LTC1235CS datasheet(10/16 Pages) LINER

10 / 16 page

LTC1235

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

capacitance on the VBATT pin. The oscillation cycle is as

follows: When VBATT reaches within 50mV of VCC, the

LTC1235 switches to battery backup or battery saving

mode. In either case, the battery supply current 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, two

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

capacitor from VBATT to GND will allow time for battery

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:

where TREQ'D is the maximum time required to replace the

backup battery. With VCC = 4.5V, VBATT = 3V and TREQ'D =

3 sec, the value for external capacitor is 2

µF. Second, 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 exter-

nal resistor required to hold VBATT below VCC is:

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

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

resets as described above, these spikes can take the VBATT

pin below GND violating the LTC1235 absolute maximum

ratings. A 0.1

µF capacitor from VBATT to GND is recom-

mended to eliminate these potential spikes when battery

replacement is made through long wires.

Figure 5. Charging External Battery Through VOUT

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 backup instead of a battery. For capacitor backup,

see Typical Applications. The charging resistor for re-

charging rechargeable batteries should be connected to

VOUT through a diode since this eliminates the discharge

path that exists when VCCcollapses and RAM is not backed

up (Figure 5).

Figure 4. Conditional Battery Backup Operation

+5V

+3V

0.1µF

VBATT

VCC

LTC1235

VOUT

GND

LTC1235 F05

VOUT – VBATT – VD

I =

BACKUP

I/O LINE

µP

RAM

1N4148

VOUT IN BATTERY SAVING MODE

LOGIC LOW

BACKUP

VCC

RESET VOLTAGE THRESHOLD

VOUT

Hi-Z

VBATT

VOUT IN BATTERY BACKUP MODE

LOGIC

HIGH

RESET VOLTAGE THRESHOLD

VBATT

BACKUP

VCC

VOUT

VOUT = VBATT

LTC1235 F04

V– V

EXT

REQ'D

BATT

≥









– 50mV

≤

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