Datasheet

LTC694-3.3/LTC695-3.3
10
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APPLICATIONS INFORMATION
The LTC694-3.3/LTC695-3.3 are protected for safe area
operation with short-circuit limit. Output current is limited
to approximately 200mA. If the device is overloaded for
a 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 oscillating 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 LTC694-3.3/LTC695-3.3 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
V
BATT
pin is strictly junction leakage.
A 125 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 1.5V
to 2.75V. The charging resistor for 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 Back-Up Battery
When changing the back-up 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 LTC694-3.3/LTC695-3.3 switch to battery backup.
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 so the external resistor
required to hold V
BATT
below V
CC
is:
R ≤
V
CC
– 50mV
1µA
With V
CC
= 3V, a 2.7M resistor will work. With a 2V battery,
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
3.3V
2.4V
0.1µF
0.1µF
V
BATT
V
CC
LTC695-3.3
V
OUT
GND
4
3
1
2
5
ANY PNP POWER TRANSISTOR
694/5-3.3 F02
BATT ON
Figure 3. Charging External Battery Through V
OUT
3.3V
2.4V
0.1µF
0.1µF
V
BATT
V
CC
LTC694-3.3
LTC695-3.3
V
OUT
GND
694/5-3.3 F03
V
OUT
– V
BATT
R
I =
R