LTC1235
Microprocessor
Supervisory Circuit
FEATURES
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DESCRIPTION
The LTC
®
1235 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 pushbutton 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 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.
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, which forces the reset pins to active states when
the watchdog input is not toggled prior to the time-out
period.
Guaranteed Reset Assertion at V
CC
= 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
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
Intelligent Instruments
Battery-Powered Computers and Controllers
Automotive Systems
L,
LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
TYPICAL APPLICATION
V
IN
≥ 7.5V
LT1086-5
BATTERY LIFE (NORMALIZED)
V
IN
ADJ
V
OUT
+5V
10
Battery Life vs
Backup Duty Cycle
V
CC
V
OUT
LTC1235
V
BATT
BACKUP
RESET
PFI
PB RST
PFO
WDI
0.1μF
POWER TO
μP
CMOS RAM POWER
I/O LINE
μP
RESET
μP NMI
I/O LINE
μP
SYSTEM
9
8
7
6
5
4
3
2
1
0
0
20
60
80
40
BACKUP DUTY CYCLE (%)
100
1235 TA02
+
10μF
+
100μF
LTC1235
0.1μF
51k
+3V
10k
1235 TA1
LTC695
(WITHOUT
CONDITIONAL
BATTERY
BACKUP)
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.
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LTC1235
ABSOLUTE MAXIMUM RATINGS
(Notes 1 and 2)
Terminal Voltage
V
CC
....................................................... –0.3V to 6.0V
V
BATT
.................................................... –0.3V to 6.0V
All Other Inputs ........................–0.3V to (V
CC
+ 0.3V)
Input Current
V
CC
..................................................................200mA
V
BATT
.................................................................50mA
V
OUT
Output Current ................... Short Circuit Protected
Power Dissipation ...............................................500mW
Operating Temperature Range
LTC1235C ................................................ 0°C to 70°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec.) ................. 300°C
PIN CONFIGURATION
TOP VIEW
V
BATT
V
OUT
V
CC
GND
BATT ON
LOW LINE
PB RST
BACKUP
1
2
3
4
5
6
7
8
LTC1235
16 RESET
15
14
13
12
11
10
9
RESET
WDO
CE
IN
CE
OUT
WDI
PFO
PFI
V
BATT
V
OUT
V
CC
GND
BATT ON
LOW LINE
PB RST
BACKUP
1
2
3
4
LTC1235
5
6
7
8
12
11
10
9
SW PACKAGE
16-LEAD PLASTIC SOL
TOP VIEW
16
15
14
13
PRESET
PRESET
WDO
CE
IN
CE
OUT
WDI
PFO
PFI
N PACKAGE
16-LEAD PLASTIC DIP
T
JMAX
= 110°C,
θ
JA
= 130°C/W
T
JMAX
= 110°C,
θ
JA
= 130°C/W
CONDITIONS:PCB MOUNT ON FR4 MATERIAL,
STILL AIR AT 25°C COPPER TRACE
ORDER INFORMATION
LEAD FREE FINISH
LTC1235CN#PBF
LTC1235CSW#PBF
TAPE AND REEL
LTC1235CN#TRPBF
LTC1235CSW#TRPBF
PART MARKING
LTC1235CN
LTC1235CSW
PACKAGE DESCRIPTION
16-Lead Plastic DIP
16-Lead Plastic SOL
TEMPERATURE RANGE
0°C to 70°C
0°C to 70°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to:
http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to:
http://www.linear.com/tapeandreel/
PRODUCT SELECTION GUIDE
PINS
LTC1235
LTC690
LTC691
LTC694
LTC695
LTC699
LTC1232
16
8
16
8
16
8
8
RESET
X
X
X
X
X
X
X
WATCHDOG
TIMER
X
X
X
X
X
X
X
X
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BATTERY
BACKUP
X
X
X
X
X
POWER FAIL
WARNING
X
X
X
X
X
RAM WRITE
PROTECT
X
X
X
PUSH-BUTTON
RESET
X
CONDITIONAL
BATTERY BACKUP
X
2
LTC1235
ELECTRICAL CHARACTERISTICS
PARAMETER
Battery Backup Switching
Operating Voltage Range
V
CC
V
BATT
V
OUT
Output Voltage
I
OUT
= 1mA
I
OUT
= 50mA
BACKUP Input Threshold
V
CC
> Reset Voltage Threshold
Logic Low
Logic High
I
OUT
= 250μA, V
CC
< V
BATT
V
CC
< V
BATT
1MΩ Pulldown on V
OUT
I
OUT
≤ 50mA
V
CC
= 0V, V
BATT
= 2.8V
5.5 > V
CC
> V
BATT
+ 0.2V
Power-Up
Power-Down
I
SINK
= 3.2mA
BATT ON = V
OUT
Sink Current
BATT ON = 0V Source Current
Logic Low
Logic High
l
l
l
l
l
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= Full Operating Range, V
BATT
= 2.8V, Backup = No Connection,
T
A
= 25°C, unless otherwise noted.
