LTC2955
14
2955fa
For more information www.linear.com/LTC2955
applicaTions inForMaTion
Adjusting the Forced Turn-Off Timing
The LTC2955 allows the user to force the system power to
turn off if the µP fails to respond during fault conditions. As
shown by the solid lines in Figure 3 and Figure 5, when the
µP fails to bring the KILL pin low after the interrupt signal
is asserted, the user can force a turn-off by holding down
the pushbutton. The length of time that PB must be held
low is given by a fixed 64ms delay (t
DB(OFF)
+ t
INT(MIN)
)
plus an adjustable power-down timer delay (t
TMR
). The
adjustable delay is set by placing an optional external
capacitor on the TMR pin. Use the following equation to
calculate the capacitance for the desired delay. C
TMR
is
the external capacitor at the TMR pin:
C
TMR
= 0.19 • t
TMR
[µF/sec]
As an example, if the required turn-off debounce time is
one second:
t
TMR
= (1000ms – 64ms)
C
TMR
= 0.19 • 0.936
Required C
TMR
is 0.178µF
The ON pin turn-off debounce time is 32ms less than the
PB pin debounce time since there is no t
DB(OFF)
. If the
TMR pin is left open, the turn-off debounce time defaults
to 64ms for the PB pin and 32ms for the ON pin.
Voltage Monitoring with KILL Input
The KILL pin can also be used as a voltage monitor input.
Figure 8 shows an application where the KILL pin has a dual
function. It is driven by a low leakage open drain output
of the µP. It is also connected to a resistive divider that
monitors battery voltage (V
IN
). When the battery voltage
falls below the set value, the voltage at the KILL pin falls
below 0.8V and the EN pin is quickly pulled low. Note that
the resistor values should be as large as possible, but
small enough to keep leakage currents from tripping the
0.8V KILL comparator.
Operation Without µP
If there is no circuitry available to drive the KILL pin,
this pin can be connected to a voltage regulator output
through a resistive divider or RC network as shown in
Figure 9. The KILL pin acts as a voltage monitor pin that
shuts down the regulator when its output voltage drops
below a certain level.
The minimum pulse width needed to trigger KILL is 30µs.
If there are glitches on the resistor pull-up voltage that
are wider than 30µs and transition below 0.8V, then an
appropriate bypass capacitor should be connected to the
KILL pin.
Figure 8. Input Voltage Monitoring with KILL Input
Figure 9. Application without µP
*OPTIONAL
PB
INT
KILL
V
IN
LTC2955TS8-1
GND TMR
C
TMR*
1µF
ON
EN
C4
0.1µF
V
IN
= 9V
2955 F09
R1
10k
V
OUT
V
IN
V
OUT
LT3060
SHDN
C3*
0.01µF
+
*OPTIONAL
PB
INT
KILL
V
IN
LTC2955TS8-1
GND TMR
C
TMR*
1µF
ON
EN
C4
0.1µF
R3
583k
1%
V
IN
= 9V
R2
100k
1%
2955 F08
µP
R1
10k
KILL
(OPEN DRAIN)
INT
V
OUT
V
IN
V
OUT
LT3060
SHDN
