LTC1235
Microprocessor
Supervisory Circuit
FEATURES
DESCRIPTION
TheLTC®1235providescompletepowersupplymonitoring
and battery control functions for microprocessor reset,
batterybackup,RAMwriteprotection,powerfailurewarn-
ingandwatchdogtiming. TheLTC1235hasalltheLTC695
featuresplusconditionalbatterybackupandexternalreset
control.Whenanout-of-tolerancepowersupplycondition
occurs, the reset outputs are forced to active states and
the Chip Enable output write-protects external memory.
The RESET output is guaranteed to remain logic low with
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Guaranteed Reset Assertion at V = 1V
CC
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1.5mA Maximum Supply Current
Fast (35ns Max.) Onboard Gating of RAM Chip
Enable Signals
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Conditional Battery Backup Extends Battery Life
4.65V Precision Voltage Monitor
Power OK/Reset Time Delay: 200ms
External Reset Control
Minimum External Component Count
1μA Maximum Standby Current
Voltage Monitor for Power Fail or Low Battery
Warning
V
as low as 1V. External reset control is provided by a
CC
debounced pushbutton reset input.
TheLTC1235powerstheactiveCMOSRAMswithacharge
pumped NMOS power switch to achieve low dropout and
low supply current. When primary power is lost, auxiliary
power,connectedtothebatteryinputpin,providesbackup
power to the RAMs. The LTC1235 can be programmed by
a P signal to either back up the RAMs or not. This extends
the battery life in situations where RAM data need not
always be saved when power goes down.
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Thermal Limiting
Performance Specified Over Temperature
All the LTC695 Features Plus Conditional Battery
Backup and External Reset Control
APPLICATIONS
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Critical μP Power Monitoring
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Intelligent Instruments
For an early warning of impending power failure, the
LTC1235 provides an internal comparator with a user-
defined threshold. An internal watchdog timer is also
available,whichforcestheresetpinstoactivestateswhen
the watchdog input is not toggled prior to the time-out
period.
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Battery-Powered Computers and Controllers
Automotive Systems
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L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
Battery Life vs
Backup Duty Cycle
TYPICAL APPLICATION
10
9
LT1086-5
V
IN
≥ 7.5V
+5V
+
POWER TO
μP
V
V
OUT
V
V
V
LTC1235
IN
CC
OUT
+
CMOS RAM POWER
8
7
6
5
4
3
2
0.1μF
+3V
0.1μF
100μF
10μF
51k
ADJ
LTC1235
I/O LINE
BACKUP
BATT
RESET
μP RESET
LTC695
μP
SYSTEM
μP NMI
(WITHOUT
CONDITIONAL
BATTERY
BACKUP)
PFO
PFI
PB RST
I/O LINE
WDI
10k
1235 TA1
1
0
THE LTC1235 EXTENDS BATTERY LIFE BY PROVIDING BATTERY POWER ONLY WHEN REQUIRED TO BACK UP RAM DATA.
IT SAVES THE BATTERY WHEN NO DATA BACKUP IS NEEDED. THE μP REQUESTS BACKUP WITH THE BACKUP PIN.
0
20
40
60
80
100
BACKUP DUTY CYCLE (%)
1235 TA02
1235fa
1