LTC1235CS Linear Technology, LTC1235CS Datasheet - Page 10

• Manufacturer Part Number: LTC1235CS
• Manufacturer: Linear Technology

Specifications of LTC1235CS

• Number Of Elements: 1
• Monitored Voltage 1 (typ): 4.65V
• Battery Backup Switching: Yes
• Watchdog Timer: Yes
• Chip Enable Signals: Yes
• Reset Active Time: 280ms
• Manual Reset: Yes
• Package Type: SOL
• Operating Supply Voltage (min): 4.75V
• Operating Supply Voltage (max): 5.5V
• Reset Threshold Voltage (max): 4.75V
• Reset Threshold Voltage (min): 4.5V
• Power Dissipation: 500mW
• Operating Temp Range: 0C to 70C
• Operating Temperature Classification: Commercial
• Power Fail Detection: Yes
• Mounting: Surface Mount
• Pin Count: 16
• Supply Current: 1.5mA
• Lead Free Status / Rohs Status: Not Compliant

APPLICATIONS INFORMATION
LTC1235
The operating voltage at the V
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
discharge path that exists when V
is not backed up (Figure 5).
10
BACKUP
BACKUP
V
V
V
V
OUT
OUT
CC
CC
Figure 5. Charging External Battery Through V
+5V
Figure 4. Conditional Battery Backup Operation
0.1μF
OUT
+3V
I =
through a diode since this eliminates the
V
OUT
LOGIC
HIGH
LOGIC LOW
V
V
OUT
OUT
– V
V
V
V
V
BATT
CC
BATT
BATT
R
R
BATT
LTC1235
IN BATTERY BACKUP MODE
IN BATTERY SAVING MODE
GND
BACKUP
– V
4
V
RESET VOLTAGE THRESHOLD
RESET VOLTAGE THRESHOLD
D
OUT
BATT
Hi-Z
1N4148
CC
pin ranges from 2.0V
collapses and RAM
0.1μF
V
OUT
I/O LINE
= V
RAM
μP
BATT
1235 F05
OUT
1235 F04
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
is as follows: When V
the LTC1235 switches to battery backup or battery sav-
ing mode. In either case, the battery supply current pulls
V
The leakage current then charges up the V
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
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:
where T
backup battery. With V
= 3 sec, the value for external capacitor is 2μF. Second,
a resistor from V
changing the battery. For example, the battery standby
current is 1μA maximum over temperature and the external
resistor required to hold V
With V
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
pin below GND violating the LTC1235 absolute maximum
ratings. A 0.1μF capacitor from V
mended to eliminate these potential spikes when battery
replacement is made through long wires.
BATT
C
R ≤
EXT
low and the device goes back to normal operation.
CC
V
REQ’D
CC
= 4.5V, a 4.3M resistor will work. With a 3V bat-
T
1μA
REQ'D
± 50mV
is the maximum time required to replace the
BATT
BATT
V
CC
BATT
to GND will allow time for battery
to GND will hold the pin low while
1μA
± V
CC
BATT
= 4.5V, V
BATT
BATT
reaches within 50mV of V
below V
pin. The oscillation cycle
BATT
BATT
to GND is recom-
CC
= 3V and T
BATT
is:
pin again
REQ’D
BATT
1235fa
CC
,