LTC1235 Datasheet, PDF (1/16 Pages) Linear Technology – Microprocessor Supervisory Circuit

FEATURES
s _{Guaranteed Reset Assertion at VCC = 1V}
s _{1.5mA Maximum Supply Current}
s _{Fast (35ns Max.) Onboard Gating of RAM Chip}
Enable Signals
s _{Conditional Battery Backup Extends Battery Life}
s _{4.65V Precision Voltage Monitor}
s _{Power OK/Reset Time Delay: 200ms}
s _{External Reset Control}
s _{Minimum External Component Count}
s _{1µA Maximum Standby Current}
s _{Voltage Monitor for Power Fail or Low Battery}
Warning
s _{Thermal Limiting}
s _{Performance Specified Over Temperature}
s _{All the LTC695 Features Plus Conditional Battery}
Backup and External Reset Control
APPLICATI S
s _{Critical µP Power Monitoring}
s _{Intelligent Instruments}
s _{Battery-Powered Computers and Controllers}
s _{Automotive Systems}
LTC1235
Microprocessor
Supervisory Circuit
DESCRIPTIO
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 V_CC as
low as 1V. External reset control is provided by a debounced
push-button reset input.
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.
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.
TYPICAL APPLICATI
V_IN ≥ 7.5V
+
10µF
LT1086-5
V_IN
V_OUT
ADJ
+5V
+
100µF
51k
10k
V_CC
V_OUT
0.1µF
LTC1235
V_BATT BACKUP
+3V
RESET
PFO
PFI
PB RST
WDI
POWER TO µP
0.1µF ^{CMOS RAM POWER}
I/O LINE
µP RESET
µP NMI
I/O LINE
µP
SYSTEM
LTC1235 TA1
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.
Battery Life vs
Backup Duty Cycle
10
9
LTC1235
8
7
6
5
LTC695
4
(WITHOUT
CONDITIONAL
3
BATTERY
BACKUP)
2
1
0
0
20
40
60
80 100
BACKUP DUTY CYCLE (%)
LTC1235 TA02
1