LTC694IN8-3.3-PBF Ver la hoja de datos (PDF) - Linear Technology

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LTC694IN8-3.3-PBF

3.3V Microprocessor Supervisory Circuits

Linear Technology 

LTC694-3.3/LTC695-3.3

APPLICATIONS INFORMATION

ANY PNP POWER TRANSISTOR

3.3V

5

3

BATT ON

VCC

VOUT

2

0.1μF

LTC695-3.3

1

VBATT

2.4V

GND

4

0.1μF

694/5-3.3 F02

Figure 2. Using BATT ON to Drive External PNP Transistor

I

=

VOUT

– VBATT

R

3.3V

0.1μF

2.4V

R

VCC

VOUT

LTC694-3.3

LTC695-3.3

VBATT

GND

0.1μF

694/5-3.3 F03

Figure 3. Charging External Battery Through VOUT

The LTC694-3.3/LTC695-3.3 are protected for safe area

operation with short-circuit limit. Output current is limited

to approximately 200mA. If the device is overloaded for

a long period of time, thermal shutdown turns the power

switch off until the device cools down. The threshold

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 LTC694-3.3/LTC695-3.3 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

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 1.5V

to 2.75V. The charging resistor for 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 Back-Up Battery

When changing the back-up 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 LTC694-3.3/LTC695-3.3 switch to battery backup.

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 so the external resistor

required to hold VBATT below VCC is:

R

≤

VCC

– 50mV

1µA

With VCC = 3V, a 2.7M resistor will work. With a 2V battery,

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

is negligible in most cases.

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