LTC2925

14

2925f

Ratiometric Tracking Example

This example converts the coincident tracking example to

the ratiometric tracking profile shown in Figure 13, using

three supplies without an external FET. The ramp rate of

the master signal remains unchanged (Step 1) and there

is no delay in ratiometric tracking (Step 3), so only the

result of step 2 in the 3-step design procedure needs to be

considered. In this example, the ramp rate of the 1.8V

slave 1 supply ramps up at 60V/s, the 2.5V slave 2 supply

ramps up at 85V/s, and the 1.5V slave 3 supply ramps up

at 50V/s. Always verify that the chosen ramp rate will allow

the supplies to ramp-up completely before RAMPBUF

it would only reach 1.65V because the RAMPBUF signal

supply reached 1.8V.

2.

Solve for the pair of resistors that provide the desired

slave supply behavior, assuming no delay.

From Equation 2:

Rk

Vs

Vs

k

≈

16 5

100

60

27 4

..

APPLICATIO S I FOR ATIO

Figure 13. Ratiometric Tracking from Figure 14

SLAVE2

SLAVE1

SLAVE3

0.1

µF

0.82

µF

10k

3.3V

3.3V

154k

27.4k

100k

41.2k

2.5V

SLAVE2

88.7k

38.3k

174k

63.4k

10k

100k

RAMPBUF

TRACK1

TRACK2

TRACK3

FB2

LTC2925

PGTMR

0.082

µF

SDTMR

0.41

µF

SCTMR

GND

2925 F14

100k

1.5V

SLAVE3

86.6k

DC/DC

IN

FB = 0.8V

OUT

DC/DC

IN

FB = 0.8V

RUN/SS

RUN/SS

OUT

FB3

3.3V

PGI

35.7k

1.8V

SLAVE1

16.5k

SUPPLY

MONITOR

DC/DC

IN

FB = 1.235V

RUN/SS

OUT

FB1

FAULT

ON

10k

STATUS

REMOTE

RST

0.1

µF

GATE

RAMP

SD3

SD2

SD1

Figure 14. Ratiometric Tracking Example

10ms/DIV

2925 F13b

10ms/DIV

2925 F13a

1V/DIV

1V/DIV

From Equation 3:

R

V

V

k

V

k

V

k

k

+

≈

08

1 235

16 5

1 235

35 7

08

27 5

10

.

.

.

.

.

–

.

.

Step 3 is unnecessary because there is no delay, so