LTC692/LTC693
10
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The LTC692/LTC693 are protected for safe area operation
with a short-circuit limit. Output current is limited to ap-
proximately 200mA. If the device is overloaded for long
periods of time, thermal shutdown turns the power switch
off until the device cools down. The threshold temperature
for thermal shutdown is approximately 155°C with about
10°C of hysteresis which prevents the device from oscil-
lating in and out of shutdown.
The PNP switch used in competitive devices was not chosen
for the internal power switch because it injects unwanted
current into the substrate. This current is collected by the
V
BATT
pin in competitive devices and adds to the charging
current of the battery which can damage lithium batteries.
The LTC692/LTC693 use a charge-pumped NMOS power
switch to eliminate unwanted charging current while
achieving low dropout and low supply current. Since no
current goes to the substrate, the current collected by the
V
BATT
pin is strictly junction leakage.
A 125Ω PMOS switch connects the V
BATT
input to V
OUT
in
battery backup mode. The switch is designed for very low
dropout voltage (input-to-output differential). This feature
is advantageous for low current applications such as bat-
tery backup in CMOS RAM and other low power CMOS
circuitry. The supply current in battery backup mode is
1μA maximum.
The operating voltage at the V
BATT
pin ranges from 2.0V to
4.0V. High value capacitors, such as electrolytic or farad-
size double layer capacitors, can be used for short-term
memory backup instead of a battery. The charging resistor
for the rechargeable batteries should be connected to V
OUT
since this eliminates the discharge path that exists when
the resistor is connected to V
CC
(Figure 3).
Replacing the Backup Battery
When changing the backup battery with system power on,
spurious resets can occur while the battery is removed
due to battery standby current. Although battery standby
current is only a tiny leakage current, it can still charge up
the stray capacitance on the V
BATT
pin. The oscillation cycle
is as follows: When V
BATT
reaches within 50mV of V
CC
,
the LTC692/LTC693 switch to battery backup. V
OUT
pulls
V
BATT
low and the devices go back to normal operation.
The leakage current then charges up the V
BATT
pin again
and the cycle repeats.
If spurious resets during battery replacement pose no
problems, then no action is required. Otherwise, a resistor
from V
BATT
to GND will hold the pin low while changing
the battery. For example, the battery standby current is
1μA maximum over temperature and the external resistor
required to hold V
BATT
below V
CC
is:
R
V – 50mV
1A
CC
≤
μ
With V
CC
= 4.25V, a 3.9M resistor will work. With a 3V
battery, this resistor will draw only 0.77μA from the bat-
tery, which is negligible in most cases.
APPLICATIONS INFORMATION
5V
3V
0.1μF
0.1μF
V
BATT
V
CC
LTC693
V
OUT
GND
4
3
1
2
5
ANY PNP POWER TRANSISTOR
692_3 • F02
BATT ON
5V
3V
0.1μF
0.1μF
692_3 • F03
V
OUT
– V
BATT
R
I =
R
V
BATT
V
CC
LTC692
LTC693
V
OUT
GND
Figure 2. Using BATT ON to Drive External PNP Transistor Figure 3. Charging External Battery Through V
OUT
