Datasheet

LTC2950-1/LTC2950-2
12
295012fd
The following equations describe the turn on and turn
off times. C
ONT
and C
OFFT
are the external programming
capacitors:
t
BD,ON
+ t
ONT
= 32ms + 1ms + (6.7x10
6
) • C
ONT
t
BD,OFF
+ t
OFFT
= 32ms + 1ms + (6.7x10
6
) • C
OFFT
High Voltage Pins
The V
IN
and PB pins can operate at voltages up to 26.4V.
PB can, additionally, operate below ground (–6V) without
latching up the device. PB has an ESD HBM rating of ±10kV.
If the pushbutton switch connected to PB exhibits high
leakage current, then an external pull-up resistor to V
IN
is
recommended. Furthermore, if the pushbutton switch is
physically located far from the LTC2950 PB pin, parasitic
capacitances may couple onto the high impedance PB
input. Additionally, parasitic series inductance may cause
unpredictable ringing at the PB pin. Placing a 5k resistor
from the PB pin to the pushbutton switch would mitigate
parasitic inductance problems. Placing a 0.1µF capacitor
on the PB pin would lessen the impact of parasitic capaci-
tive coupling.
Voltage Monitoring with KILL Input
The KILL pin can be used as a voltage monitor. Figure 5
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 resistor divider that monitors battery
voltage (V
IN
). When the battery voltage falls below the set
value, the voltage at the KILL pin falls below 0.6V 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 0.6V KILL comparator.
The DC/DC converter shown has an internal pull-up current
on its SHDN pin. A pull-up resistor on EN is thus not needed.
Operation Without µP
Figure 6 shows how to connect the KILL pin when there
is no circuitry available to drive it. The minimum pulse
width detected is 30µs. If there are glitches on the resistor
pull-up voltage that are wider than 30µs and transition
Figure 5. Input Voltage Monitoring with KILL Input Figure 6. No µP Application
PB
V
IN
SHDN**
V
IN
V
OUT
EN
INT
KILL
INT
KILL
(OPEN DRAIN)
LTC2950-1
GND ONT OFFT
LT1767-3.3
µP
9V
R1
10k
3.3V
2950 F05
C4
0.1µF
R3
806k
1%
R2
100k
1%
5.4V THRESHOLD
V
IN
C
ONT
*
0.033µF
C
OFFT
*
0.033µF
*OPTIONAL
**SHDN INTERNALLY PULLED UP BY THE LT1767-3.3
V
IN
SHDN**
V
IN
V
OUT
LT1767-3.3
9V
3.3V
R1
100k
C3*
0.01µF
2950 F06
+
C4
0.1µF
PB
EN
INT
KILL
LTC2950-1
GND ONT OFFT
V
IN
C
ONT
*
0.033µF
C
OFFT
*
0.033µF
*OPTIONAL
**SHDN INTERNALLY PULLED UP BY THE LT1767-3.3
applicaTions inForMaTion
Typical applicaTions