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Fabricante

LTC2924CGN

Quad Power Supply Sequencer

Linear Technology 

LTC2924

Operation

logic, which starts the charge pump and enables OUT1

to turn on the first power supply. The power good timer

circuit is also enabled by turning off the switch that is

shorting the external capacitor to ground and enabling a

5µA current source to charge the CPGT capacitor.

The output circuit responds by opening a switch, which

is shorting the OUT1 pin to ground and enabling a 10µA

current source, which is connected to the charge pump.

The OUT1 pin can be connected to either the shutdown pin

of a power supply or the gate of a N-channel MOSFET that

is in series with the output of the sequenced power supply.

As the power supply turns on, the resistive divider con­

nected to the IN1 pin starts to drive up the voltage at the

IN1 pin. When the voltage at this pin exceeds 0.61V, the

comparator signals the logic that the first power supply

is on. At this time a current is sourced out of the IN1

pin which serves as the hysteresis current for the input

comparator. This allows the application to choose a lower

Power Off voltage sense during the Power Off sequence.

The power good timer (PGT) circuit is signaled and resets

the PGT capacitor. The timer circuit is enabled and the

cycle repeats until the last power supply has turned on.

When the last power supply has turned on, the DONE pin

pull-down switch is turned on to signal that the Power On

sequence has completed.

If a power supply fails to turn on after it is enabled and the

voltage at the PGT pin exceeds 1V, the LTC2924 will disable

all power supplies by pulling all OUT pins to ground. A fault

condition will be indicated by the FAULT pin pulling low.

The hysteresis current sourced at the ON pin and each IN

pin is set at the HYS/CFG pin. The current is determined

by an external resistor nominally pulled to ground. The

hysteresis current is 0.5V/RHYS.

The Power Off sequence is initiated by pulling the ON pin

below 0.61V after a Power On sequence has completed

(see Figure 3). The Power Off sequence turns off the power

supplies in the reverse order of the Power On sequence.

OUT4 is turned off first. The timer function is used between

each supply being sequenced down. The PGT is not used.

The end of the Power Off sequence is indicated by the

LTC2924 floating the DONE pin.

ON

0.61V

TMR

OUT4

IN4

0.61V

OUT3

IN3

0.61V

OUT2

IN2

0.61V

OUT1

IN1

0.61V

DONE

2924 F03

Figure 3. 4-Power Supply Power Off Sequence

For more information www.linear.com/LTC2924

2924fc

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