LTC694CS8 datasheet(9/16 Pages) LINER

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LTC690/LTC691

LTC694/LTC695

APPLICATI

I FOR ATIO

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 VOUT since this eliminates the discharge

path that exists when the resistor is connected to VCC

(Figure 3).

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 VCC pin to the

VOUT pin to supply the extra current.

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 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 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 cur-

rent. Since no current goes to the substrate, the current

collected by VBATT pin is strictly junction leakage.

A 125

Ω 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

µA maximum.

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

4.25V. High value capacitors, such as electrolytic or farad-

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 1

µA

maximum over temperature and the external resistor

required to hold VBATT below VCC is:

With VCC = 4.5V, a 4.3M resistor will work. With a 3V

battery, this resistor will draw only 0.7

µA from the battery,

which is negligible in most cases.

– 50mV

≤

0.1

µF

0.1

µF

VBATT

VCC

LTC691

LTC695

VOUT

GND

ANY PNP POWER TRANSISTOR

690 F02

BATT ON

0.1

µF

0.1

µF

VBATT

VCC

LTC690

LTC691

LTC694

LTC695

VOUT

GND

690 F03

VOUT – VBATT

I =

Figure 3. Charging External Battery Through VOUT

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