LTC3823IGN datasheet(17/26 Pages) LINER

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LTC3823

3823fd

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

the ratiometric tracking, the ratio of the divider should be

exactly the same as the master IC’s feedback divider. Note

that the pin Z1/SSENABLE of the slave IC should be tied to

INTVCC so that the internal soft-start current is disabled

in both tracking modes or it will introduce a small error

on the tracking voltage depending on the absolute values

of the tracking resistive divider.

By selecting different resistors, the LTC3823 can achieve

different modes of tracking including the two in Figure 7.

So which mode should be programmed? While either

mode in Figure 7 satisﬁes most practical applications,

there do exist some tradeoffs. The ratiometric mode saves

a pair of resistors, but the coincident mode offers better

output regulation. This can be better understood with the

help of Figure 9. At the input stage of the slave IC’s error

ampliﬁer, two common anode diodes are used to clamp

the equivalent reference voltage and an additional diode

is used to match the shifted common mode voltage. The

top two current sources are of the same amplitude. In the

coincident mode, the TRACK/SS voltage is substantially

higher than 0.6V at steady state and effectively turns off

D1. D2 and D3 will therefore conduct the same current

and offer tight matching between VFB2 and the internal

precision 0.6V reference. In the ratiometric mode, however,

TRACK/SS equals 0.6V at steady state. D1 will divert part

of the bias current to make VFB2 slightly lower than 0.6V.

Although this error is minimized by the exponential I-V

characteristic of the diode, it does impose a ﬁnite amount

of output voltage deviation. Furthermore, when the master

IC’s output experiences dynamic excursion (under load

transient, for example), the slave IC output will be affected

as well. For better output regulation, use the coincident

tracking mode instead of ratiometric.

APPLICATIONS INFORMATION

TIME

(7a) Coincident Tracking

VOUT1

VOUT2

TIME

3823 F07

(7b) Ratiometric Tracking

VOUT1

VOUT2

Figure 7. Two Different Modes of Output Voltage Tracking

VOUT2

(8a) Coincident Tracking Setup

VFB1

TRACK/SS2

VFB2

VOUT1

VOUT2

3823 F08

(8b) Ratiometric Tracking Setup

VFB1

TRACK/SS2

VFB2

VOUT1

Figure 8. Setup for Coincident and Ratiometric Tracking

–

TRACK/SS2

0.6V

VFB2

3823 F09

EA2

Figure 9. Equivalent Input Current of Error Ampliﬁer

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