SC416 datasheet(15/34 Pages) SEMTECH

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SC416

The general shutdown waveforms are shown in Figure

300mV:

switching stops

VOUT linear

down-ramp

Soft-discharge

into 16 ohms

Linear down-ramp

+5V

750mV: VOUT

down-ramp begins

810mV: SS linear

down-ramp begins

Rapid CSS discharge

into 4Kohm

VOUT

TSD_DELAY

TSD_RAMP

CSS

SC416

Figure 8

Note that due to the discharging of CSS from +5V to

750mV, there is a small delay between EN going low and

VOUT beginning to ramp down. The delay is given by

the equation:

SD_DELAY

= Css × 19

SD_DELAY

in μsec, Css in nF)

After this delay, the time for the output to ramp down to

the switch-off point of 300mV is given by:

SD_RAMP

= Css × 150 • (1 – 300mV/VOUT)

SD_ RAMP

in μsec, Css in nF)

Once the output reaches 300mV, the shutdown sequence

is complete. DH and DH stop switching, the internal 16

ohm discharge pulldown at VOUT is energized, and that

side will go to the lowest-power state.

Voltage Tracking

The SC416 provides voltage tracking during both start-

up and shutdown.

The use of the SS pins determines

the type of tracking. The two most common tracking

schemes are proportional and coincident tracking.

Proportional Tracking (FB tracking)

Proportional tracking causes the output waveforms

to have the same startup and shutdown ramp time as

shown in Figure 9.

VOUT1

TSTART

300mV

EN1

EN2

SS1

SS2

750mV

Rapid

discharge of

SS capacitor

+5V

VOUT2

EN1

EN2

SS1

SS2

SC416

CSS

TSD_RAMP

Figure 9

For proportional tracking, the two SS pins are tied togeth-

er to share the same voltage ramp. The two EN pins must

also be tied together. Since the SS pins are tied together,

the startup and shutdown ramps for each side will track

the same signal. Note that during startup and shutdown,

the FB pins for each side will track the same SS signal; this

method can also be called FB tracking.

Startup for proportional tracking (FB tracking) is as fol-

lows. When both EN pins are set high simultaneously,

each SS pin sources 5uA. By connecting the SS pins to-

gether, the startup ramp rates for both FB1 and FB2 are

forced to be the same. Since the total current source is

Applications Information (continued)

SC416 Datasheet(PDF) 15 Page - Semtech Corporation

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