LTC692(Rev0) 查看數據表（PDF） - Linear Technology

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LTC692
(Rev.:Rev0)

Microprocessor Supervisory Circuits

Linear Technology 

LTC692/LTC693

APPLICATI S I FOR ATIO

ANY PNP POWER TRANSISTOR

5V

0.1µF

5

3

BATT ON

VCC

VOUT

2

LTC693

1

VBATT

3V

GND

4

0.1µF

LTC692/3 • F02

Figure 2. Using BATT ON to Drive External PNP Transistor

I

=

VOUT

– VBATT

R

5V

0.1µF

3V

R

VCC

VOUT

LTC692

LTC693

VBATT

GND

0.1µF

LTC692/3 • F03

Figure 3. Charging External Battery Through VOUT

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 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 cur-

rent. 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

battery 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 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 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

battery, which is negligible in most cases.

9

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