SC286 datasheet(16/20 Pages) SEMTECH

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SC286

Applications Information (continued)

Soft-Start

The soft-start mode is activated for each channel after

AVIN reaches it’s UVLO voltage threshold and CTL

XA/B is set

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.

OUT

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.

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