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LTC2927CDDB

Single Power Supply Tracking Controller

Linear Technology 

LTC2927

APPLICATIO S I FOR ATIO

Power Supply Tracking and Sequencing

The LTC2927 handles a variety of power-up profiles to

satisfy the requirements of digital logic circuits including

FPGAs, PLDs, DSPs and microprocessors. These require-

ments fall into one of the four general categories illustrated

in Figures 1 to 4.

Some applications require that the potential difference

between two power supplies must never exceed a speci-

fied voltage. This requirement applies during power-up

and power-down as well as during steady-state operation,

often to prevent destructive latch-up in a dual supply

ASIC. Typically, this is achieved by ramping the supplies

up and down together (Figure 1). In other applications it

is desirable to have supplies ramp up and down with fixed

voltage offsets between them (Figure 2) or to have them

ramp up and down ratiometrically (Figure 3).

Certain applications require one supply to come up after

another. For example, a system clock may need to start

before a block of logic. In this case, the supplies are se-

quenced as in Figure 4 where the 2.5V supply ramps up

after the 1.8V supply is completely powered.

Operation

The LTC2927 provides a simple solution to all of the

power supply tracking and sequencing profiles shown in

Figures 1 to 4. A single LTC2927 controls a single sup-

ply that tracks to a “master” signal. With two resistors,

a slave supply is configured to ramp up as a function of

the master signal. This master signal can be a separate

supply or it can be a ramp signal generated by tying the

RAMP pin to an external capacitor.

Tracking Cell

The LTC2927’s operation is based on the tracking cell

shown in Figure 5, which uses a proprietary wide-range

current mirror. The tracking cell shown in Figure 5 servos

the TRACK pin at 0.8V. The current supplied by the TRACK

1V/DIV

MASTER

SLAVE1

SLAVE2

1V/DIV

MASTER

SLAVE1

SLAVE2

10ms/DIV

2927 F01

Figure 1. Coincident Tracking

10ms/DIV

2927 F02

Figure 2. Offset Tracking

1V/DIV

MASTER

SLAVE1

SLAVE2

1V/DIV

MASTER

SLAVE1

SLAVE2

10ms/DIV

2927 F03

Figure 3. Ratiometric Tracking

6

10ms/DIV

2927 F04

Figure 4. Supply Sequencing

2927fb

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