LTC2923IDE datasheet(7/20 Pages) LINER

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LTC2923

2923fa

APPLICATIO S I FOR ATIO

Power Supply Tracking and Sequencing

The LTC2923 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 illus-

trated 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 desir-

able to have the 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

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

after the 1.8V supply is completely powered.

Operation

The LTC2923 provides a simple solution to all of the power

supply tracking and sequencing profiles shown in Figures

1 to 4. A single LTC2923 controls up to three supplies with

two “slave” supplies that track a “master” signal. With just

two resistors, a slave supply is configured to ramp up as

a function of the master signal. This master signal can be

a third supply that is ramped up through an external FET,

whose ramp rate is set with a single capacitor, or it can be

a signal generated by tying the GATE and RAMP pins to an

external capacitor.

1V/DIV

1ms/DIV

2923 F01

Figure 1. Coincident Tracking

1V/DIV

1ms/DIV

2923 F02

Figure 2. Offset Tracking

1V/DIV

1ms/DIV

2923 F03

Figure 3. Ratiometric Tracking

1V/DIV

1ms/DIV

2923 F04

Figure 4. Supply Sequencing

MASTER

SLAVE1

SLAVE2

MASTER

SLAVE1

SLAVE2

MASTER

SLAVE1

SLAVE2

SLAVE1

SLAVE2

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