Datasheet

LTC2955
13
2955f
OPERATION
As shown in Figure 7, if the ON pin is high when power is
first applied to the LTC2955-1 at time t
1
, the EN pin will
go high (at t
2
) after the one second EN lockout time. At
time t
3
, the pushbutton is pressed to activate turn-off
while the ON pin remains high. The LTC2955-1 will wait
for a rising edge at the ON pin or a low at the PB pin to
activate the next turn-on. As shown at time t
4
, after the
one second lockout time, the EN stays low with ON high.
At time t
5
, only the rising edge of the ON pin or a PB event
will trigger the next turn-on again.
LTC2955-1, LTC2955-2 Versions
The LTC2955-1 and LTC2955-2 differ only by the polarity
of the high voltage (36V absolute maximum) enable pin.
The LTC2955-1 EN pin is a high true output designed to
drive the SHUTDOWN pin of most voltage regulators. In
turn-on mode, this pin is pulled high by a pull-up current of
2µA powered by an internal LDO, which gives a high level
output voltage (V
OH
) of typically 4.3V. In turn-off mode,
this pin is pulled low by an internal N-channel MOSFET. If
a higher V
OH
or higher pull-up current is required, connect
the EN pin to an external source through a pull-up resistor.
The LTC2955-2 EN pin is a low true enable output designed
to drive the gate of an external P-channel MOSFET. In
turn-on mode, the EN pin is pulled low by an internal
N-channel MOSFET. In turn-off mode, this pin is pulled
high to V
IN
through an internal 900k resistor. An external
pull-up resistor can be connected between this pin and
V
IN
to increase the pull-up current.
Figure 7. Power First Applied to LTC2955-1
TURN-ON BY ON
RISING EDGE
TURN-OFF BY
PUSHBUTTON
ON PIN
PB PIN
V
IN
A HIGH AT ON PIN IS IGNORED AFTER
THE FIRST TURN-ON/OFF SEQUENCE
ON RISING EDGE
POWER FIRST APPLIED TO
LTC2955 V
IN
PIN
EN PIN
2955 F07
t
1
t
2
t
3
t
4
t
5
TURN-ON BY ON
HIGH LEVEL
t
EN(LOCKOUT)
t
EN(LOCKOUT)