Datasheet

LTC1235
10
1235fa
APPLICATIONS INFORMATION
Figure 4. Conditional Battery Backup Operation
Figure 5. Charging External Battery Through V
OUT
The operating voltage at the V
BATT
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 V
OUT
through a diode since this eliminates the
discharge path that exists when V
CC
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 V
BATT
pin. The oscillation cycle
is as follows: When V
BATT
reaches within 50mV of V
CC
,
the LTC1235 switches to battery backup or battery sav-
ing mode. In either case, the battery supply current pulls
V
BATT
low and the device goes back to normal operation.
The leakage current then charges up the V
BATT
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 V
BATT
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:
C
EXT
 T
REQ'D
1μA
V
CC
±V
BATT






where T
REQ’D
is the maximum time required to replace the
backup battery. With V
CC
= 4.5V, V
BATT
= 3V and T
REQ’D
= 3 sec, the value for external capacitor is 2μF. Second,
a resistor from V
BATT
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 V
BATT
below V
CC
is:
R≤
V
CC
± 50mV
1μA
With V
CC
= 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 V
BATT
pin below GND violating the LTC1235 absolute maximum
ratings. A 0.1μF capacitor from V
BATT
to GND is recom-
mended to eliminate these potential spikes when battery
replacement is made through long wires.
V
OUT
IN BATTERY SAVING MODE
LOGIC LOW
BACKUP
V
CC
RESET VOLTAGE THRESHOLD
V
OUT
Hi-Z
V
BATT
V
OUT
IN BATTERY BACKUP MODE
LOGIC
HIGH
RESET VOLTAGE THRESHOLD
V
BATT
BACKUP
V
CC
V
OUT
V
OUT
= V
BATT
1235 F04
+5V
+3V
0.1μF
0.1μF
V
BATT
V
CC
LTC1235
V
OUT
GND
4
1235 F05
V
OUT
– V
BATT
–V
D
R
I =
R
BACKUP
I/O LINE
μP
RAM
1N4148