ltc690cs8-trpbf Linear Technology Corporation, ltc690cs8-trpbf Datasheet - Page 9

• Manufacturer Part Number: ltc690cs8-trpbf
• Description: Microprocessor Supervisory Circuits
• Manufacturer: Linear Technology Corporation

A
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 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 LTC690 family uses 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 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
4.25V. High value capacitors, such as electrolytic or farad-
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
V
OUT
PPLICATI
Figure 2. Using BATT ON to Drive External PNP Transistor
pin to supply the extra current.
5V
0.1 F
BATT
BATT
3V
O
ANY PNP POWER TRANSISTOR
pin is strictly junction leakage.
pin in competitive devices and adds to
U
3
1
S
V
V
BATT
CC
BATT ON
I FOR ATIO
LTC691
LTC695
U
GND
4
5
BATT
V
OUT
pin ranges from 2.0V to
2
W
BATT
0.1 F
690 F02
input to V
CC
pin to the
U
OUT
in
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
cycle is as follows: When V
V
V
The leakage current then charges up the V
and the cycle repeats.
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 0.7 A from the battery,
which is negligible in most cases.
size double layer capacitors, can be used for short term
memory back-up instead of a battery. The charging resis-
tor for both capacitors and rechargeable batteries should
be connected to V
path that exists when the resistor is connected to V
(Figure 3).
CC
BATT
R
, the LTC690 switches to battery back-up. V
low and the device goes back to normal operation.
Figure 3. Charging External Battery Through V
BATT
CC
V
CC
5V
= 4.5V, a 4.3M resistor will work. With a 3V
0.1 F
to GND will hold the pin low while changing the
1 A
– 50mV
3V
BATT
OUT
since this eliminates the discharge
I =
below V
V
V
CC
BATT
V
OUT
LTC690
LTC691
LTC694
LTC695
GND
R
BATT
LTC694/LTC695
LTC690/LTC691
– V
R
V
OUT
BATT
CC
BATT
reaches within 50mV of
is:
pin. The oscillation
0.1 F
690 F03
BATT
OUT
pin again
OUT
pulls
9
CC