies. This is because the speed of
the transition greatly affects the power dissipation. It may be
a good trade-off to select a MOSFET with a somewhat
higher RDS,on, if by so doing a much smaller gate charge is
available. For high current applications, it may be necessary
to use two MOSFETs in parallel for the high-side for each
slice.
At the FAN5092’s highest operating frequencies, it may be
necessary to limit the total gate charge of both the high-side
and low-side MOSFETs together, to avert excess power
dissipation in the IC.
REV. 1.0.7 6/20/02
15