LTC1235CSW datasheet(10/16 Pages) LINER

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LTC1235

1235fa

APPLICATIONS INFORMATION

Figure 4. Conditional Battery Backup Operation

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

faradsize 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 recharging rechargeable batteries should be con-

nected to VOUT through a diode since this eliminates the

discharge path that exists when VCC collapses and RAM

is not backed up (Figure 5).

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 sav-

ing 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 1μA maximum over tem-

perature and the external capacitor required to slow the

charge rate is:

CEXT TREQ'D

1μA

VCC ±VBATT

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 2μF. Second,

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 ≤

CC ± 50mV

1μA

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

tery, this resistor will draw only 0.7μA 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 0.1μF capacitor from VBATT to GND is recom-

mended to eliminate these potential spikes when battery

replacement is made through long wires.

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

1235 F04

+5V

+3V

0.1μF

VBATT

VCC

LTC1235

VOUT

GND

1235 F05

VOUT – VBATT –VD

I =

BACKUP

I/O LINE

μP

RAM

1N4148

LTC1235CSW Datasheet(PDF) 10 Page - Linear Technology

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