FAN5362 Datasheet, PDF (9/14 Pages) Fairchild Semiconductor – 3MHz, 500mA / 750mA Synchronous Buck Regulator

Operation Description
FAN5362 is a 500mA or 750mA, step-down switching
voltage regulator that delivers a fixed output voltage from an
input voltage supply up to 5.5V. Using a proprietary
architecture with synchronous rectification, FAN5362 is
capable of delivering a peak efficiency above 96%, while
maintaining efficiency above 90% 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 inductor and 4.7µF for the output
capacitor. High efficiency is maintained at light load with
single-pulse PFM mode.
Control Scheme
The FAN5362 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 FAN5362 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 18mV at VOUT during the transition between
DCM and CCM modes.
Combined with exceptional transient response
characteristics, the very low quiescent current of the
controller (45µA) maintains high efficiency, even at very light
loads, while preserving fast transient response for
applications requiring tight output regulation.
100% Duty Cycle Operation
When VIN approaches VOUT, the regulator increases its duty
cycle until 100% duty cycle is reached. As the duty cycle
approaches 100%, the switching frequency declines due to
the minimum off-time (tOFF(MIN)) of about 35ns imposed by the
control circuit. When 100% duty cycle is reached, VOUT
follows VIN with a drop-out voltage (VDROPOUT) determined by
the total resistance between VIN and VOUT:
( ) V_DROPOUT = I_LOAD • PMOS R_DS(ON) + DCR_L
(1)
To calculate the worst-case VDROPOUT, use the maximum
PMOS RDS(ON) at high temperature from Figure 5.
Enable and Soft Start
When the EN pin is LOW, the IC is shut down and the part
draws very little current. In addition, during shutdown, FB is
actively discharged to ground through a 230Ω path. Raising
EN above its threshold voltage activates the part and starts
the soft-start cycle. During soft-start, the internal reference is
ramped using an exponential RC shape to prevent any
overshoot of the output voltage. Current limiting minimizes
inrush during soft-start.
Synchronous rectification is inhibited during soft-start,
allowing the IC to start into a pre-charged load.
The IC may fail to start if heavy load is applied during startup
and/or if excessive COUT is used. This is due to the current-
limit fault response, which protects the IC in the event of an
over-current condition present during soft-start.
The current required to charge COUT during soft-start is
commonly referred to as “displacement current” and given as:
I_DISP
=
C_OUT
•
dV
dt
(2)
dV
where refers to the soft-start slew rate.
dt
To prevent shutdown during soft-start, the following condition
must be met:
I_DISP + I_LOAD < I_MAX(DC)
(3)
where IMAX(DC) is the maximum load current the IC is
guaranteed to support (500mA or 750mA).
MODE Pin
Logic 1 on this pin forces the IC to stay in PWM mode. A
logic 0 allows the IC to automatically switch to PFM during
light loads. If the MODE pin is toggled, the converter
synchronizes its switching frequency to four times the
frequency on the mode pin (fMODE).
At startup, the mode pin must be held LOW or HIGH for at
least 10μs to ensure that the converter does not attempt to
synchronize to this pin.
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.
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
causes the regulator to shut down and stay off for about
2900μs before attempting a restart.
In the event of a short circuit, the soft-start circuit attempts to
restart at 240μs, which results in a duty cycle of less than
10%, providing current into a short.
The closed-loop peak-current limit, ILIM(PK), is not the same
as the open-loop tested current limit, ILIM(OL), in the Electrical
Characteristics table. This is primarily due to the effect of
propagation delays of the IC current limit comparator.
© 2009 Fairchild Semiconductor Corporation
FAN5362 • Rev. 1.0.1
9
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