LTC695-3.3 LINER [Linear Technology], LTC695-3.3 Datasheet - Page 9

• Manufacturer Part Number: LTC695-3.3
• Description: 3.3V Microprocessor Supervisory Circuits
• Manufacturer: LINER [Linear Technology]

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 threshhold
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 col-
lected by the V
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 charg-
ing current while achieving low dropout and low supply
current. Since no current goes to the substrate, the current
collected by V
A 125 PMOS switch connects the V
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
1 A maximum.
The operating voltage at the V
2.75V. The charging resistor for rechargeable batteries
should be connected to V
discharge path that exists when the resistor is connected
to V
A
PPLICATI
Figure 2. Using BATT ON to Drive External PNP Transistor
CC
(Figure 3).
3.3V
BATT
BATT
O
2.4V
ANY PNP POWER TRANSISTOR
0.1 F
pin is strictly junction leakage.
pin in competitive devices and adds to
U
3
1
S
V
V
BATT
CC
LTC695-3.3
BATT ON
I FOR ATIO
GND
OUT
U
4
BATT
5
V
OUT
since this eliminates the
pin ranges from 1.5V to
2
W
BATT
0.1 F
694/5-3.3 F02
input to V
U
OUT
in
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
cycle is as follows: When V
V
up. V
normal operation. The leakage current then charges up the
V
If spurious resets during battery replacement pose no
problems, then no action is required. Otherwise, a resistor
from V
battery. For example, the battery standby current is 1 A
maximum over temperature so the external resistor re-
quired to hold V
With V
this resistor will draw only 0.7 A from the battery, which
is negligible in most cases.
If battery connections are made through long wires, a 10
to 100 series resistor and a 0.1 F capacitor are recom-
mended to prevent any overshoot beyond V
lead inductance (Figure 4).
CC
BATT
R
, the LTC694-3.3/LTC695-3.3 switch to battery back-
OUT
pin again and the cycle repeats.
CC
Figure 3. Charging External Battery Through V
BATT
V
3.3V
CC
= 3V, a 2.7M resistor will work. With a 2V battery,
pulls V
0.1 F
to GND will hold the pin low while changing the
1 A
– 50mV
LTC694-3.3/LTC695-3.3
2.4V
BATT
BATT
below V
low and the device goes back to
I =
V
V
BATT
LTC694-3.3
LTC695-3.3
CC
V
OUT
GND
BATT
R
CC
– V
R
V
OUT
BATT
BATT
is:
reaches within 50mV of
pin. The oscillation
694/5-3.3 F03
0.1 F
CC
due to the
OUT
9