SC198 datasheet(8/16 Pages) SEMTECH

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Applications Information (Cont.)

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SC198

POWER MANAGEMENT

VOUT

Upper

Threshold Limit

PSAVE Mode at Light Load

PWM Mode at Medium/

High Load

PSAVE Exit

Threshold

BURST

OFF

Higher Load

Applied

0 A

Inductor Current

Time

Lower

Threshold Limit

PSAVE Operation

In PSAVE mode, V

OUT regulation is controlled by bursts

in switching. While the output voltage is between the

low and high voltage thresholds, switching is stopped to

reduce loss in the regulator due to switching. When the

voltage drops to the lower threshold, the switch bursts

at a ﬁxed on time until the upper threshold is met. At

this point the output voltage is allowed to ramp down to

the lower threshold again without switching to conserve

power. In the PSAVE mode, less switching is used. Less

switching results in less power loss which can be seen

in the difference of efﬁciency between PWM mode and

PSAVE mode at light loads.

The PSAVE switching burst frequency is controlled such

that the inductor current ripple during the burst is similar

to that in PWM mode. The minimum switching frequency

during this period is limited to 650kHz.

The SC198 automatically detects when to exit PSAVE

mode. The device sets a maximum peak current in

PSAVE mode, and then detects when V

OUT falls due to the

load increasing above the level that the peak current will

support.

For the SC198 to exit PSAVE mode, the load must be

increased, which causes V

OUT to decrease until the PSAVE

exit threshold is reached. PSAVE levels are set high to

minimize the undershoot when the SC198 exits PSAVE.

Protection Features

The SC198 provides the following protection features:

Thermal Shutdown

Current Limit

Over-Voltage Protection

Soft-Start

Thermal Shutdown

The device has a thermal shutdown feature to protect the

SC198 if the junction temperature exceeds 145°C. In

thermal shutdown the on-chip power devices are disabled,

tri-stating the LX output. Switching will resume when the

temperature drops by 10°C. During this time if the output

voltage decreases by more than 60% of its programmed

value, a soft-start will be invoked.

Current Limit

The PMOS and NMOS power devices of the buck switcher

stage are protected by current limit functions. In the case

of a short to ground on the output, the part enters frequen-

cy foldback mode, that causes the switching frequency to

divide by a factor determined by the output voltage. This

prevents the inductor current from “staircasing”.

Over-Voltage Protection

Over-voltage protection is provided on the SC198.

Hysteresis is provided to prevent toggling in and out of

operation.

Soft-Start

Thesoft-startmodeoperatesbylimitingthecurrentthrough

the inductor and controlling the switching frequency. The

current limit is increased over several milliseconds the in

discrete steps. This has the desired effect of limiting in-

rush current from the input supply. The soft-start mode is

cancelled once the output reaches regulation. Soft-start

is only re-enabled by power cycling, toggling enable, UVLO

event, or shutdown.

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