LTC2924 Linear Technology, LTC2924 Datasheet - Page 8

• Manufacturer Part Number: LTC2924
• Description: Quad Power Supply Sequencer
• Manufacturer: Linear Technology

LTC2924
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 C
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  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
8
U
PGT
capacitor.
by an external resistor nominally pulled to ground. The
hysteresis current is 0.5V/R
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  D  O  N  E pin.
DONE
OUT4
OUT3
OUT2
OUT1
TMR
IN4
IN3
IN2
IN1
ON
Figure 3. 4-Power Supply Power Off Sequence
0.61V
0.61V
0.61V
0.61V
HYS
.
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0.61V
2924fa
2924 F03