sc286ultrt Semtech Corporation, sc286ultrt Datasheet - Page 16

• Manufacturer Part Number: sc286ultrt
• Description: Dual Channel 4a Synchronous Step-down Regulator
• Manufacturer: Semtech Corporation

Applications Information (continued)
Soft-Start
The soft-start mode is activated for each channel after
AVIN reaches it’s UVLO voltage threshold and CTL
high to enable the part. Recovery from a thermal shut-
down event will also activate the soft start sequence. The
soft-start mode controls the slew-rate of the output
voltage during start-up thus limiting in-rush current on
the input supply. During start-up, the reference voltage
for the error amplifier is clamped by the voltage on SS pin.
The output voltage slew rate during soft-start is deter-
mined by the value of the external capacitor connected to
the SS pin and the internal 5µA charging current. The
device requires a minimum soft-start time from enable to
final regulation in the order of 200µs, including the 50µs
enable delay. As a result the soft start capacitor, Css, should
be higher than 1.5nF. During start up, the chip operates in
forced PWM mode.
100% Duty-Cycle Operation
SC286 is capable of operating at 100% duty-cycle. When
the difference between the input voltage and output
voltage is less than the minimum dropout voltage, the
PMOS switch is turned completely on, operating in 100%
duty-cycle. The minimum dropout voltage is the output
current multiplied by the on-resistance of the internal
PMOS switch and the DC-resistance of the inductor when
the PMOS switch is on continuously.
Output L-C filter Selection
SC286 has fixed internal loop-gain compensation for
each channel. It is optimized for X5R or X7R ceramic
output capacitors and an output L-C filter corner
frequency of less than 34kHz. The output L-C corner
frequency can be determined by the following
equation.
f
C
2
L
1
C
OUT
XA/B
is set
In general, the inductor is chosen to set the inductor ripple
current to approximately 30% of the maximum output
current. It is recommended to use a typical inductor value
of 1μH to 2.2μH with output ceramic capacitors of 44μF or
higher capacitance. Lower inductance should be consid-
ered in applications where faster transient response is
required. More output capacitance will reduce the output
deviation for a particular load transient. When using low
inductance, the maximum peak inductor current at any
condition (normal operation and start up) can not exceed
5A which is the guaranteed minimum current limit. The
saturation current rating of the inductor needs to be at
least larger than the peak inductor current which is the
maximum output current plus half of inductor ripple
current.
SC286
16