SC3102

Figure 3 — Exiting PSAVE with Heavy Load

Protection Features

The SC3102 provides the following protection features:

Current Limit

Over-Voltage Protection

Soft-Start Operation

Thermal Shutdown

UVLO

Current Limit & OCP

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 8 consecutive cycles,

the part enters foldback current limit mode and the output

current is limited to the current limit holding current

(I

tion, the output voltage will be the product of I

the load resistance. When the load presented falls below

the current limit holding level, the output will charge to

the upper PSAVE voltage threshold and return to normal

operation. The SC3102 is capable of sustaining an indefi-

nite short circuit without damage. During soft-start, if

current limit has occurred before the SS voltage has

reached 400mV, the part enters foldback current limit

mode. Foldback current limit mode will be disabled during

soft-start after the SS voltage is higher than 400mV.

Over-Voltage Protection

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

PWM drive is disabled with the LX pin floating. Switching

does not resume until the output voltage falls below the

nominal Vout regulation voltage.

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•

•

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Applications Information (continued)

Soft-Start

The soft-start mode is activated after AVIN reaches it’s

UVLO voltage threshold and EN is set high to enable the

part. A thermal shutdown event will also activate the soft-

start sequence. The soft-start mode controls the slew-rate

of the output voltage during startup 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 the SS pin. The output voltage slew rate during soft-

start is determined by the value of the external capacitor

connected to the SS pin and the internal 5μA charging

current. The SC3102 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. The value of

Css for the desired soft-start time, tss, can be determined

by Equation 1.

A

V

C

t

SS

SS

μ

5

5

.

0

×

=

(1)

The SC3102 is capable of starting up into a pre-biased

output.When the output is pre-charged by another supply

rail, the SC3102 will not discharge the output during the

soft-start period.

Thermal Shutdown

The device has a thermal shutdown feature to protect

the SC3102 if the junction temperature exceeds 160°C.

During thermal shutdown, the on-chip power devices are

disabled, floating the LX output. When the temperature

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

normal operation.

Under-Voltage Lockout

Under-Voltage Lockout (UVLO) is enabled when the

input voltage drops below the UVLO threshold. This

prevents the device from entering an ambiguous state

in which regulation cannot be maintained. Hysteresis of

approximately 300mV is included to prevent chattering

near the threshold. When the AVIN voltage rises back to

the turn-on threshold and EN is high, a soft-start sequence

is initiated.

Power Good

The power good (PGOOD) is an open-drain output. When

Vout

Inductor Current

0A

Load Current

MODE = Low

MODE = High

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