LTC693 LINER [Linear Technology], LTC693 Datasheet - Page 9

• Manufacturer Part Number: LTC693
• Description: Microprocessor Supervisory Circuits
• Manufacturer: LINER [Linear Technology]

The LTC692/LTC693 are protected for safe area operation
with a short circuit limit. Output current is limited to
approximately 200mA. If the device is overloaded for long
periods of time, thermal shutdown turns the power switch
off until the device cools down. The threshhold tempera-
ture 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 LTC692/LTC693 use a charge pumped
NMOS power switch to eliminate unwanted charging
current while achieving low dropout and low supply cur-
rent. Since no current goes to the substrate, the current
collected by the V
A 125 PMOS switch connects the V
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
battery backup in CMOS RAM and other low power CMOS
circuitry. The supply current in battery backup mode is
1 A maximum.
The operating voltage at the V
4.0V. High value capacitors, such as electrolytic or farad-
size double layer capacitors, can be used for short term
A
Figure 2. Using BATT ON to Drive External PNP Transistor
PPLICATI
5V
0.1µF
BATT
3V
O
BATT
ANY PNP POWER TRANSISTOR
pin in competitive devices and adds to
U
3
1
pin is strictly junction leakage.
S
V
V
BATT
CC
BATT ON
LTC693
GND
I FOR ATIO
U
4
5
BATT
V
OUT
pin ranges from 2.0V to
2
BATT
W
LTC692/3 • F02
input to V
0.1µF
U
OUT
in
memory backup instead of a battery. The charging resistor
for the rechargeable batteries should be connected to
V
when the resistor is connected to V
Replacing the Backup Battery
When changing the backup battery with system power
on, spurious resets can occur while the battery is re-
moved 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
oscillation cycle is as follows: When V
50mV of V
backup. 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 and the external resistor
required to hold V
With V
battery, this resistor will draw only 0.77 A from the
battery, which is negligible in most cases.
OUT
R
BATT
since this eliminates the discharge path that exists
BATT
Figure 3. Charging External Battery Through V
CC
V
5V
CC
0.1µF
OUT
= 4.25V, a 3.9M resistor will work. With a 3V
pin again and the cycle repeats.
to GND will hold the pin low while changing the
1 A
– 50mV
CC
pulls V
, the LTC692/LTC693 switch to battery
3V
BATT
BATT
I =
below V
V
V
BATT
CC
V
low and the devices go back to
LTC692
LTC693
OUT
GND
R
LTC692/LTC693
– V
R
V
OUT
BATT
CC
is:
CC
LTC692/3 • F03
BATT
(Figure 3).
reaches within
BATT
0.1µF
OUT
pin. The
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