SC120 datasheet(15/21 Pages) SEMTECH

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PRELIMINARY

SC120

Applications Information (continued)

VOUT

+0.8%

+1.8%

PSAVE Mode at

Moderate Load

PWM Mode at

High Load

-2%

BURST

OFF

(> 5μs)

Higher Load

Applied

Inductor

Current

Time

Prog’d

Voltage

BURST

PWM Mode

PSAVE exit due to

off-time reduction

240mA

OFF

(< 5μs)

Figure 3 — PSAVE Operation With Exit to PWM Due To Off-time < 5μs

OUT

< 0.98 × V

REG

, PWM operation will resume. The second

PSAVE exit criterion, illustrated in Figure 3, depends on the

rate of discharge of the output capacitor between PSAVE

bursts. If the time between bursts is less than 5μs, then

PWM operation resumes. The output capacitance value

will affect the second criterion, but not the first. Reducing

the output capacitor will reduce the output load at which

PSAVE mode exits to PWM mode.

Within each on/off cycle of a PSAVE burst, the rate of

decrease of the inductor current during the off-state is

proportional to (V

OUT

− V

). If V

is sufficiently close to V

OUT

the decrease in current during the off-state may not over-

come the increase in current during the minimum on-time

of the on-state, approximately 100ns. This can result in

the peak inductor current rising above the PSAVE mode

n-channel FET current limit.

(Normally, when the

n-channel FET current limit is reached, the on-state ends

immediately and the off-state begins. This sets the duty

cycle on a cycle-by-cycle basis.) This inductor current rise

accumulates with each successive cycle in the burst. The

result is that the output load current that can be sup-

ported in PSAVE under this high V

condition will be

greater than occurs if the 240mA current limit can be

enforced. Therefore the PSAVE exit load due to the first

exit criterion (Figure 2) can increase significantly. This

phenomenon is advantageous. Reverting to PWM opera-

tion with high V

can result in V

OUT

rising above V

REG

, due

to the PWM minimum duty cycle. PSAVE operation avoids

this voltage rise because of its voltage-threshold on/off

control. If the load remains low enough to remain in

PSAVE, V

can approach and even slightly exceed V

OUT

. To

initally enter PSAVE mode, the initial startup load must be

small enough to cause discontinuous mode PWM opera-

tion. This PSAVE mode startup load upper limit can be

increased if needed by reducing the inductance (refer to

the section Inductor Selection). Sufficiently large output

capacitance will prevent PSAVE exit due to the second exit

criterion (Figure 3).

PSAVE V

OUT

ripple may increase due to parasitic capaci-

tance on the external FB pin network. If using external

feedback programming, it is prudent to add a small capaci-

tor between OUT and FB to the circuit board layout. When

operating the SC120 in the final configuration in PSAVE,

observe the amplitude of PSAVE ripple. If the ripple

exceeds 50mV for the expected range of input voltage, a

small-value capacitor should be tried. Capacitance on the

order of a few picofarads is often sufficient to bring the

ripple amplitude to approximately 50mV.

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