SC198A

11

In PSAVE mode (see Figure 1), VOUT regulation is con-

trolled by bursts in switching. While the output voltage is

between the low and high voltage thresholds, switching

is stopped to reduce loss. When the voltage drops to the

lower threshold, the switch bursts at a fixed on time until

the upper threshold is reached. At this point the output

voltage is allowed to ramp down to the lower threshold

without switching to conserve power. In the PSAVE mode,

less switching is used. This results in less power loss which

can be seen in the difference of efficiency between PWM

mode and PSAVE mode at light loads.

Lower

Thershold

Limit

Upper

Thershold

Limit

PSAVE Mode at

Moderate Load

PWM Mode at

High Load

PSAVE

Exit

Threshold

BURST

OFF

Higher Load

Applied

0A

Inductor

Current

Time

BURST

OFF

BURST

PWM Mode

PSAVE Mode at

High Load

Figure 1 — PSAVE Operation

The PSAVE switching burst frequency is controlled such

that the inductor current ripple during the burst is similar

to that in PWM mode.

The SC198A automatically detects when to exit PSAVE

mode. The device sets a maximum peak current in PSAVE

mode, and then detects when VOUT falls due to the load

increasing above the level that the peak current will

support.

For the SC198A to exit PSAVE mode, the load must be

increased, which causes VOUT to decrease until the PSAVE

exit threshold is reached. PSAVE levels are set high to

minimize the undershoot when the SC198A exits PSAVE.

Figure 1 shows PSAVE mode operation and exiting into

PWM mode at increased load.

Protection Features

The SC198A provides the following protection features:

Thermal Shutdown

Current Limit

Over-Voltage Protection

Soft-Start

Thermal Shutdown

The SC198A has a thermal shutdown feature to protect

against damage if the junction temperature exceeds

145°C. In thermal shutdown, the on-chip power devices

are disabled, tri-stating the LX output. Switching resumes

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 internal power devices are protected by current limit

functions. In the event of a short to ground on the output,

the part enters frequency foldback mode. This causes the

switching frequency to decrease by a factor determined

by the output voltage. This prevents the inductor current

from staircasing.

Over-Voltage Protection

In the event of an over-voltage on the output in switcher

mode, the PWM drive is disabled, effectively tri-stating

the LX output. The part will not resume switching until

the output voltage has fallen below 2% of the regulation

voltage.

Soft-Start

The soft-start mode operates by limiting the current

through the inductor and controlling the switching fre-

quency. The current limit is increased over several

milliseconds 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, a UVLO event, or shutdown.

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Application Information (continued)