10

LTC1235

S

APPLICATI

I FOR ATIO

Replacing the Backup Battery with Power On

When changing the backup 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

LTC1235 switches to battery backup or battery saving

mode. In either case, the battery supply current pulls

and the cycle repeats.

If spurious resets during battery replacement pose no

problems, then no action is required. Otherwise, two

methods can be used to eliminate this problem. First, a

replacement by slowing the charge rate. For example, the

battery standby current is 1

µA maximum over tempera-

ture and the external capacitor required to slow the charge

rate is:

3 sec, the value for external capacitor is 2

µF. Second, a

changing the battery. For example, the battery standby

current is 1

µA maximum over temperature and the exter-

battery, this resistor will draw only 0.7

µA from the battery,

which is negligible in most cases.

If the battery connections are made with long wires or PC

traces, inductive spikes can be generated during battery

replacement. Even if a resistor is used to prevent spurious

pin below GND violating the LTC1235 absolute maximum

ratings. A 0.1

mended to eliminate these potential spikes when battery

replacement is made through long wires.

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

size double layer capacitors, can be used for short term

memory backup instead of a battery. For capacitor backup,

see Typical Applications. The charging resistor for re-

charging rechargeable batteries should be connected to

up (Figure 5).

Figure 4. Conditional Battery Backup Operation

+5V

+3V

0.1µF

0.1µF

LTC1235

GND

4

LTC1235 F05

R

I =

R

BACKUP

I/O LINE

µP

RAM

1N4148

LOGIC LOW

BACKUP

RESET VOLTAGE THRESHOLD

Hi-Z

LOGIC

HIGH

RESET VOLTAGE THRESHOLD

BACKUP

LTC1235 F04

CT

1A

V– V

EXT

REQ'D

CC

BATT

≥

µ









R

V

– 50mV

1A

CC

≤

µ