SC183C

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14

current (I

condition, the output voltage will be the product of I

CL_HOLD

and the load resistance. The current limit holding current

(I

The load presented must fall below the current limit

holding current for the part to exit foldback current limit

mode. Figure 2 shows how the typical current limit

holding current varies with output voltage. The SC183C 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. The current limit functionality is shown

in Figure 4.

Current Limit Holding Current over Vout

0

50

100

150

200

250

300

1.0

1.5

2.0

2.5

3.0

3.5

Output Voltage (V)

Figure 2 — Typical Current Limit Holding Current

Over-Voltage Protection

In the event of a 15% over-voltage on the output, the

PWM drive is disabled with LX pin floating.

Soft-Start

The soft-start mode is activated after AVIN 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 in-rush current. The

PMOS current limit is stepped through four soft start levels

of approximately 20%, 25%, 40%, & 100%. Each step is

maintained for 200μs following an internal reference

start up 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

the end of the soft-start period, the SC183C will switch

to PWM mode operation. Figure 3 shows the typical

diagram of soft start operation.

The SC183C is capable of starting up into a pre-biased

output. When the output is precharged by another

supply rail, the SC183C will not discharge the output

during the soft start interval.

Shut Down

When all CTL pins are low, the SC183C will run in shut-

down 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

SC183C 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 SC183C converter has internal loop compensation.

The compensation is designed to work with a output

filter corner frequency of less than 40kHz for a V

and 50kHz for a V

The corner frequency of the output filter can be defined

by Equation 3.

OUT

C

C

L

f

⋅

=

π

2

1

(3)

Values outside this range may lead to instability, malfunc-

tion, or out-of-specification performance.

In general, the inductance is chosen by making the induc-

tor 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.

Applications Information (continued)