Data Sheet

ADP1851

Rev. B | Page 21 of 24

VOLTAGE TRACKING

The ADP1851 includes a tracking feature that tracks a master

voltage. In all tracking configurations, the output can be set as

low as 0.6 V for a given operating condition.

Two tracking configurations are possible with the ADP1851:

coincident and ratiometric tracking.

Coincident Tracking

The most common application is coincident tracking, used

in core vs. I/O voltage sequencing and similar applications. As

shown in Figure 30, coincident tracking forces the ramp rate of

the output voltage to be the same for the master and slave until

the slave output reaches its regulation voltage. Connect the slave

SS/TRK input to a resistor divider from the master voltage that

is the same as the divider used on the slave FB pin. This forces the

slave voltage to be the same as the master voltage. For coincident

TIME

MASTER VOLTAGE

SLAVE VOLTAGE

Figure 30. Coincident Tracking

SS/

TRK

FB

ADP1851

1.8V

20kΩ

0.6V

10kΩ

3.3V

20kΩ

1.1V

10kΩ

Figure 31. Example of a Coincident Tracking Circuit

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

function of the two dividers.













+













+

=

TRKB

TRKT

BOT

TOP

MASTER

OUT

SLAVE

OUT

R

R

R

R

V

V

1

1

_

_

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

Eventually, the slave voltage reaches its regulation voltage, at

which point the internal reference takes over the regulation

while the SS/TRK input continues to increase, thus removing

itself from influencing the output voltage.

To ensure that the output voltage accuracy is not compromised

by the SS/TRK pin being too close in voltage to the reference

Ratiometric Tracking

Ratiometric tracking limits the output voltage to a fraction of

the master voltage, as illustrated in Figure 32 and Figure 33. The

final SS/TRK voltage of the slave channel should be set to at

TIME

MASTER VOLTAGE

SLAVE VOLTAGE

Figure 32. Ratiometric Tracking

SS/

TRK

FB

ADP1851

1.8V

20kΩ

0.6V

10kΩ

3.3V

41.2kΩ

0.65V

10kΩ

Figure 33. Example of a Ratiometric Tracking Circuit

PCB LAYOUT GUIDELINES

The recommended board layout practices for the synchronous

buck controller are described in the AN-1119 Application Note.