SC120 datasheet(14/21 Pages) SEMTECH

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PRELIMINARY

SC120

Applications Information (continued)

the inductance value, the input voltage, and the on-time

(D×T). Therefore, the instantaneous inductor current will

be alternately larger and smaller than the average. If the

average output current is sufficiently small, the minimum

inductor current can reach zero during the off-state. If

the energy stored in the inductor is depleted (if the induc-

tor current decreases to zero) during the off-state, both

FETs turn off for the remainder of the off-state. If this dis-

continuous mode (DM) operation persists, the SC120

transitions to PSAVE operation.

PSAVE Operation

At light loads, the SC120 will operate in PSAVE mode.

PSAVE mode ensures proper regulation when V

is too

close to V

OUT

while the output load is too small to keep the

duty cycle above its minimum value. At very low output

load, PSAVE mode will operate more efficiently than PWM

mode. PSAVE operation is triggered by 256 consecutive

cycles of DM operation in PWM mode, when the output

of the P

LIM

amplifier falls to 0V during the off-state due to

low load current.

PSAVE mode requires fewer circuit resources than PWM

mode. All unused circuitry is disabled to reduce quies-

cent power dissipation. In PSAVE mode, the OUT pin

voltage monitoring circuit remains active and the output

voltage error amplifier operates as a comparator. PSAVE

regulation is shown in Figure 2. When V

OUT

< 1.008xV

REG

where V

REG

is the programmed output voltage, a burst of

fixed-period switching occurs to boost the output voltage.

The n-channel FET turns on (on-state) until the inductor

current rises to approximately 240mA. Then the n-channel

FET turns off and the p-channel FET turns on to transfer

the inductor energy to the output capacitor and load for

the duration of the off-state. This cycle repeats until

OUT

> 1.018×V

REG

, at which point bot FETs are turned off.

The output capacitor then discharges into the load until

OUT

< 1.008×V

REG

, and the burst cycle repeats.

When the output current increases above a predeter-

mined level, either of two PSAVE exit conditions will force

the resumption of PWM operation. The first PSAVE exit

criterion is shown in Figure 2. If the PSAVE burst cycle

cannot provide sufficient current to the output, the

output voltage will decrease during the burst. If

VOUT

+0.8%

+1.8%

PSAVE Mode at

Moderate Load

PWM Mode at

High Load

-2%

BURST

OFF

Higher Load

Applied

Inductor

Current

Time

Prog’d

Voltage

BURST

OFF

BURST

PWM Mode

PSAVE exit due

to output decay

240mA

Figure 2 — PSAVE Operation With Exit to PWM Due To Output Voltage Decay

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