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LTC3839

3839fa

APPLICATIONS INFORMATION

As voltage on the RUN pin increases, typically beyond

3V, its bias current will start to reverse direction and flow

into the RUN pin. Keep in mind that the RUN pin can sink

up to 50μA; Even if a RUN pin may slightly exceed 6V

when sinking 50μA, the RUN pin should never be forced

to higher than 6V by a low impedance voltage source to

prevent faulty conditions.

Soft-Start and Tracking

The LTC3839 has the ability to either soft-start by itself

with a capacitor or track an external supply. Note that the

soft-start and tracking features are achieved not by limit-

ing the maximum output current of the controller, but by

controlling the output ramp voltage according to the ramp

rate on the TRACK/SS pin.

When the LTC3839 is configured to soft-start by itself, a

capacitor 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)

Figure 9a. Two Different Modes of Output Tracking

Figure 9b. Setup for External Supply Tracking

TIME

Coincident Tracking

EXTERNAL SUPPLY

VOUT

EXTERNAL SUPPLY

VOUT

TIME

3839 F09a

Ratiometric Tracking

RFB2(1)

RFB1(1)

RFB2(2)

VOUT

RFB1(2)

VOUTSENSE+

VOUTSENSE–

TRACK/SS

EXTERNAL SUPPLY

3839 F09b

LTC3880 Datasheet(PDF) 27 Page - Linear Technology

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