ADP1877ACPZ-R7 datasheet(24/32 Pages) AD

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ADP1877

Rev. 0 | Page 24 of 32

The ratio of the slave output voltage to the master voltage is a

function of the two dividers.

RATIOMETRIC TRACKING

Ratiometric tracking limits the output voltage to a fraction of

the master voltage. For ratiometric tracking, the simplest

configuration is to tie the TRK pin of the slave channel to the

FBx pin of the master channel. However, because of the large

internal offset between TRKx and FBx, this ratiometric tracking

configuration is not recommended. A tracking configuration

that requires the TRKx voltage of the slave channel below 0.6 V

is not recommended because of the large internal TRKx to FBx

offset voltage.

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

TRKB

TRKT

BOT

TOP

MASTER

OUT

SLAVE

OUT

As the master voltage rises, the slave voltage rises identically.

Eventually, the slave voltage reaches its regulation voltage,

where the internal reference takes over the regulation while the

TRKx input continues to increase and thus removes itself from

influencing the output voltage.

Another ratiometric tracking configuration is having the slave

channel rise more quickly than the master channel, as shown in

Figure 38 and Figure 39. The tracking circuits in Figure 37 and

Figure 38 are virtually identical with the exception that RTRKB

> RTRKT, as shown in Figure 38.

To ensure that the output voltage accuracy is not compromised

by the TRKx pin being too close in voltage to the 0.6 V

reference, make sure that the final value of the TRKx voltage of

the slave channel is at least 0.7 V.

CSS1

100nF

CSS2

20nF

3.3V

VOUT1_MASTER

FB1

RBOT

10kΩ

FB2

SS1

ADP1877

EN1

EN2

SS2

VCCO

TRK1

TRK2

RTOP

20kΩ

1.8V

VOUT2_SLAVE

RTRKB

10kΩ

RTRKT

5kΩ

2.2V

10kΩ

45.3kΩ

1MΩ

Figure 38. A Simple Ratiometric Tracking Circuit (Slave Channel Has

a Faster Ramp Rate)

TIME

SLAVE VOLTAGE

MASTER VOLTAGE

Figure 39. Ratiometric Tracking (Slave Channel Has a Faster Ramp Rate)

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