LTC2923IDE datasheet(14/20 Pages) LINER

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LTC2923

2923fa

Offset Tracking Example

APPLICATIO S I FOR ATIO

Figure 15. Offset Tracking Example

Converting the circuit in the coincident tracking example

to the offset tracking shown in Figure 14 is relatively

simple. Here the 1.8V slave 1 supply ramps up 1V below

the master. The ramp rate remains the same (1000V/s), so

there are no changes necessary to steps 1 and 2 of the

3-step design procedure. Only step 3 must be considered.

Be sure to verify that the chosen voltage offsets will allow

the slave supplies to ramp up completely. In this example,

if the voltage offset were 2V, the slave supply would only

ramp up to 3.3V – 2V = 1.3V.

Choose RTA to obtain the desired delay.

First, convert the desired voltage offset, VOS, to a delay,

tD, using the ramp rate:

1000

(6)

From Equation 4:

V s

TA′′ =

Ω

0 8

16 5

1000

13 2

.•

•/

From Equation 5:

RTA = 13.1kΩ||13.2kΩ ≈ 6.65kΩ

1ms/DIV

2923 F14b

1V/DIV

1ms/DIV

2923 F14a

1V/DIV

Figure 14. Offset Tracking (from Figure 15)

SLAVE2

SLAVE1

MASTER

CGATE

10nF

VCC

RONB

138k

3.3V

RTB1

16.5k

RTB2

887k

RFA1

35.7k

1.8V

SLAVE1

3.3V

MASTER

RFB1

16.5k

RTA2

316k

RTA1

6.65k

RONA

100k

FB1

FB2

GATE

LTC2923

GND

2923 F15

RAMP

RFA2

412k

2.5V

SLAVE2

RFB2

887k

RAMPBUF

TRACK1

TRACK2

DC/DC

3.3V

FB = 1.235V OUT

DC/DC

FB = 0.8V

OUT

0.1µF

10Ω

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