FAN5361 Fairchild Semiconductor, FAN5361 Datasheet - Page 12

• Manufacturer Part Number: FAN5361
• Description: The FAN5361 is a 600mA or 750mA, step-down, switching voltage regulator that delivers a fixed output from an input voltage supply of 2
• Manufacturer: Fairchild Semiconductor

The FAN5361 is a 600mA or 750mA, step-down, switching
voltage regulator that delivers a fixed output from an input
voltage supply of 2.3V to 5.5V. Using a proprietary
architecture with synchronous rectification, the FAN5361 is
capable of delivering a peak efficiency of 92%, while
maintaining efficiency over 80% at load currents as low as
1mA. The regulator operates at a nominal frequency of
6MHz at full load, which reduces the value of the external
components to 470nH for the inductor and 4.7µF for the
output capacitor.
Control Scheme
The FAN5361 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 FAN5361 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 V
DCM and CCM modes.
Combined
characteristics, the very low quiescent current of the
controller (35µA) maintains high efficiency; even at very light
loads,
applications requiring tight output regulation.
Enable and Soft-Start
When EN is LOW, all circuits in FAN5361 are off and the IC
draws ~50nA of current. When EN is HIGH and V
its UVLO threshold, the regulator begins a soft-start cycle. The
output ramp during soft-start is a fixed slew rate of 50mV/s
from 0 to 1 V
setpoint. Regardless of the state of the MODE pin, PFM mode
is enabled to prevent current from being discharged from C
if soft-start begins when C
The IC may fail to start if heavy load is applied during startup
and/or if excessive C
limit fault response, which protects the IC in the event of an
over-current condition present during soft-start.
The current required to charge C
commonly referred to as “displacement current” is given as:
where the
Operation Description
© 2008 Fairchild Semiconductor Corporation
FAN5361 • Rev. 1.1.2
I
DISP

C
while
OUT
dV
dt
OUT

with
term refers to the soft-start slew rate above.
preserving
dV
, then 12.5mV/s until the output reaches its
dt
OUT
exceptional
OUT
is used. This is due to the current-
OUT
is charged.
fast
during the transition between
transient
OUT
transient
during soft-start is
response
IN
response
is above
OUT
(1)
for
12
To prevent shut-down during soft-start, the following condition
must be met:
where I
guaranteed to support (600mA or 750mA).
Table 1 shows combinations of C
successfully with the minimum R
Table 1. Minimum R
Various C
Startup into Large C
Multiple soft-start cycles are required for no-load startup if
C
load to ensure the FAN5361 starts appropriately. The IC
shuts down for 85s when I
21s of current limit. The IC then begins a new soft-start
cycle. Since C
reaches regulation after multiple soft-start attempts.
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 (f
The MODE pin is internally buffered with a Schmitt trigger,
which allows the MODE pin to be driven with slow rise and
fall times. An asymmetric duty cycle for frequency
synchronization is also permitted as long as the minimum
time below V
Current Limit, Fault Shutdown, and Restart
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 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s before attempting a restart.
If the fault was caused by short circuit, the soft-start circuit
attempts to restart and produces an over-current fault after
about 32s, which results in a duty cycle of less than 30%,
limiting power dissipation.
The closed-loop peak-current limit, I
the open-loop tested current limit, I
Characteristics table. This is primarily due to the effect of
propagation delays of the IC current limit comparator.
I
DISP
OUT
2 X 4.7F, 0402
is greater than 15F. Large C

4.7F, 0402
10F, 0603
10F, 0805
I
MAX(DC)
LOAD
OUT
C
OUT
IL(MAX)
Values
OUT

is the maximum load current the IC is
I
retains its charge when the IC is off, the IC
MAX
or above V
LOAD
(
DC
OUT
Values for Soft-Start with
)
DISP
MODE
IH(MAX)
LOAD
exceeds I
OUT
).
LIM(PK)
that can be supported.
is 100ns.
Minimum R
that allow the IC to start
OUT
LIM(OL)
V
V
V
V
OUT
OUT
OUT
OUT
requires light initial
, is not the same as
LIMIT
, in the Electrical
www.fairchildsemi.com
/ 0.60
/ 0.60
/ 0.60
/ 0.50
for more than
LOAD
(2)