SC194B datasheet(8/17 Pages) SEMTECH

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SC194B

POWER MANAGEMENT

Applications Information (Cont.)

In PSAVE mode V

OUT is driven from a lower level to an

upper level by a switching burst. Once the upper level has

been reached the switching is stopped and the quiescent

current is reduced. V

OUT falls from the upper to lower levels

in this low current state as the load current discharges

the output capacitor. The burst-to-off period in PSAVE will

decrease as the load current reduces.

The PSAVE switching burst frequency is controlled so

that the inductor current ripple is similar to that in PWM

mode.

The SC194B automatically detects when to exit PSAVE

mode by monitoring V

OUT. For the SC194B to exit PSAVE

mode, the load must be increased, causing V

OUT to

decrease until the power save exit threshold is reached.

PSAVE levels are set high to minimize the undershoot

when exiting PSAVE. The lower PSAVE comparator level

is set +0.7% above V

OUT, and the upper comparator level

at +1.5% above V

OUT, with the exit threshold at -2% below

OUT.

If PSAVE operation is required then a 22μF output

capacitor must be used.

100% Duty Cycle Operation

The 100% duty cycle mode may be selected by connecting

the MODE pin high. This will allow the SC194B to maintain

output regulation under low input voltage/high output

voltage conditions.

In 100% duty cycle operation, as the input supply drops

toward the output voltage, the PMOS on-time increases

linearly above the maximum value in ﬁxed-frequency

operation until the PMOS is active continuously. Once

the PMOS is switched on continuously, the output voltage

tracks the input voltage minus the voltage drop across

the PMOS power device and inductor according to the

following relationship:

where,

OUT

= Output voltage

= Input voltage

OUT

= Output current

DSP

= PMOS switch ON resistance

IND

= Series resistance of the inductor

Inductor Selection

The SC194B is designed for use with a 4.7μH inductor.

The magnitude of the inductor current ripple is dependent

on the inductor value and can be determined by the

following equation:

OUT

osc

OUT

This equation demonstrates the relationship between

input voltage, output voltage, and inductor ripple

current.

The inductor should have a low DCR to minimize the

conduction losses and maximize efﬁciency.

As a

minimum requirement, the DC current rating of the

inductor should be equal to the maximum load current

plus half of the inductor current ripple as shown by the

following equation:

L(PK)

UT(MAX)

)

IND

DSP

OUT

)

IND

DSP

OUT

VOUT

0.7%

1.5%

PSAVE Mode at Light Load

PWM Mode at Medium/

High Load

-2%

BURST

OFF

Higher Load

Applied

0 A

Inductor Current

Time

Power Save Operation

SC194B Datasheet(PDF) 8 Page - Semtech Corporation

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