CONDITIONS
MIN
TYP
MAX
UNITS
4.75
2.00
V
CC
– 0.05
V
CC
– 0.1
V
CC
– 0.5
V
CC
– 0.005
V
CC
– 0.005
V
CC
– 0.25
5.50
4.25
V
V
V
V
V
0.8
2.0
3
V
BATT
– 0.1 V
BATT
– 0.02
0
0.6
0.6
0.04
0.04
–0.1
–1.0
70
50
20
0.4
0.5
35
1
25
0.8
2.0
40
4.5
60
140
1.2
1.0
4.65
40
200
200
1.6
1.6
1
8
4.75
240
280
2.00
2.25
1.5
2.5
1
5
+0.02
+0.10
V
V
μA
V
V
V
mA
mA
μA
μA
μA
μA
mV
mV
mV
V
mA
μA
V
V
ms
V
mV
ms
ms
sec
sec
ms/V
ms/V
ns
BACKUP Pullup Current (Note 4)
V
OUT
in Battery Backup Mode (Note 5)
V
OUT
in Battery Saving Mode (Note 5)
V
CC
Supply Current (excluding I
OUT
)
Battery Supply Current in Battery Backup Mode
and Battery Saving Mode (Note 5)
Battery Standby Current
(+ = Discharge, – = Charge)
Battery Switchover Threshold
V
CC
– V
BATT
Battery Switchover Hysteresis
BATT ON Output Voltage (Note 6)
BATT ON Output Short Circuit Current (Note 6)
Push-Button Reset
PB RST Input Threshold
PB RST Input Low Time (Notes 4, 7)
Reset and Watchdog Timer
Reset Voltage Threshold
Reset Threshold Hysteresis
Reset Active Time
Watchdog Time-out Period
Reset Active Time PSRR
Watchdog Time-out Period PSRR
Minimum WDI Input Pulse Width
RESET
Output Voltage At V
CC
= 1V
RESET
and LOW LINE Output Voltage
(Note 6)
RESET and
WDO
Output Voltage
(Note 6)
V
IL
= 0.4V, V
IH
= 3.5V
I
SINK
= 10μA, V
CC
= 1V
I
SINK
= 1.6mA, V
CC
= 4.25V
I
SOURCE
= 1μA, V
CC
= 5V
I
SINK
= 1.6mA, V
CC
= 5V
I
SOURCE
= 1μA, V
CC
= 4.25V
3.5
l
l
V
CC
= 5V
V
CC
= 5V
l
l
200
4
200
0.4
0.4
3.5
mV
V
V
V
V
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LTC1235
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= Full Operating Range, V
BATT
= 2.8V, Backup = No Connection,
T
A
= 25°C, unless otherwise noted.
PARAMETER
RESET,
RESET, WDO,
LOW LINE
Output Short Circuit Current (Note 6)
WDI Input Threshold
WDI Input Current
Power Fail Detector
PFI Input Threshold
PFI Input Threshold PSRR
PFI Input Current
PFO
Output Voltage (Note 6)
PFO
Short Circuit Source Current
(Note 6)
PFI Comparator Response Time (falling)
PFI Comparator Response Time (rising)
(Note 6)
Chip Enable Gating
CE
IN Threshold
CE
IN Pullup Current (Note 4)
CE
OUT Output Voltage
I
SINK
= 3.2mA
I
SOURCE
= 3.0mA
I
SOURCE
= 1μA, V
CC
= 0V
V
CC
= 5V, CL = 20pF
Output Source Current
Output Sink Current
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ELECTRICAL CHARACTERISTICS
The
l
denotes the specifications which apply over the full operating
CONDITIONS
Output Source Current
Output Sink Current
Logic Low
Logic High
WDI = V
OUT
WDI = 0V
V
CC
= 5V
l
l
l
MIN
1
TYP
3
25
MAX
25
0.8
UNITS
μA
mA
V
V
μA
μA
V
mV/V
nA
V
V
μA
mA
μs
μs
μs
2.0
–50
1.25
4
–8
1.3
0.3
±0.01
±25
0.4
3.5
1
3
30
2
40
8
0.8
2.0
3
0.4
V
OUT
– 1.50
V
OUT
– 0.05
20
20
30
35
35
45
25
50
1.35
I
SINK
= 3.2mA
I
SOURCE
= 1A
PFI = HIGH,
PFO
= 0V
PFI = LOW,
PFO
= V
OUT
ΔV
IN
= –20mV, V
OD
= 15mV
ΔV
IN
= 20mV, V
OD
= 15mV
with 10kΩ Pullup
V
IL
V
IH
V
V
μA
V
V
V
ns
ns
mA
mA
CE
Propagation Delay
CE
OUT Output Short Circuit Current
Note 1:
Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2:
All voltage values are with respect to GND.
