LTC7803 datasheet(22/34 Pages) AD

22 / 34 page

LTC7802-3.3

Rev. 0

For more information www.analog.com

Figure 7. During start-up VOUT will track VX according to

the ratio set by the resistor divider:

VOUT

RTRACKA

•

RTRACKA +RTRACKB

RA +RB

Set RTRACKA = RA and RTRACKB = RB for coincident track-

ing (VOUT = VX during start-up).

APPLICATIONS INFORMATION

Figure 5. Using the RUN Pins As a UVLD

Figure 6. Two Different Modes of Output Voltage Tracking

(b) Ratiometric Tracking

(a) Coincident Tracking

780233 F07a

VX(MASTER)

VOUT(SLAVE)

TIME

780233 F07b

VX(MASTER)

VOUT(SLAVE)

TIME

78033 F08

LTC7802-3.3

VFB

TRACK/SS1,2

VOUT

RTRACKB

RTRACKA

780233 F06

1/2 LTC7802-3.3

RUN

VIN

Figure 7. Using the TRACK/SS Pin for Tracking

The current that flows through the R1-R2 divider directly

adds to the shutdown, sleep, and active current of the

LTC7802-3.3, and care should be taken to minimize the

impact of this current on the overall efficiency of the appli-

cation circuit. Resistor values in the MΩ range may be

required to keep the impact on quiescent shutdown and

sleep currents low.

Soft-Start and Tracking (TRACK/SS Pins)

The start-up of each VOUT is controlled by the voltage on

the TRACK/SS pin (TRACK/SS1 for channel 1, TRACK/SS2

for channel 2). When the voltage on the TRACK/SS pin

is less than the internal 0.8V reference, the LTC7802-3.3

regulates the negative input of the Error Amplifier (EA–)

to the voltage on the TRACK/SS pin instead of the internal

reference. The TRACK/SS pin can be used to program

an external soft-start function or to allow VOUT to track

another supply during start-up.

Soft-start is enabled by simply connecting a capacitor

from the TRACK/SS pin to ground. An internal 12.5μA cur-

rent source charges the capacitor, providing a linear ramp-

ing voltage at the TRACK/SS pin. The LTC7802-3.3 will

regulate VOUT proportional to the voltage on the TRACK/

SS pin, allowing VOUT to rise smoothly from 0V to its final

regulated value. For a desired soft-start time, tSS, select a

soft-start capacitor CSS = tSS • 15μF/sec.

Alternatively, the TRACK/SS pins can be used to track two

or more supplies during start-up, as shown qualitatively

in Figure 6a and Figure 6b. To do this, a resistor divider

should be connected from the master supply (VX) to the

TRACK/SS pin of the slave supply (VOUT), as shown in

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