FAN5358 Datasheet, PDF (9/13 Pages) Fairchild Semiconductor – 2MHz, 500mA, SC70 Synchronous Buck Regulator

Operation Description
The FAN5358 is a step-down switching voltage regulator
that delivers a fixed output from an input voltage supply of
2.7V to 5.5V. Using a proprietary architecture with
synchronous rectification, the device is capable of delivering
500mA and maintaining a very high efficiency of over 80% at
load currents as low as 1mA. The regulator operates at a
nominal frequency of 2MHz, which reduces the value of the
external components to as low as 2.2μH for the output
inductor and 4.7µF for the output capacitor.
Control Scheme
The FAN5358 uses a proprietary, non-linear, quasi fixed-
frequency PWM modulator to deliver a fast load transient
response, while maintaining a nominal switching frequency
over a wide range of load conditions. The regulator
performance is independent of the output capacitor ESR,
allowing the use of ceramic output capacitors.
For very light loads, the device operates in discontinuous
current (DCM) single-pulse PFM mode, which produces low
output ripple compared with other PFM architectures.
Transition between PWM and PFM is near seamless,
exhibiting very little VOUT glitch.
Combined with exceptional transient response
characteristics, the very low quiescent current of the
controller (25µA) maintains high efficiency, even at very light
loads, while preserving fast transient response for
applications requiring very tight output regulation.
Enable and Soft Start
Maintaining the EN pin LOW keeps the FAN5358 in non-
switching mode in which all circuits are off and the part
draws ~50nA of current. Increasing EN above its threshold
voltage activates the part and starts the soft-start cycle.
During soft start, the output is ramped using a slow RC time
constant. This minimizes any large surge currents on the
input and prevents any overshoot of the output voltage.
Current limit is enforced in case the output cannot keep pace
with the reference or in case of a shorted output.
The current-limit fault response protects the IC in the event
of an over-current condition present during soft-start. This
protection can cause the IC to fail to start if heavy load is
applied during startup or if excessive COUT is used.
Table 2 shows combinations of COUT that allow the IC to start
successfully with the minimum RLOAD that can be supported.
Table 2. Minimum RLOAD Values for Soft-Start with
Various COUT Values
COUT
4.7μF
10μF
Minimum RLOAD
No restriction
VOUT / 0.40
Under-Voltage Lockout (UVLO)
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.
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 current from
increasing further.
After 12 consecutive PWM cycles that terminate in current
limit, the IC shuts down. About 275μs after shutting down,
the IC attempts to restart. If the fault has not cleared, the IC
continues to shut down, then attempts to restart as shown in
Figure 16.
Thermal Shutdown
When the die temperature increases, due to a heavy load
condition and/or high ambient temperature, output switching
is disabled until the die temperature falls sufficiently. The
junction temperature at which the thermal shutdown
activates is nominally 150°C with a 20°C hysteresis. Upon
cooling, the output is enabled and goes through the regular
soft start.
© 2009 Fairchild Semiconductor Corporation
FAN5358 • Rev. 1.0.1
9
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