LTC1235
Microprocessor
Supervisory Circuit
U
DESCRIPTIO
EATURE
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F
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The LTC1235 provides complete power supply monitoring
and battery control functions for microprocessor reset,
battery backup, RAM write protection, power failure warn-
ing and watchdog timing. The LTC1235 has all the LTC695
features plus conditional battery backup and external reset
control. When an out-of-tolerance power supply condition
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 VCC as
lowas1V.Externalresetcontrolisprovidedbyadebounced
push-button reset input.
Guaranteed Reset Assertion at VCC = 1V
1.5mA Maximum Supply Current
Fast (35ns Max.) Onboard Gating of RAM Chip
Enable Signals
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
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The LTC1235 powers the active CMOS RAMs with a charge
pumped NMOS power switch to achieve low dropout and
low supply current. When primary power is lost, auxiliary
power, connected to the battery input pin, provides backup
power to the RAMs. The LTC1235 can be programmed by
a µP 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
O U
PPLICATI
A
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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 avail-
able, which forces the reset pins to active states when the
watchdog input is not toggled prior to the time-out period.
Critical µP Power Monitoring
Intelligent Instruments
Battery-Powered Computers and Controllers
Automotive Systems
U
O
TYPICAL APPLICATI
Battery Life vs
Backup Duty Cycle
10
LT1086-5
V
≥ 7.5V
9
IN
+5V
+
LTC1235
POWER TO
µP
V
V
OUT
V
V
V
IN
CC
OUT
8
7
6
5
4
3
2
+
CMOS RAM POWER
0.1µF
0.1µF
100µF
10µF
51k
ADJ
LTC1235
I/O LINE
BACKUP
RESET
BATT
+3V
LTC695
µP RESET
(WITHOUT
CONDITIONAL
BATTERY
BACKUP)
µP
SYSTEM
µP NMI
PFO
WDI
PFI
PB RST
I/O LINE
10k
1
0
LTC1235 TA1
0
20
40
60
80
100
BACKUP DUTY CYCLE (%)
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.
LTC1235 TA02
1