SC284 datasheet(14/20 Pages) SEMTECH

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SC284

Applications Information (continued)

limit mode. Figure 4 shows how the typical current limit

holding current varies with output voltage. The SC284 is

capable of sustaining an indefinite short circuit without

damage and will resume normal operation when the fault

is removed. The foldback current limit mode is disabled

during soft-start. Current limit functionality is shown in

Figure 5 at the end of this section.

Current Limit Holding Current over Vout

100

150

200

250

300

1.0

1.5

2.0

2.5

3.0

3.5

Output Voltage (V)

TA= 25°C

VIN= 5.0V

VIN= 3.6V

VIN= 3.3V

Figure 4 — Typical Current Limit Holding Current

vs. Output Voltage

Over-Voltage Protection

In the event of a 5% over-voltage on the output, the

PWM drive is disabled leaving the LX pin floating.

Soft-Start

The soft-start mode is activated after VIN reaches its UVLO

and one or more CTL pins are set high to enable the part.

A thermal shutdown event will also activate the soft start

sequence. Soft-start mode controls the maximum current

during startup thus limiting inrush current. The PMOS

current limit is stepped through four soft start levels of

approximately 20%, 25%, 40%, & 00%. Each step is main-

tained for 200μs following an internal reference start up

duration of 50μs giving a total nominal startup period of

850μs. During startup, the chip operates by controlling

the inductor current swings between 0A and current limit.

If at any time V

OUT reaches 86% of the target or at the end

of the soft-start period, the SC284 will switch to PWM

mode operation. Figure 6 at the end of this section shows

the typical diagram of soft start operation.

The SC284 is capable of starting up into a pre-biased

output. When the output is precharged by another supply

rail, the SC284 will not discharge the output during the

soft start interval.

Shut Down

When all CTL pins of each channel are low, the channel will

run in shutdown mode, drawing less than μA from the

input power supply. The internal switches and bandgap

voltage will be immediately turned off.

Thermal Shutdown

The device has a thermal shutdown feature to protect

the SC284 if the junction temperature exceeds 60°C.

During thermal shutdown, the on-chip power devices are

disabled, tri-stating the LX output. When the temperature

drops by 0°C, it will initiate a soft start cycle to resume

normal operation.

Inductor Selection

The SC284 converter has internal loop compensation.

The compensation is designed to work with an output

filter corner frequency of less than 40kHz for a V

IN of 5V

and 50KHz for a V

IN of 3.3V over any operating condition.

The corner frequency of the output filter is shown in the

following equation.

OUT

Values outside this range may lead to instability,

malfunction, or out-of-specification performance.

In general, the inductance is chosen by making the

inductor ripple current to be less than 30% of maximum

load current. When choosing an inductor, it is important

to consider the change in inductance with DC bias

current. The inductor saturation current is specified as

the current at which the inductance drops a specific

percentage from the nominal value. This is approximately

30%. Except for short-circuit or other fault conditions,

the peak current must always be less than the saturation

current specified by the manufacturer. The peak current is

the maximum load current plus one half of the inductor

ripple current at the maximum input voltage. Load and/or

line transients can cause the peak current to exceed this

level for short durations. Maintaining the peak current

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