LTC4449 datasheet(25/38 Pages) LINER

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LTC3774

3774fc

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Figure 9. Two Different Modes of Output Voltage Tracking

Figure 10. Setup and Coincident and Ratiometric Tracking

TIME

(9a) Coincident Tracking

VOUT1

VOUT2

VOUT1

VOUT2

TIME

3774 F09

(9b) Ratiometric Tracking

VOUT2

(10a) Coincident Tracking Setup

VOSNS1+

TK/SS2

VOSNS2+

VOUT1

VOUT2

3774 F10

(10b) Ratiometric Tracking Setup

VOSNS1+

TK/SS2

VOSNS2+

VOUT1

APPLICATIONS INFORMATION

or ratiometrically track another supply’s output, as shown

in Figure 9. In the following discussions, VOUT2 refers to

the LTC3774’s channel 2 as a slave and VOUT1 refers to

channel 1 as a master. To implement the coincident track-

ing in Figure 9a, connect an additional resistive divider to

VOUT1 and connect its mid-point to the TK/SS pin of the

slave controller. The ratio of this divider should be the

same as that of the slave controller’s feedback divider

shown in Figure 10a. In this tracking mode, VOUT1 must

be set higher than VOUT2. To implement the ratiometric

tracking in Figure 9b, the ratio of the VOUT2 divider should

be exactly the same as the master controller’s feedback

divider shown in Figure 10b . By selecting different resis-

tors, the LTC3774 can achieve different modes of tracking

including the two in Figure 9.

So which mode should be programmed? While either

mode in Figure 9 satisfies most practical applications,

some trade-offs exist. The ratiometric mode saves a pair

of resistors, but the coincident mode offers better output

regulation. Under ratiometric tracking, when the master

controller’s output experiences dynamic excursion (under

load transient, for example), the slave controller output

will be affected as well. For better output regulation, use

the coincident tracking mode instead of ratiometric.

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