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Data Sheet

ADP1850

Rev. C | Page 23 of 32

VOLTAGE TRACKING

The ADP1850 includes a tracking feature that tracks a master

voltage. This feature is especially important when the ADP1850

is providing separate power supply voltages to a single integrated

circuit, such as the core and I/O voltages of a DSP, FPGA, or

microcontroller. In these cases, improper sequencing can cause

damage to the load IC.

In all tracking configurations, the output can be set as low as 0.6 V

for a given operating condition. The soft start time setting of

the master voltage should be longer than the soft start of the

slave voltage. This forces the rise time of the master voltage to

be imposed on the slave voltage. If the soft start setting of the

slave voltage is longer, the slave comes up more slowly, and the

tracking relationship is not seen at the output.

Two tracking configurations are possible with the ADP1850:

coincident and ratiometric trackings.

Coincident Tracking

The most common application is coincident tracking, used in

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

Coincident tracking forces the slave output voltage’s ramp rate

to be the same as the master’s until the slave output reaches its

regulation. Connect the slave TRKx input to a resistor divider

from the master voltage that is the same as the divider used on

the slave FBx pin. This forces the slave voltage to be the same as

the master voltage. For coincident tracking, use RTRKT = RTOP

and RTRKB = RBOT, as shown in Figure 37.

TIME

SLAVE VOLTAGE

MASTER VOLTAGE

Figure 36. Coincident Tracking

ADP1850

EN2

EN1

FB2

FB1

TRK1

SS1

VCCO

TRK2

SS2

10kΩ

45.3kΩ

RTRKB

10kΩ

RTRKT

20kΩ

1.1V

3.3V

VOUT1_MASTER

CSS2

20nF

CSS1

100nF

RBOT

10kΩ

RTOP

20kΩ

1.8V

VOUT2_SLAVE

Figure 37. 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

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.

To ensure that the output voltage accuracy is not compromised

by the TRKx pin being too close in voltage to the reference voltage

(VFB, typically 0.6 V), make sure that the final value of the TRKx

voltage of the slave channel is at least 30 mV above VFB.

Ratiometric Tracking

Ratiometric tracking limits the output voltage to a fraction of

the master voltage, as illustrated in Figure 38 and Figure 39. The

final TRKx voltage of the slave channel should be set to at least

30 mV below the FB voltage of the master channel. When the

TRKx voltage of the slave channel drops to a level that’s below

the minimum on-time condition, the slave channel operates in

pulse skip mode while keeping the output regulated and tracked

to the master channel. Also, when TRKx or FBx drops below

the PGOOD undervoltage threshold, the PGOOD signal gets

tripped and becomes active low.

TIME

SLAVE VOLTAGE

MASTER VOLTAGE

Figure 38. Ratiometric Tracking

ADP1850

EN2

EN1

FB2

FB1

TRK1

SS1

VCCO

TRK2

SS2

10kΩ

45.3kΩ

RTRKB

10kΩ

RTRKT

49.9kΩ

0.6V

0.55V

3.3V

VOUT1_MASTER

CSS2

20nF

CSS1

37nF

RBOT

10kΩ

RTOP

22.6kΩ

1.8V

VOUT2_SLAVE

0.55V

Figure 39. Example of a Ratiometric Tracking Circuit

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