1

LTC1235

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

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.

Microprocessor

Supervisory Circuit

S

APPLICATI

DESCRIPTIO

S

FEATURE

s

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

1

µA 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

s

Critical

µP Power Monitoring

s

Intelligent Instruments

s

Battery-Powered Computers and Controllers

s

Automotive Systems

APPLICATI

TYPICAL

BACKUP DUTY CYCLE (%)

0

6

8

10

80

LTC1235 TA02

4

2

0

20

40

60

100

1

3

5

7

9

LTC1235

LTC695

(WITHOUT

CONDITIONAL

BATTERY

BACKUP)

Battery Life vs

Backup Duty Cycle

+

+

ADJ

10µF

+5V

100µF

0.1µF

LTC1235

PFI

WDI

µP NMI

µP

POWER

POWER TO

CMOS RAM

I/O LINE

µP RESET

µP

SYSTEM

0.1µF

+3V

51k

10k

LTC1235 TA1

RESET

PFO

LT1086-5

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.

I/O LINE

BACKUP

PB RST