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LTC3838

3838fa

APPLICATIONS INFORMATION

When a channel is configured to soft-start by itself, a ca-

pacitor should be connected to its TRACK/SS pin. TRACK/

SS is pulled low until the RUN pin voltage exceeds 1.2V

and UVLO is released, at which point an internal current

of 1μA charges the soft-start capacitor, CSS, connected

to the TRACK/SS pin. Current-limit foldback is disabled

during this phase to ensure smooth soft-start or track-

ing. The soft-start or tracking range is defined to be the

voltage range from 0V to 0.6V on the TRACK/SS pin. The

total soft-start time can be calculated as:

tSS(SEC)= 0.6(V)•

CSS(µF)

1(µA)

When one particular channel is configured to track an

external supply, a voltage divider can be used from the

external supply to the TRACK/SS pin to scale the ramp

rate appropriately. Two common implementations are co-

incidental tracking and ratiometric tracking. For coincident

tracking, make the divider ratio from the external supply

the same as the divider ratio for the differential feedback

voltage. Ratiometric tracking could be achieved by using

a different ratio than the differential feedback.

Note that the 1μA soft-start capacitor charging current is

still flowing, producing a small offset error. To minimize

this error, select the tracking resistive divider values to be

small enough to make this offset error negligible.

The LTC3838 allows the user to program how its two

channels’ outputs track each other ramping up or down.

In the following discussions, VOUT1 refers to the LTC3838’s

output 1 as a master channel and VOUT2 refers to the

LTC3838’s output 2 as a slave channel. In practice though,

either channel can be used as the master.

By selecting different resistors, the LTC3838 can achieve

different modes of tracking including the two in Figure 9.

To implement the coincident tracking, connect an addi-

tional resistive divider to VOUT1 and connect its midpoint

to the TRACK/SS pin of the slave channel. The ratio of this

divider should be the same as that of the slave channel’s

feedback divider shown in Figure 9b. In this tracking mode,

Figure 9a. Two Different Modes of Output Tracking

Figure 9b. Setup for Coincident and Ratiometric Tracking

TIME

Coincident Tracking

VOUT1

VOUT2

VOUT1

VOUT2

TIME

3838 F09a

Ratiometric Tracking

RFB2(1)

RFB1(1)

RFB2(2)

RFB1(2)

RFB2(2)

VOUT2

RFB1(2)

Coincident Tracking Setup

VOUTSENSE1+

VOUTSENSE1–

TRACK/SS2

VFB2

VOUT1

RFB2(1)

RFB1(1)

RFB2(2)

VOUT2

RFB1(2)

3838 F09b

Ratiometric Tracking Setup

VOUTSENSE1+

VOUTSENSE1–

TRACK/SS2

VFB2

VOUT1

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