SC416 datasheet(16/34 Pages) SEMTECH

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SC416

10μA instead of 5μA, the CSS capacitor must be twice as

large as in the independent case. It is acceptable to use

only one SS capacitor for both SS pins, and the startup

time follows the equation:

START

= Css × 75

START

in μsec, Css in nF)

Once the SS pins reach 750mV, each FB comparator

switches over to the internal 750mV reference, and the

SS pin is ignored. CSS will continue to charge to +5V, just

as in the independent case.

Shutdown for proportional tracking is similar to the inde-

pendent case. Both EN pins are simultaneously pulled to

GND to start the shutdown sequence. When the EN pins

are set low, the two SS 5μA current sources are disabled,

and the internal 4k pull-down of each SS pin is energized.

This begins a quick discharge of CSS from the initial +5V

starting point.

When CSS discharges down to 810mV, each 4k resistor

is removed and each SS 5μA current sink is energized, to

cause a more gradual ramp down to 750mV. When SS

reaches 750mV, each FB comparator ignores the inter-

nal 750mV reference and uses the falling SS ramp as the

reference. This causes both FB pins to fall at a constant

linear rate. This in turn causes both VOUTs to fall linearly.

This continues for each VOUT until it reaches 300mV, at

which point all switching for that output will stop.

Note that the output ramp-down is controlled only

until the point where the output voltage (as sensed at

the VOUT pin, not the FB pin) reaches 300mV, and then

switching stops. Also note that the two output voltages

may not reach 300mV at the same time. With propor-

tional tracking, the lower voltage will reach 300mV first;

at that point, the lower voltage will enter soft-discharge

into 16 ohms, but the other output will continue on the

SS ramp-down until it also reaches 300mV. For the ma-

jority of applications this is acceptable since 300mV is

typically too small to cause a voltage-differential prob-

lem for the load.

Applications Information (continued)

Coincident Tracking (VOUT tracking)

Coincident tracking has the lower output actively match-

ing the higher output during startup and shutdown.

During startup, the two voltages track identically until

the lower VOUT reaches its nominal point and then re-

mains at that point. The higher voltage continues on a

linear ramp until reaching its final value.

During shutdown, the higher voltage initially starts to fall

while the lower voltage stays in regulation. When the

higher voltage has dropped to the same level as the low-

er voltage, the lower voltage joins the ramp-down, and

both voltages fall at the same rate. When the outputs

reach 300mV, switching stops and the outputs will soft-

discharge into their VOUT pins.

The waveforms and schematic for coincident tracking

are shown in Figure 10.

VOUT1

TSTART

300mV

EN1

EN2

SS1

750mV

Rapid

discharge of

SS capacitor

+5V

VOUT2

EN1

EN2

SS1

SC416

CSS

TSD_RAMP

SS2

VOUT1

VOUT2

FB2

Figure 10

Note that both EN pins are tied together. For coincident

tracking, the higher output voltage must act as the con-

trol, and the lower output must be the follower. Either

side can act as controller or follower. In this case, VOUT1

is the higher output voltage. When the EN pins are set

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