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LTC690

Microprocessor Supervisory Circuits

Linear Technology 

LTC690/LTC691

LTC694/LTC695

APPLICATIONS INFORMATION

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 VCC pin to the

VOUT pin to supply the extra current.

ANY PNP POWER TRANSISTOR

5V

0.1μF

3V

5

3

BATT ON

VCC

VOUT

2

LTC691

1 LTC695

VBATT

GND

4

0.1μF

690 F02

Figure 2. Using BATT ON to Drive External PNP Transistor

The LTC690 family is protected for safe area operation

with short-circuit limit. Output current is limited to

approximately 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

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 LTC690 family uses 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

VBATT pin is strictly junction leakage.

A 125Ω PMOS switch connects the VBATT input to VOUT

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 VBATT pin ranges from 2.0V

to 4.25V. High value capacitors, such as electrolytic or

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

term memory back-up instead of a battery. The charging

resistor for both capacitors and rechargeable batteries

should be connected to VOUT since this eliminates the

discharge path that exists when the resistor is connected

to VCC (Figure 3).

I

=

VOUT

– VBATT

R

5V

0.1μF

3V

R

VCC

VOUT

LTC690

LTC691

LTC694

LTC695

VBATT

GND

0.1μF

690 F03

Figure 3. Charging External Battery Through VOUT

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 VBATT pin. The oscillation

cycle is as follows: When VBATT reaches within 50mV of

VCC, the LTC690 switches to battery back-up. VOUT pulls

VBATT low and the device goes 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 = 4.5V, a 4.3M resistor will work. With a 3V

battery, this resistor will draw only 0.7μA from the battery,

which is negligible in most cases.

690fe

10

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