LTC2925

7

2925f

APPLICATIO S I FOR ATIO

Power Supply Tracking and Sequencing

The LTC2925 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 IC.

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 se-

quenced as in Figure 4 where the 1.8V supply ramps up

completely followed by the 2.5V supply and then the 1.5V

supply.

Operation

The LTC2925 provides a simple solution to all of the power

supply tracking and sequencing profiles shown in Figures

1 to 4. A single LTC2925 controls up to four supplies with

three “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 fourth 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.

SLAVE1

SLAVE2

SLAVE3

10ms/DIV

2925 F02

10ms/DIV

2925 F01

1V/DIV

Figure 1. Coincident Tracking

Figure 3. Ratiometric Tracking

Figure 2. Offset Tracking

Figure 4. Supply Sequencing

SLAVE1

SLAVE2

SLAVE3

10ms/DIV

2925 F04

10ms/DIV

2925 F03

2V/DIV

1V/DIV

2V/DIV

MASTER

MASTER

SLAVE1

SLAVE2

SLAVE3

MASTER

SLAVE2

SLAVE1

SLAVE3