LTC2925

13

2925f

APPLICATIO S I FOR ATIO

Coincident Tracking Example

A typical four supply application is shown in Figure 12. The

master signal is a 3.3V module. The slave 1 supply is a

1.8V switching power supply, the slave 2 supply is a 2.5V

switching power supply, and the slave 3 supply is a 1.5V

supply. All three slave supplies track coincidently with the

3.3V supply that is controlled with an external FET. The

ramp rate of the supplies is 100V/s. The 3-step design

procedure detailed previously can be used to determine

component values. Only the slave 1 supply is considered

here as the procedure is the same for the other supplies.

1.

Set the ramp rate of the master signal.

From Equation 1:

C

A

Vs

F

µ

10

100

01

µ

.

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

100

16 5

.•

.

From Equation 3:

R

V

V

k

V

k

V

k

k

+

≈

08

1 235

16 5

1 235

35 7

08

16 5

13

.

.

.

.

.

–

.

.

3.

In this example, all supplies remain low while the ON pin

is held below 1.23V. When the ON pin rises above 1.23V,

10

the gate of the FET rises, the source follows and pulls up

the output to 3.3V at 100V/s. This output serves as the

master signal and is buffered from the RAMP pin to the

RAMPBUF pin. As this output and the RAMPBUF pin rise,

the current from the TRACKx pins is reduced. Conse-

quently, the voltages at the slave supplies’ outputs in-

crease, and the slave supplies track the master supply.

When the ON pin is again pulled below 1.23V, 10

µA will

loads on the outputs are sufficient, all outputs will track

down coincidently at 100V/s.

MASTER

SLAVE2

SLAVE1

SLAVE3

0.015

Ω

Q1

Si4412ADY

0.1

µF

0.82

µF

10k

3.3V

3.3V

154k

16.5k

88.7k

41.2k

2.5V

SLAVE2

MASTER

88.7k

41.2k

86.6k

100k

13k

100k

RAMPBUF

TRACK1

TRACK2

TRACK3

FB2

GATE

LTC2925

PGTMR

0.082

µF

SDTMR

0.47

µF

SCTMR

GND

2925 F12

RAMP

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

SD3

SD2

SD1

0.1

µF

10

Ω

Figure 12. Coincident Tracking Example

10ms/DIV

2925 F11b

10ms/DIV

2925 F11a

1V/DIV

1V/DIV

Figure 11. Coincident Tracking from Figure 12