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
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
µ
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 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
U
S
A
O
PPLICATI
D
U
ESCRIPTIO
S
FEATURE
s
Guaranteed Reset Assertion at V
CC
= 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
1
µ
A 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
µ
P Power Monitoring
s
Intelligent Instruments
s
Battery-Powered Computers and Controllers
s
Automotive Systems
U
A
O
PPLICATI
TYPICAL
BACKUP DUTY CYCLE (%)
0
BATTERY LIFE (NORMALIZED)
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
V
IN
V
OUT
10µF
V
IN
7.5V
+5V
100µF
0.1µF
LTC1235
V
OUT
V
CC
V
BATT
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
µ
P REQUESTS BACKUP WITH THE BACKUP PIN.
I/O LINE
BACKUP
PB RST
2
LTC1235
A
U
G
W
A
W
U
W
A
R
BSOLUTE
XI
TI
S
(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
W
U
U
PACKAGE/ORDER I FOR ATIO
(Note 3)
ORDER PART
NUMBER
PRODUCT SELECTIO GUIDE
U
LTC1235
16
X
X
X
X
X
X
X
LTC690
8
X
X
X
X
LTC691
16
X
X
X
X
X
LTC694
8
X
X
X
X
LTC695
16
X
X
X
X
X
LTC699
8
X
X
LTC1232
8
X
X
X
CONDITIONAL
WATCHDOG
BATTERY
POWER FAIL
RAM WRITE
PUSH-BUTTON
BATTERY
PINS
RESET
TIMER
BACKUP
WARNING
PROTECT
RESET
BACKUP
ORDER PART
NUMBER
1
2
3
4
5
6
7
8
TOP VIEW
WDO
V
CC
V
OUT
V
BATT
GND
LTC1235
CE IN
RESET
N PACKAGE
16-LEAD PLASTIC DIP
9
16
15
14
13
12
11
10
BATT ON
PB RST
LOW LINE
BACKUP
RESET
CE OUT
WDI
PFI
PFO
T
JMAX
= 110
°
C,
JA
= 130
°
C/W
LTC1235CS
LTC1235CN
LTC1235
1
2
3
4
5
6
7
8
TOP VIEW
WDO
V
CC
V
OUT
V
BATT
GND
S PACKAGE
16-LEAD PLASTIC SOL
CE IN
PRESET
9
16
15
14
13
12
11
10
BATT ON
PB RST
LOW LINE
BACKUP
PRESET
CE OUT
WDI
PFI
PFO
T
JMAX
= 110
°
C,
JA
= 130
°
C/W
CONDITIONS: PCB MOUNT ON FR4 MATERIAL,
STILL AIR AT 25
°
C, COPPER TRACE
3
LTC1235
PARAMETER
CONDITONS
MIN
TYP
MAX
UNITS
Battery Backup Switching
Operating Voltage Range
V
CC
4.75
5.50
V
V
BATT
2.00
4.25
V
OUT
Output Voltage
I
OUT
= 1mA
V
CC
0.05
V
CC
0.005
V
q
V
CC
0.1
V
CC
0.005
I
OUT
= 50mA
V
CC
0.5
V
CC
0.25
BACKUP Input Threshold
V
CC
> Reset Voltage Threshold
Logic Low
0.8
V
Logic High
2.0
BACKUP Pullup Current (Note 4)
3
µ
A
V
OUT
in Battery Backup Mode (Note 5)
I
OUT
= 250
µ
A, V
CC
< V
BATT
V
BATT
0.1
V
BATT
0.02
V
V
OUT
in Battery Saving Mode (Note 5)
V
CC
< V
BATT
0
V
1M
Pulldown on V
OUT
V
CC
Supply Current (excluding I
OUT
)
I
OUT
50mA
0.6
1.5
mA
q
0.6
2.5
Battery Supply Current in Battery Backup Mode and
V
CC
= 0V, V
BATT
= 2.8V
0.04
1
µ
A
Battery Saving Mode (Note 5)
q
0.04
5
Battery Standby Current
5.5 > V
CC
> V
BATT
+ 0.2V
0.1
+0.02
µ
A
(+ = Discharge, = Charge)
q
1.0
+0.10
Battery Switchover Threshold
Power Up
70
mV
V
CC
V
BATT
Power Down
50
Battery Switchover Hysteresis
20
mV
BATT ON Output Voltage (Note 6)
I
SINK
= 3.2mA
0.4
V
BATT ON Output Short Circuit Current (Note 6)
BATT ON = V
OUT
Sink Current
35
mA
BATT ON = 0V Source Current
0.5
1
25
µ
A
Push-Button Reset
PB RST Input Threshold
Logic Low
0.8
V
Logic High
2.0
PB RST Input Low Time (Notes 4, 7)
q
40
ms
Reset and Watchdog Timer
Reset Voltage Threshold
q
4.5
4.65
4.75
V
Reset Threshold Hysteresis
40
mV
Reset Active Time
V
CC
= 5V
160
200
240
ms
q
140
200
280
Watchdog Time-out Period
V
CC
= 5V
1.2
1.6
2.00
sec
q
1.0
1.6
2.25
Reset Active Time PSRR
1
ms/V
Watchdog Time-out Period PSRR
8
ms/V
Minimum WDI Input Pulse Width
V
IL
= 0.4V, V
IH
= 3.5V
q
200
ns
RESET Output Voltage At V
CC
= 1V
I
SINK
= 10
µ
A, V
CC
= 1V
4
200
mV
RESET and LOW LINE Output Voltage
I
SINK
= 1.6mA, V
CC
= 4.25V
0.4
V
(Note 6)
I
SOURCE
= 1
µ
A, V
CC
= 5V
3.5
ELECTRICAL C
C
HARA TERISTICS
V
CC
= Full Operating Range, V
BATT
= 2.8V, Backup = No Connection, T
A
= 25
°
C, unless otherwise noted.
