FAN5350_08 FAIRCHILD [Fairchild Semiconductor], FAN5350_08 Datasheet - Page 6

• Manufacturer Part Number: FAN5350_08
• Description: 3MHz, 600mA Step-Down DC-DC Converter in Chip-Scale and MLP Packaging
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

© 2007 Fairchild Semiconductor Corporation
FAN5350 Rev. 1.0.3
Operation Description
The FAN5350 is a step-down switching voltage regulator
that delivers a fixed 1.82V from an input voltage supply of
2.7V to 5.5V. Using a proprietary architecture with
synchronous rectification, the FAN5350 is capable of
delivering
maintaining a light load efficiency of over 80% at load
currents as low as 1mA. The regulator operates at a
nominal frequency of 3MHz at full load, which reduces the
value of the external components to 1µH for the output
inductor and 4.7µF for the output capacitor.
Control Scheme
The FAN5350 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.
For very light loads, the FAN5350 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 seamless, with a glitch of less than 14mV at V
during the transition between DCM and CCM modes.
Combined
characteristics, the very low quiescent current of the
controller (<16µA) maintains high efficiency, even at
very light loads, while preserving fast transient response
for applications requiring very tight output regulation.
600mA
with
exceptional
at
over
90%
transient
efficiency,
response
while
OUT
6
Enable and Soft Start
Maintaining the EN pin LOW keeps the FAN5350 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 current limit is
increased in discrete steps so that the inductor current
is increased in a controlled manner. This minimizes any
large surge currents on the input and prevents any
overshoot of the output voltage.
Under-Voltage Lockout
When EN is high, the under-voltage lock-out keeps the
part from operating until the input supply voltage rises
high enough to properly operate. This ensures no
misbehavior of the regulator during start-up 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 high currents from causing damage.
The peak current limit shown in Figure 16, I
slightly higher than the open-loop tested current limit,
I
primarily due to the effect of propagation delays of the
IC current limit comparator.
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.
LIM(OL)
, in the Electrical Characteristics table. This is
www.fairchildsemi.com
LIM(PK)
is