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Operation Description
The FAN4603 is a 600mA, step-down, switching-voltage
regulator that delivers a fixed output from an input voltage
To prevent shutdown during soft-start, the following condition
must be met:
supply of 2.3V to 5.5V. Using a proprietary architecture with
synchronous rectification, the FAN4603 is capable of
I DISP + I LOAD < I MAX ( DC )
(2)
delivering a peak efficiency of >90%, while maintaining
efficiency over 80% at load currents as low as 1mA. The
regulator operates at a nominal PWM frequency of 6MHz.
Control Scheme
The FAN4603 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. Regulator stability is not
where I MAX(DC) is the maximum load current the IC is
guaranteed to support (600mA).
Table 1 shows combinations of external C OUT that allow the
IC to start successfully with the minimum R LOAD that can be
supported at each.
Table 1. Minimum R LOAD Values for Soft-Start with
Various External C OUT Values
dependent on output capacitor ESR, which allows the use of
ceramic 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.
C OUT
2.2 μ F, 0402
4.7 μ F, 0402
10 μ F, 0603
Minimum R LOAD
V OUT / 0.55
V OUT / 0.50
V OUT / 0.45
For very light loads, FAN4603 incorporates a discontinuous
current (DCM) single-pulse PFM mode, which produces
lower output ripple when compared with other PFM
architectures. Transition between PWM and PFM is
seamless, with a glitch of less than 20mV at V OUT during the
transition between DCM and CCM modes.
Combined with exceptional transient response
characteristics, the very low quiescent current of the
controller (35μA) maintains high efficiency; even at very light
loads, while preserving fast transient response for
applications requiring tight output regulation.
Enable and Soft-Start
When EN is LOW, all circuits in FAN4603 are off and the IC
draws ~100nA of current. When EN is HIGH and V IN is
above its UVLO threshold, the regulator begins a soft-start
cycle. The output ramp during soft-start is a fixed slew rate
of 50mV/ μ s from 0 to 1 V OUT , then 12.5mV/ μ s until the output
reaches its setpoint.
PWM mode operation is prohibited during the soft-start cycle
to prevent C OUT from being discharged. This allows glitchless
starting into a pre-charged output.
Startup into Large C OUT
The IC may fail to start if heavy load is applied during startup
and a large external C OUT is present. This is due to the
current-limit fault response, which protects the IC in an over-
current condition during soft-start.
The current required to charge C OUT during soft-start,
referred to as “displacement current,” is given as:
Multiple soft-start cycles may be required for C OUT >10 μ f
(15 μ f with no load). The IC shuts down for 85 μ s when I DISP +
I LOAD exceeds I LIMIT for more than 21 μ s of current limit. The
IC then begins a new soft-start cycle. Subsequent soft-start
cycles begin with any charge retained by C OUT while the IC is
off, allowing V OUT to incrementally reach regulation over
multiple soft-start attempts.
Current Limit, Fault Shutdown, and Restart
A heavy load or short circuit on the output causes the current
to increase until a maximum current threshold is reached.
Upon reaching this point, the high-side switch turns off,
preventing high currents from causing damage. The
regulator continues to limit the current cycle-by-cycle. After
21μs of current limit, the regulator triggers an over-current
fault, causing the regulator to shut down for about 85 μ s
before attempting an automatic restart.
If the fault is caused by short circuit, the soft-start circuit
attempts to restart and produces an over-current fault after
about 32 μ s, which results in a duty cycle of less than 30%,
limiting power dissipation.
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.
Thermal Shutdown
When the die temperature increases, due to a high load
condition and/or a high ambient temperature, the output
I DISP = C OUT ?
dV
dt
(1)
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 15°C
where
dV
dt
refers to the soft-start slew rate.
hysteresis. After cooling, the IC automatically restarts, with a
soft-start cycle.
? 2009 Fairchild Semiconductor Corporation
FAN4603 ? Rev. 1.0.1
9
www.fairchildsemi.com
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