LTC2923

13

2923fa

APPLICATIO S I FOR ATIO

Ratiometric Tracking Example

Figure 13. Ratiometric Tracking Example

This example converts the coincident tracking example to

the ratiometric tracking profile shown in Figure 12, using

two 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 consid-

ered. In this example, the ramp rate of the 1.8V slave 1

supply ramps up at 600V/s and the 2.5V slave 2 supply

ramps up at 850V/s. Always verify that the chosen ramp

rate will allow the supplies to ramp-up completely before

up at 500V/s it would only reach 1.65V because the

before the slave 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

Ω≈

Ω

16 5

1000

600

27 4

.•

/

/

.

From Equation 3:

R

V

V

k

V

k

V

k

k

Ω

+

ΩΩ

=

Ω

08

1 235

16 5

1 235

35 7

08

27 5

10

.

.

.

.

.

–

.

.

Step 3 is unnecessary because there is no delay, so

1ms/DIV

2923 F12b

1V/DIV

1ms/DIV

2923 F12a

1V/DIV

Figure 12. Ratiometric Tracking (from Figure 13)

SLAVE2

SLAVE1

10nF

0.1µF

138k

3.3V

27.4k

1M

35.7k

1.8V

SLAVE1

16.5k

383k

10k

100k

ON

FB1

GATE

LTC2923

GND

2923 F13

RAMP

412k

2.5V

SLAVE2

887k

RAMPBUF

TRACK1

TRACK2

FB2

DC/DC

IN

3.3V

3.3V

FB = 1.235V OUT

DC/DC

IN

FB = 0.8V

OUT