Datasheet

LTC2955
12
2955f
OPERATION
PB vs ON Pin
With the DFN package, if the turn-on sequence is initi-
ated by the pushbutton, the ON input is ignored until the
pushbutton is pressed again to initiate a turn-off sequence.
A s s h o w n i n F i g u r e 6 , t h e PB pin initiates a turn-on sequence
at time t
1
before the ON pin goes high at time t
2
. Once EN
goes high (t
3
), the ON input is ignored, so a falling edge
at the ON pin (t
4
) will not initiate a turn-off sequence even
if the SEL pin is high. This allows the system to remain
powered up if it is plugged in and out of a secondary power
source while the system is already turned on.
If the turn-on sequence is initiated by the ON pin, both
the pushbutton and the ON pin falling edge can initiate
the turn-off sequence. For the TS8 package, ON pin falling
edge is always ignored regardless of which pins initiate
the turn-on.
The PB pin has priority over the ON pin, so if the PB pin
goes low and ON pin goes high at the same time, the
LTC2955-1 will monitor the PB pin for the 32ms debounce
time and ignore the ON pin unless the PB pin bounces
during the 32ms.
ON Pin Initial Turn-On
If the LTC2955-1 powers up with the ON pin already high,
the EN pin will go high after a power-up delay of about
one second. Subsequently, if the pushbutton is pressed
to force EN low, the LTC2955-1 will not initiate another
turn-on even if the ON pin remains high. The LTC2955-1
needs a rising edge at the ON pin to initiate the subsequent
turn-on.
Figure 6. Pushbutton Is Pressed Before the ON Pin Goes High
TURN-ON BY PB
PB PIN
PGD PIN
ON PIN
ON GOING LOW IS IGNORED,
SINCE THE EN IS ASSERTED
BY THE PUSHBUTTON
ON PIN IS IGNORED, SINCE
PUSHBUTTON IS PRESSED
FIRST
EN PIN
2955 F06
t
1
t
2
t
3
t
4
t
5