LTC7821 datasheet(25/36 Pages) LINER

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LTC7821

Rev A

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APPLICATIONS INFORMATION

Figure 17. Output Voltage Set By EXT_REF Pin

TIME

VOUT1

VOUT2

VOUT1

VOUT2

(15a) COINCIDENT TRACKING

(15b) RATIO-METRIC TRACKING

7821 F15

(FROM LTC7821)

EXTERNAL SUPPLY

(FROM LTC7821)

EXTERNAL SUPPLY

VOUT1

VOUT2

TRACK/SS

PIN OF

LTC7821

VFB1

VFB2

VOUT1

VOUT2

TRACK/SS

PIN OF

LTC7821

VFB1

VFB2

(16a) COINCIDENT TRACKING SETUP

(16b) RATIO-METRIC TRACKING SETUP

7821 F16

Figure 15. Two Different Methods of Output Voltage Tracking

Figure 16. Setup for Coincident and Ratio-Metric Tracking

To implement the ratiometric tracking in Figure 15b, the

ratio of the VOUT2 divider should be exactly the same as

the master channel’s feedback divider ratio shown in

Figure 16b.

In order to track down another channel or supply after

the soft-start has successfully reached 82.5% of 0.8V

or VEXT_REF, it is recommended to set the LTC7821 into

force continuous mode operation by setting the MODE/

PLLIN = 0V. For no load condition, the LTC7821 should

be in force continuous mode to ensure good tracking of

the master supply.

By selecting different resistors, the LTC7821 can achieve

different modes of tracking including the two in Figure

15. The ratio-metric mode uses one less pair of resistors

compared to the coincident mode but has lesser output

accuracy on VOUT2 and is also fully coupled to any varia-

tions in VOUT1. In both modes, there is an error in output

voltage setting cause by the pin current of TRACK/SS. To

minimizethiserror,usesmallerresistorvaluesinthedivider.

Output Voltage Setting

The LTC7821 uses its internal reference of 0.8V when the

VEXT_REF ≥ 1.3V. The output voltage is given by:

VOUT = 0.8 • 1+

⎛

⎝⎜

⎞

⎠⎟

If the applied voltage to the EXT_REF is less than 1.3V,

then VOUT will track EXT_REF voltages between 0.4V and

0.9V, as indicated by the characteristic in Figure 17.

0.4V

7821 F17

0.9V

0.4•(1+R2/R1)

0.9•(1+R2/R1)

VEXT_REF

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