4
LTC1235
PARAMETER
CONDITONS
MIN
TYP
MAX
UNITS
RESET and WDO Output Voltage
I
SINK
= 1.6mA, V
CC
= 5V
0.4
V
(Note 6)
SOURCE
= 1
µ
A, V
CC
= 4.25V
3.5
RESET, RESET, WDO, LOW LINE
Output Source Current
1
3
25
µ
A
Output Short Circuit Current (Note 6)
Output Sink Current
25
mA
WDI Input Threshold
Logic Low
0.8
V
Logic High
2.0
WDI Input Current
WDI = V
OUT
q
4
50
µ
A
WDI = 0V
q
50
8
Power Fail Detector
PFI Input Threshold
V
CC
= 5V
q
1.25
1.3
1.35
V
PFI Input Threshold PSRR
0.3
mV/V
PFI Input Current
±
0.01
±
25
nA
PFO Output Voltage (Note 6)
I
SINK
= 3.2mA
0.4
V
I
SOURCE
= 1
µ
A
3.5
PFO Short Circuit Source Current
PFI = HIGH, PFO = 0V
1
3
25
µ
A
(Note 6)
PFI = LOW, PFO = V
OUT
30
mA
PFI Comparator Response Time (falling)
V
IN
= 20mV, V
OD
= 15mV
2
µ
s
PFI Comparator Response Time (rising)
V
IN
= 20mV, V
OD
= 15mV
40
µ
s
(Note 6)
with 10k
Pullup
8
Chip Enable Gating
CE IN Threshold
V
IL
0.8
V
V
IH
2.0
CE IN Pullup Current (Note 4)
3
µ
A
CE OUT Output Voltage
I
SINK
= 3.2mA
0.4
V
I
SOURCE
= 3.0mA
V
OUT
1.50
I
SOURCE
= 1
µ
A, V
CC
= 0V
V
OUT
0.05
CE Propagation Delay
V
CC
= 5V, C
L
= 20pF
20
35
ns
q
20
45
CE OUT Output Short Circuit Current
Output Source Current
30
mA
Output Sink Current
35
V
CC
= Full Operating Range, V
BATT
= 2.8V, Backup = No Connection, T
A
= 25
°
C, unless otherwise noted.
ELECTRICAL C
C
HARA TERISTICS
The
q
denotes specifications which apply over the operating temperature
range.
Note 1: Absolute maximum ratings are those values beyond which the life
of device may be impaired.
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 3
µ
A. 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.
5
LTC1235
V
OUT
vs I
OUT
LOAD CURRENT (mA)
0
OUTPUT VOLTAGE (V)
4.90
4.95
5.00
40
LTC1235 G01
4.85
4.80
4.75
10
20
30
50
SLOPE = 5
V
CC
= 5V
V
BATT
= 2.8V
T
A
= 25
°
C
TEMPERATURE (
°
C)
50
PFI INPUT THRESHOLD (V)
1.304
1.306
1.308
25
75
LTC1235 G03
1.302
1.300
25
0
50
100
125
1.298
1.296
1.294
V
CC
= 5V
C
C
HARA TERISTICS
U
W
A
TYPICAL PERFOR
CE
Power Failure Input Threshold
vs Temperature
V
OUT
vs I
OUT
LOAD CURRENT (µA)
0
OUTPUT VOLTAGE (V)
2.76
2.78
400
LTC1235 G02
2.74
2.72
100
200
300
500
2.80
SLOPE = 125
BACKUP MODE
SELECTED
V
CC
= 0V
V
BATT
= 2.8V
T
A
= 25
°
C
SUPPLY VOLTAGE (V)
0
RESET OUTPUT VOLTAGE (V)
3
4
5
4
2
1
0
1
2
3
5
T
A
= 25°C
EXTERNAL PULLUP = 10µA
V
BATT
= 0V
LTC1235 G04
Reset Active Time vs
Temperature
RESET Output Voltage vs Supply
Voltage
Reset Voltage Threshold
vs Temperature
TEMPERATURE (
°
C)
50
RESET VOLTAGE THRESHOLD (V)
4.64
4.65
4.66
25
75
LTC1235 G06
4.63
4.62
25
0
50
100
125
4.61
4.60
TEMPERATURE (
°
C)
50
RESET ACTIVE TIME (ms)
216
224
232
25
75
LTC1235 G05
208
200
25
0
50
100
125
192
184
V
CC
= 5V
Power Fail Comparator
Response Time
Power Fail Comparator
Response Time
Power Fail Comparator Response
Time with Pullup Resistor
TIME (µs)
0
4
5
6
4
LTC1235 G07
3
2
0
1
2
3
5
1
1.305V
1.285V
8
7
6
V
CC
= 5V
T
A
= 25
°
C
+
V
PFI
1.3V
PFO
30pF
V
PFI
= 20mV STEP
PFO OUTPUT VOLTAGE (V)
TIME (µs)
0
4
5
6
40
LTC1235 G08
3
2
0
20
60
1
1.315V
1.295V
120
100
80
V
PFI
= 20mV STEP
180
160
140
V
CC
= 5V
T
A
= 25
°
C
+
V
PFI
1.3V
PFO
30pF
PFO OUTPUT VOLTAGE (V)
TIME (µs)
0
4
5
6
4
LTC1235 G09
3
2
0
2
6
1
1.315V
1.295V
12
10
8
V
PFI
= 20mV STEP
18
16
14
V
CC
= 5V
T
A
= 25
°
C
+
V
PFI
1.3V
PFO
30pF
10k
+5V
PFO OUTPUT VOLTAGE (V)
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