LTC690/LTC691
LTC694/LTC695
10
690ff
For more information www.linear.com/690
applicaTions inForMaTion
These products provide BATT ON output to drive the base
of external PNP transistor (Figure 2). If higher currents
are needed with the LTC690 and LTC694, a high current
Schottky diode can be connected from the V
CC
pin to the
V
OUT
pin to supply the extra current.
5V
3V
0.1µF
0.1µF
V
BATT
V
CC
LTC691
LTC695
V
OUT
GND
4
3
1
2
5
ANY PNP POWER TRANSISTOR
690 F02
BATT ON
The LTC690 family is protected for safe area operation
with short-circuit limit. Output current is limited to ap-
proximately 200mA. If the device is overloaded for long
period 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 LTC690 family uses 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
V
BATT
pin is strictly junction leakage.
A 125Ω PMOS switch connects the V
BATT
input to V
OUT
in battery back-up mode. The switch is designed for very
low dropout voltage (input-to-output differential). This
feature is advantageous for low current applications such
as battery back-up
in CMOS RAM and other low power
CMOS
circuitry. The supply current in battery back-up
mode is 1µA maximum.
The operating voltage at the V
BATT
pin ranges from 2.0V
to 4.25V. High value capacitors, such as electrolytic or
farad-size double layer capacitors, can be used for short
term memory back-up instead of a battery. The charging
resistor for both capacitors and 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).
5V
3V
0.1µF
0.1µF
V
BATT
V
CC
LTC690
LTC691
LTC694
LTC695
V
OUT
GND
690 F03
V
OUT
– V
BATT
R
I =
R
Replacing the Back-Up Battery
When changing the back-up battery with system power
on, spurious resets can occur while 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 LTC690 switches to battery back-up. V
OUT
pulls
V
BATT
low and the device goes 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 ≤
CC
1µA
With V
CC
= 4.5V, a 4.3M resistor will work. With a 3V bat-
tery, this resistor will draw only 0.7µA from the battery,
which is negligible in most cases.
Figure 2. Using BATT ON to Drive External PNP Transistor
Figure 3. Charging External Battery Through V
OUT