Note 3:
For military temperature range parts, consult the factory.
Note 4:
The input pins of PB RST, BACKUP and
CE
IN, have weak internal
pullups which pull to the supply when the input pins are floating.
Note 5:
The LTC1235 can be programmed either to provide or not to
provide battery backup power to the V
OUT
pin during power failure.
The power down condition of V
OUT
is selected by the logic level of the
BACKUP pin which is latched internally when V
CC
falls through the reset
voltage threshold. If the latched logic level of the BACKUP pin is high,
V
OUT
will be in Battery Backup Mode and will be switched to V
BATT
when
V
CC
falls below V
BATT
. If the latched logic level of the BACKUP pin is low,
V
OUT
will be in Battery Saving Mode when V
CC
falls below V
BATT
.
Note 6:
The output pins of BATT ON, LOW LINE,
PFO, WDO, RESET
and
RESET have weak internal pullups of typically 3A. However, external
pullup resistors may be used when higher speed is required.
Note 7:
The push-button reset input requires an active low signal.
Internally, this input signal is debounced and timed for a minimum of
40ms. When this condition is satisfied, the reset outputs go to the active
states. The reset outputs will remain in active states for a minimum of
140ms from the moment the push-button reset input is released from
logic low level.
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LTC1235
TYPICAL PERFORMANCE CHARACTERISTICS
V
OUT
vs I
OUT
5.00
V
CC
= 5V
V
BATT
= 2.8V
T
A
= 25
°
C
OUTPUT VOLTAGE (V)
2.80
V
CC
= 0V
V
BATT
= 2.8V
T
A
= 25
°
C
BACKUP MODE
SELECTED
V
OUT
vs I
OUT
1.308
1.306
PFI INPUT THRESHOLD (V)
1.304
1.302
1.300
1.298
1.296
Power Failure Input Threshold
vs Temperature
V
CC
= 5V
4.95
OUTPUT VOLTAGE (V)
4.90
SLOPE = 5Ω
4.85
2.78
SLOPE = 125Ω
2.76
2.74
4.80
4.75
0
10
30
40
20
LOAD CURRENT (mA)
50
1235 G01
2.72
0
100
300
400
200
LOAD CURRENT (μA)
500
1235 G02
1.294
–50 –25
50
25
75
0
TEMPERATURE (
˚
C)
100
125
1235 G03
RESET
Output Voltage vs
SupplyVoltage
5
T
A
= 25°C
EXTERNAL PULLUP = 10μA
V
BATT
= 0V
232
Reset Active Time vs
Temperature
4.66
V
CC
= 5V
224
RESET ACTIVE TIME (ms)
216
208
200
192
184
–50 –25
RESET VOLTAGE THRESHOLD (V)
4.65
4.64
4.63
4.62
4.61
Reset Voltage Threshold vs
Temperature
RESET
OUTPUT VOLTAGE (V)
4
3
2
1
0
0
1
3
4
2
SUPPLY VOLTAGE (V)
5
1235 G04
50
25
75
0
TEMPERATURE (
°
C)
100
125
4.60
–50 –25
50
25
75
0
TEMPERATURE (
°
C)
100
125
1235 G05
1235 G06
Power Fail Comparator
Response Time
PFO OUTPUT VOLTAGE (V)
PFO OUTPUT VOLTAGE (V)
6
5
4
3
2
1
0
V
PFI
1.3V
+
–
PFO
30pF
Power Fail Comparator
Response Time
PFO OUTPUT VOLTAGE (V)
6
5
4
3
2
1
0
V
PFI
1.3V
+
–
PFO
30pF
Power Fail Comparator Response
Time with Pullup Resistor
6
5
4
3
2
1
0
+5V
V
PFI
1.3V
+
–
PFO
10k
30pF
V
CC
= 5V
T
A
= 25
°
C
V
CC
= 5V
T
A
= 25
°
C
V
CC
= 5V
T
A
= 25
°
C
1.305V
1.285V
0
1
V
PFI
= 20mV STEP
1.315V
1.295V
6
7
8
1235 G07
V
PFI
= 20mV STEP
1.315V
1.295V
0
2
V
PFI
= 20mV STEP
2
3 4 5
TIME (μs)
0
20 40
60 80 100 120 140 160 180
TIME (μs)
1235 G08
4
6
8 10 12 14 16 18
TIME (μs)
1235 G09
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