LTC2924IGN datasheet(8/20 Pages) LINER

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LTC2924

2924fa

OPERATIO

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.AtthistimeacurrentissourcedoutoftheIN1pinwhich

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.Thetimercircuitisenabledandthecyclerepeats

until the last power supply has turned on.

When the last power supply has turned on, the DONE 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 FAULT

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.

OUT4isturnedoffﬁrst.Thetimerfunctionisusedbetween

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

The end of the Power Off sequence is indicated by the

LTC2924 ﬂoating the DONE pin.

Figure 3. 4-Power Supply Power Off Sequence

TMR

OUT4

0.61V

OUT3

OUT2

OUT1

IN4

IN3

IN2

IN1

DONE

0.61V

2924 F03

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