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LTC2927CDDB

Single Power Supply Tracking Controller

Linear Technology 

LTC2927

APPLICATIO S I FOR ATIO

Ratiometric Tracking Example

1V/DIV

MASTER

SLAVE2

SLAVE1

1V/DIV

10ms/DIV

10ms/DIV

2927 F11

Figure 11. Ratiometric Tracking (from Figure 12)

This example converts the coincident tracking example to

the ratiometric tracking profile shown in Figure 11. 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 and the 2.5V slave 2

supply ramps up at 85V/s. Always verify that the chosen

ramp rate will allow the supplies to ramp-up completely

before RAMPBUF reaches VCC. If the 1.8V supply were

to ramp-up at 50V/s it would only reach 1.65V because

the RAMPBUF signal would reach its final value of VCC =

3.3V 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:

RTB

=

16.5kΩ • 100V /s

60V /s

=

27.4kΩ

From Equation 3:

RTA′

=

1.235V

+

0.8V

1.235V

−

0.8V

≈ 10kΩ

16.5kΩ 35.7kΩ 27.4kΩ

Step 3 is unnecessary because there is no delay, so

RTA = RTA’

EARLY VIN

3.3V

RONB

138k

RONA

100k

RTB1

27.4k

RTA1

10k

VCC

ON

RAMP

LTC2927

SDO

RAMPBUF

FB

TRACK

GND

0.1μF

0.1μF 3.3V

MASTER

3.3V

RUN/SS IN

DC/DC

FB = 1.235V

OUT

RFA1

35.7k

RFB1

16.5k

SLAVE1

1.8V

EARLY

3.3V

RTB2

1M

RTA2

383k

VCC

ON

RAMP

LTC2927

SDO

RAMPBUF

FB

TRACK

GND

2927 F12

0.1μF

3.3V

RUN/SS IN

DC/DC

FB = 0.8V

OUT

RFA2

412k

RFB2

887k

SLAVE2

2.5V

Figure 12. Ratiometric Tracking Example

2927fb

10

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