LTC2925
9
2925fd
For more information www.linear.com/LTC2925
applicaTions inForMaTion
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 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 IC.
T
ypically, 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 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.
1V/DIV
MASTER
SLAVE1
SLAVE2
SLAVE3
10ms/DIV
2925 F01
1V/DIV
MASTER
SLAVE1
SLAVE2
SLAVE3
10ms/DIV
2925 F02
2V/DIV
MASTER
SLAVE1
SLAVE2
SLAVE3
10ms/DIV
2925 F03
2V/DIV
SLAVE1
SLAVE2
SLAVE3
10ms/DIV
2925 F04
Figure 1. Coincident Tracking Figure 2. Offset Tracking
Figure 3. Ratiometric Tracking Figure 4. Supply Sequencing
