FM1233A 3-Pin
µ
C Supervisor Circuit
PRELIMINARY
May 2001
FM1233A
3-Pin
µ
C Supervisor Circuit
General Description
The FM1233A is a supervisor circuit that monitors a microproces-
sor power supply or other system voltage and issues a reset pulse
when a fault condition exists. Several different threshold voltages
are offered to accommodate 3V systems with different tolerances.
The device features a precision temperature-compensated volt-
age reference and comparator. When V
CC
falls to the threshold
voltage, a RESET pulse is issued, holding the output in the active
state. When power rises above V
TH
, the reset remains for approxi-
mately 250 ms to allow the system clock and other circuits to
stabilize. The reset output of FM1233A is of open-drain active low
type.
The FM1233A also can monitor a switch closure on its output,
enabling it to recognize an external reset from a pushbutton switch
or a
µP.
In the case of a switch, the closure will be debounced by
circuitry internal to the FM1233A.
Features
I
Precision monitoring of 3.3V and lower voltage
microprocessor systems
I
V
TH
values of 2.88V, 2.72V
I
Automatic restart of microprocessor after power failure
I
140ms (min) power-on RESET delay
I
Internal 5kΩ pull-up resistor
I
Other reset choices available: 32 to 256ms
I
Operating Temperature -40°C to +105°C
I
Monitors external pushbutton override
I
Internal switch debounce circuitry
I
SOT23-3 package
Typical Operating Circuit
V
CC
3
FM1233A
Controller
1
GND
2
/RESET
Connection Diagram
GND
1
FM1233A
3
V
CC
/RESET
2
SOT23-3 Package
© 2001 Fairchild Semiconductor Corporation
FM1233A Rev. A.1
1
www.fairchildsemi.com
FM1233A 3-Pin
µ
C Supervisor Circuit
Absolute Maximum Ratings
Voltage on any pin relative to GND
V
CC
RESET, RESET
Input Current
Output Current (RESET, RESET)
-0.3V to +6.0V
-0.3V to (V
CC
+ 0.3V)
20mA
20mA
Storage Temperature Range
Lead Temperature (soldering, 10s)
-65°C to +150°C
+300°C
Continuous Power Dissipation (T
A
= 70°C)
SOT23 (derate 4mW above 70°C)
Operating Temperature Range
300mW
-40°C to +105°C
These are stress ratings only, and functional operation is not implied for these levels or beyond. Exposure to Absolute Maximum Rating
conditions for extended periods may affect device reliability.
Electrical Characteristics (V
CC
= 3.3V; T
A
= -40
°
C to +105
°
C unless otherwise noted) (Note 1)
Parameter
Operating Voltage
Supply Current
Reset Threshold
Reset Threshold
Reset Output Voltage
Reset Output Voltage
Reset Timeout Period
Pushbutton Detect
Pushbutton Release
Symbol
V
CC
I
CC
V
TH
V
TH
V
OH
V
OL
t
RST
PBV
DET
PBV
REL
Conditions
V
CC
< 3.3V
FM1233AC
FM1233AD
FM1233A
FM1233A
FM1233A
FM1233A
FM1233A
V
CC
= 3.3V
Note 2
I
SOURCE
= 150
µA
V
CC
= V
TH
(max)
I
SINK
= 5mA
V
CC
= V
TH
(min)
Min
1.2
Typ
3.3
3
Max
5.5
6
2.97
2.86
Units
V
µA
V
V
V
2.70
2.58
0.8V
CC
2.88
2.72
0.4
140
0.8
0.3
256
560
2.0
1.5
V
ms
V
V
Note 1:
Testing at production is done at 25°C only. Limits over temperature are guaranteed by design.
Note 2:
C = 100pF, V
CC
= 3.3V. It is recommended to connect 100pF capacitor between the Reset pin and Ground pin if pushbutton reset is implemented.-
2
FM1233A Rev. A.1
www.fairchildsemi.com
FM1233A 3-Pin
µ
C Supervisor Circuit
Typical Operating Characteristics
Supply Current Vs. Temperature
10
8
10
8
6
4
2
-55C
+25C
+110C
0
I
CC
(µA)
6
4
2
0
V
CC
= 3.3V
Temperature (C)
Power up Reset Timeout Vs. Temperature
Power up Reset timeout (ms)
600
500
400
300
200
100
-40C
+25C
+105C
600
500
400
300
200
100
Temperature (C)
3
FM1233A Rev. A.1
www.fairchildsemi.com
FM1233A 3-Pin
µ
C Supervisor Circuit
Pin Descriptions
Pin Number
1
2
Name
GND
/RESET
GROUND
Function
/RESET remains LOW while V
CC
is below
V
TH
, and for at least 140ms after V
CC
rises
above V
TH
.
3
V
CC
Circuit Timing (Ex: FM1233A)
V
CC
5V
V
TH
1V
GND
RESET
5V
V
TH
1V
GND
140ms
When operating properly with 5V V
CC
(for example), /RESET will
also be about 5V. When V
CC
starts to fall, /RESET will follow it
down as shown. When V
CC
drops below V
TH
, /RESET drops to
ground ("issues a RESET") and stays there unless V
CC
also falls
below its minimum operating voltage, approx. 1V. At this point, the
supervisor loses control, and its output may rise, only to again
follow V
CC
down to the ground.
When V
CC
begins to rise, /RESET follows it until 1.0V or so is
reached, whereupon the device regains control, /RESET is pulled
to ground, etc. When V
CC
rises above V
TH
, /RESET comes out of
RESET 140 ms later.
If it is required that a lower value than GND + 1.0V is needed on
RESET signal during V
CC
≤
1V, a 100K resistor may be used on
the device output to GND.
4
FM1233A Rev. A.1
www.fairchildsemi.com
FM1233A 3-Pin
µ
C Supervisor Circuit
General Description
The FM1233A features a highly accurate voltage reference to
which V
CC
is compared. Once V
CC
is below the specified thresh-
old, it will drive the /RESET line and continue to hold it low until V
CC
returns above the threshold and the time for the RESET pulse
duration has expired. The FM1233A is immune to short negative
going transients on the V
CC
line. The placement of a 0.1µF bypass
capacitor as close as possible to the V
CC
pin provides additional
transient immunity.
For a V
CC
value below 1.0V, the FM1233A does not sink very
much current on the /RESET pin. This is not a problem in most
systems since common devices are not functional in this range. If
it is desired for the FM1233A reset to be functional below this
range, use a 100KΩ pull-down resistor between /RESET and V
SS
.
Connecting an External Reset to the FM1233A
V
CC
3
FM1233
Controller
1
GND
100pF
2
/RESET
PUSH
BUTTON
Bi-Directional Reset
The FM1233A permits an external pushbutton to initiate a reset.
Such a connection to pin 2 will be debounced, RESET will go low
and recover in typically 250ms. For proper operation, the external
switch should be paralleled by an external capacitor of 100pF to
0.01µF.
5
FM1233A Rev. A.1
www.fairchildsemi.com
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