FAN5353 Datasheet, PDF (9/13 Pages) Fairchild Semiconductor – 3MHz, 3A Synchronous Buck Regulator

Operation Description
The FAN5353 is a step-down switching voltage regulator
that delivers an adjustable output from an input voltage
supply of 2.7V to 5.5V. Using a proprietary architecture with
synchronous rectification, the FAN5353 is capable of
delivering 3A at over 80% efficiency. The regulator operates
at a nominal frequency of 3MHz at full load, which reduces
the value of the external components to 470nH for the output
inductor and 20µF for the output capacitor.
Control Scheme
The FAN5353 uses a proprietary non-linear, fixed-frequency
PWM modulator to deliver a fast load transient response,
while maintaining a constant switching frequency over a
wide range of operating conditions. The regulator
performance is independent of the output capacitor ESR,
allowing for the use of ceramic output capacitors. Although
this type of operation normally results in a switching
frequency that varies with input voltage and load current, an
internal frequency loop holds the switching frequency
constant over a large range of input voltages and load
currents.
Setting the Output Voltage
The output voltage is set by the R1, R2, and VREF (0.8V):
R1 ₌ V_OUT − V_REF
R2
V_REF
(1)
R1 must be set at or below 100KΩ. Therefore:
R2
=
R1•
(V_OUT
0.8
− 0.8)
(2)
For example, for VOUT = 1.2V, R1 = 100KΩ, R2 = 200KΩ.
Enable and Soft Start
When the EN pin is LOW, the IC is shut down, all internal
circuits are off, and the part draws very little current. Raising
EN above its threshold voltage activates the part and starts
the soft-start cycle. During soft-start, the modulator’s internal
reference is ramped slowly to minimize any large surge
currents on the input and prevents any overshoot of the
output voltage.
If large values of output capacitance are used, the regulator
may fail to start. If VOUT fails to achieve regulation within
320μs from the beginning of soft-start, the regulator shuts
down and waits 1200μs before attempting a restart. If the
regulator is at its current limit for more than about 60μs, the
regulator shuts down before restarting 1200μs later. This
limits the COUT capacitance when a heavy load is applied
during the startup. For a typical FAN5353 starting with a
resistive load:
COUT_MAX(μF) ≈ 400 − 100 ∗ I_LOAD(A)
where
I_LOAD
=
V_OUT
R_LOAD
(3)
Synchronous rectification is inhibited during soft-start,
allowing the IC to start into a pre-charged load.
PGOOD Pin
The PGOOD pin is an open drain output that indicates the IC
is in regulation when its state is open. PGOOD requires an
external pull-up resistor. PGOOD pulls LOW under the
following conditions:
1. The IC has operated in cycle-by-cycle current limit for
eight or more consecutive PWM cycles.
2. The circuit is disabled; either after a fault occurs, or
when EN is LOW.
3. The IC is performing a soft-start.
Under-Voltage Lockout
When EN is HIGH, the under-voltage lockout keeps the part
from operating until the input supply voltage rises high
enough to properly operate. This ensures no misbehavior of
the regulator during startup or shutdown.
Input Over-Voltage Protection (OVP)
When VIN exceeds VSDWN (about 6.2V) the IC stops
switching, to protect the circuitry from internal spikes above
6.5V. An internal 40μs filter prevents the circuit from shutting
down due to noise spikes. For the circuit to fully protect the
internal circuitry, the VIN slew rate above 6.2V must be
limited to no more than 15V/ms when the IC is switching.
The IC protects itself if VIN overshoots to 7V during initial
power-up as long as the VIN transition from 0 to 7V occurs in
less than 10μs (10% to 90%).
Current Limiting
A heavy load or short circuit on the output causes the current
in the inductor to increase until a maximum current threshold
is reached in the high-side switch. Upon reaching this point,
the high-side switch turns off, preventing high currents from
causing damage. 16 consecutive PWM cycles in current limit
cause the regulator to shut down and stay off for about
1200μs before attempting a restart.
In the event of a short circuit, the soft-start circuit attempts to
restart and produces an over-current fault after about 50μs,
which results in a duty cycle of less than 10%, providing
current into a short circuit.
Thermal Shutdown
When the die temperature increases, due to a high load
condition and/or a high ambient temperature, the output
switching is disabled until the temperature on the die has
fallen sufficiently. The junction temperature at which the
thermal shutdown activates is nominally 150°C with a 20°C
hysteresis.
© 2009 Fairchild Semiconductor Corporation
FAN5353 • Rev. 1.0.2
9
www.fairchildsemi.com