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LTC692CS8PBF

Microprocessor Supervisory Circuits

Linear Technology 

LTC692/LTC693

APPLICATIONS INFORMATION

ANY PNP POWER TRANSISTOR

5V

0.1μF

5

3

BATT ON

VCC

VOUT

2

LTC693

1

VBATT

3V

GND

4

692_3 • F02

0.1μF

Figure 2. Using BATT ON to Drive External PNP Transisitor

I

=

VOUT

– VBATT

R

5V

0.1μF

3V

R

VCC

VOUT

LTC692

LTC693

VBATT

GND

0.1μF

692_3 • F03

Figure 3. Charging External Battery Through VOUT

The LTC692/LTC693 are protected for safe area opera-

tion 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

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

VBATT pin in competitive devices and adds to 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 current. Since no

current goes to the substrate, the current collected by the

VBATT pin is strictly junction leakage.

A 125Ω PMOS switch connects the VBATT input to VOUT in

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 bat-

tery backup in CMOS RAM and other low power CMOS

circuitry. The supply current in battery backup mode is

1μA maximum.

The operating voltage at the VBATT pin ranges from 2.0V to

4.0V. High value capacitors, such as electrolytic or farad-

size double layer capacitors, can be used for short-term

memory backup instead of a battery. The charging resistor

for the rechargeable batteries should be connected to VOUT

since this eliminates the discharge path that exists when

the resistor is connected to VCC (Figure 3).

Replacing the Backup Battery

When changing the backup 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 VBATT pin. The oscillation cycle

is as follows: When VBATT reaches within 50mV of VCC,

the LTC692/LTC693 switch to battery backup. VOUT pulls

VBATT low and the devices go back to normal operation.

The leakage current then charges up the VBATT pin again

and the cycle repeats.

If spurious resets during battery replacement pose no

problems, then no action is required. Otherwise, a resistor

from VBATT 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 VBATT below VCC is:

R

≤

VCC

– 50mV

1μA

With VCC = 4.25V, a 3.9M resistor will work. With a 3V

battery, this resistor will draw only 0.77μA from the bat-

tery, which is negligible in most cases.

0692fa

10

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