LTC2923 Linear Technology, LTC2923 Datasheet - Page 7

• Manufacturer Part Number: LTC2923
• Description: Power Supply Tracking Controller
• Manufacturer: Linear Technology

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).
1V/DIV
1V/DIV
Figure 3. Ratiometric Tracking
Figure 1. Coincident Tracking
U
U
1ms/DIV
1ms/DIV
W
2923 F01
2923 F03
U
MASTER
SLAVE1
SLAVE2
MASTER
SLAVE1
SLAVE2
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
1V/DIV
Figure 4. Supply Sequencing
Figure 2. Offset Tracking
1ms/DIV
1ms/DIV
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LTC2923
2923 F02
2923 F04
MASTER
SLAVE1
SLAVE2
SLAVE1
SLAVE2
2923fa
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