LTC7802EUFDM datasheet(21/34 Pages) AD

21 / 34 page

LTC7802

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 7. Using the TRACK/SS Pin for Tracking

(6b) Ratiometric Tracking

(6a) Coincident Tracking

7802 F07a

VX(MASTER)

VOUT(SLAVE)

TIME

7802 F07b

VX(MASTER)

VOUT(SLAVE)

TIME

7802 F08

LTC7802

VFB

TRACK/SS1,2

VOUT

RTRACKB

RTRACKA

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

regulates the VFB pin voltage 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

current source charges the capacitor, providing a linear

ramping voltage at the TRACK/SS pin. The LTC7802 will

regulate its feedback voltage (and hence VOUT) according

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

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).

Single Output 2-Phase Operation

For high power applications, the two channels can be

operated in a 2-phase single output configuration. The

channels switch 180° out-of-phase, which reduces the

required output capacitance in addition to the required

input capacitance and power supply induced noise. To

configure the LTC7802 for 2-phase operation, tie VFB2 to

INTVCC, ITH2 to ground, and RUN2 to RUN1.

The RUN1, VFB1, ITH1, TRACK/SS1 pins are then used

to control both channels, but each channel uses its own

ICMP and IR comparators to monitor their respective

Figure 6. Two Different Modes of Output Voltage Tracking

inductor currents. Figure 10 is a typical application con-

figured for single output 2-phase operation.

INTVCC Regulators

The LTC7802 features two separate internal low dropout

linear regulators (LDOs) that supply power at the INTVCC

pin from either the VIN pin or the EXTVCC pin depending

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