SC284P datasheet(16/22 Pages) SEMTECH

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SC284P

Applications Information (continued)

Protection Features

The SC284P provides the following protection features:

Current Limit

Over-Voltage Protection

Soft-Start

Thermal Shutdown

Current Limit

The internal PMOS power device in the switching stage

is protected by a current limit feature. If the inductor

current is above the PMOS current limit for 6 consecutive

cycles, the part enters foldback current limit mode and

the output current is limited to the current limit holding

current (I

CL_HOLD) of a few hundred milliampere. Under

this condition, the output voltage will be the product of

CL_HOLD and the load resistance. The current limit holding

current will decrease when the output voltage increases.

The load presented must fall below the current limit

holding current for the part to exit foldback current

limit mode. Figure 4 shows how the typical current limit

holding current varies with output voltage. The SC284P

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.

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

Soft-start is activated once VIN reaches the 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 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 maintained for 400μs

following an internal reference start up duration of 00μs

giving a total nominal startup period of 700μs. During

startup, the chip operates by controlling the inductor

current swings between 0A and current limit. If at any time

OUT reaches 86% of the target or at the end of the soft-

start period, the SC284P will switch to PWM mode

operation.

The SC284P is capable of starting up into a pre-biased

output.

Shut Down

When all CTL pins of a channel are low, the corresponding

channel will be disabled, drawing less than μA from that

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 SC284P 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 SC284P 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.

Values outside this range may lead to instability, malfunc-

tion, or out-of-specification performance.

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

OUT

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