SC283 datasheet(14/20 Pages) SEMTECH

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SC283

Applications Information (continued)

vs. Output Voltage

Over-Voltage Protection

In the event of a 15% 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%, & 100%. 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 SC283 will switch to PWM

mode operation. Figure 5 shows the typical diagram of

soft start operation.

The SC283 is capable of starting up into a pre-biased

output. When the output is precharged by another supply

rail, the SC283 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 1μA 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 SC283 if the junction temperature exceeds 160°C.

During thermal shutdown, the on-chip power devices are

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

drops by 10°C, it will initiate a soft start cycle to resume

normal operation.

Inductor Selection

The SC283 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 Equation

OUT

⋅

(3)

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

below the inductor saturation specification keeps the

inductor ripple current and the output voltage ripple at

acceptable levels. Manufacturers often provide graphs of

actual inductance and saturation characteristics versus

applied inductor current. The saturation characteristics of

the inductor can vary significantly with core temperature.

Core and ambient temperatures should be considered

when examining the core saturation characteristics.

When the inductance has been determined, the DC

resistance (DCR) must be examined. The efficiency that

can be achieved is dependent on the DCR of the inductor.

The lower values give higher efficiency. The RMS DC

current rating of the inductor is associated with losses in

the copper windings and the resulting temperature rise of

the inductor. This is usually specified as the current which

produces a 40˚C temperature rise. Most copper windings

are rated to accommodate this temperature rise above

maximum ambient.

Magnetic fields associated with the output inductor can

interfere with nearby circuitry. This can be minimized by

the use of low noise shielded inductors which use the

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