LTC1235 Datasheet, PDF (10/16 Pages) Linear Technology – Microprocessor Supervisory Circuit

LTC1235
APPLICATI S I FOR ATIO
BACKUP
V_CC
V_OUT
V_OUT IN BATTERY SAVING MODE
LOGIC LOW
RESET VOLTAGE THRESHOLD
V_BATT
Hi-Z
BACKUP
V_CC
V_OUT IN BATTERY BACKUP MODE
LOGIC
HIGH
RESET VOLTAGE THRESHOLD
V_BATT
V_OUT
V_OUT = V_BATT
LTC1235 F04
Figure 4. Conditional Battery Backup Operation
The operating voltage at the V_BATT 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
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).
I = ^{VOUT – VBATT – VD}
R
1N4148
R
+5V
0.1µF
V_CC
V_OUT
0.1µF ^RAM
LTC1235
BACKUP
V_BATT
+3V
GND
4
I/O LINE
µP
LTC1235 F05
Figure 5. Charging External Battery Through V_OUT
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 saving
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 tempera-
ture and the external capacitor required to slow the charge
rate is:
C_EXT
≥
T_REQ'D


V_CC
1µA
– 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 exter-
nal resistor required to hold V_BATT below V_CC is:
R ≤ ^{VCC – 50mV}
1µA
With V_CC = 4.5V, a 4.3MΩ resistor will work. With a 3V
battery, 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.
10