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LTC3855

3855f

applicaTions inForMaTion

producingasmalloffseterror.Tominimizethiserror,select

the tracking resistive divider value to be small enough to

make this error negligible.

In order to track down another channel or supply after

the soft-start phase expires, the LTC3855 is forced into

continuous mode of operation as soon as VFB is below the

undervoltage threshold of 0.54V regardless of the setting

on the MODE/PLLIN pin. However, the LTC3855 should

always be set in force continuous mode tracking down

when there is no load. After TK/SS drops below 0.1V, its

channel will operate in discontinuous mode.

Output Voltage Tracking

The LTC3855 allows the user to program how its output

ramps up and down by means of the TK/SS pins. Through

these pins, the output can be set up to either coincidentally

or ratiometrically track another supply’s output, as shown

in Figure 8. In the following discussions, VOUT1 refers to

the LTC3855’s output 1 as a master channel and VOUT2

refers to the LTC3855’s output 2 as a slave channel. In

practice, though, either phase can be used as the master.

To implement the coincident tracking in Figure 8a, con-

nect an additional resistive divider to VOUT1 and connect

its midpoint to the TK/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 9a. In this

tracking mode, VOUT1 must be set higher than VOUT2. To

implementtheratiometrictrackinginFigure9b,theratioof

the VOUT2dividershouldbeexactlythesameasthemaster

channel’s feedback divider shown in Figure 9b. By select-

ing different resistors, the LTC3855 can achieve different

modes of tracking including the two in Figure 8.

So which mode should be programmed? While either

mode in Figure 8 satisfies most practical applications,

some tradeoffs exist. The ratiometric mode saves a pair

of resistors, but the coincident mode offers better output

regulation.

When the master channel’s output experiences dynamic

excursion (under load transient, for example), the slave

channel output will be affected as well. For better output

regulation, use the coincident tracking mode instead of

ratiometric.

Figure 8. Two Different Modes of Output Voltage Tracking

Figure 9. Setup for Coincident and Ratiometric Tracking

TIME

(8a) Coincident Tracking

VOUT1

VOUT2

3855 F08a

VOUT1

VOUT2

TIME

3855 F08b

(8b) Ratiometric Tracking

VOUT2

(9a) Coincident Tracking Setup

VFB1

TK/SS2

VFB2

VOUT1

VOUT2

3855 F09

(9b) Ratiometric Tracking Setup

VFB1

TK/SS2

VFB2

VOUT1

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