FM1233A 3-Pin µC Supervisor Circuit
September 2004
FM1233A
3-Pin µC Supervisor Circuit
General Description
The FM1233A is a supervisor circuit that monitors a micropro-
cessor power supply or other system voltage and issues a reset
pulse when a fault condition exists. Several different threshold
voltages are offered to accommodate 3.3V and 3.0V systems
with different tolerances.
FM1233A 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
asserted for a minimum period (t
RST
) 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
■
Precision monitoring of 3.3V and lower voltage
microprocessor systems
■
V
TH
values of 3.08V, 2.88V and 2.72V
■
Automatic restart of microprocessor after power failure
■
Reset Timeout Period Choice:
– Options AC and AD: 140ms (min)/256ms (typ)
– Option ABA: 17ms (min)/32ms (typ)
– Available on request: 64ms (typ) and 128ms (typ)
■
Internal 5k
Ω
pull-up resistor
■
Operating Temperature -40°C to +105°C
■
Monitors external pushbutton override
■
Internal switch debounce circuitry
■
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
©2004 Fairchild Semiconductor Corporation
1
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FM1233A Rev. E
FM1233A 3-Pin µC Supervisor Circuit
Absolute Maximum Ratings
Voltage on any pin relative to GND
V
CC
RESET
Input Current
Output Current (RESET)
-0.3V to +6.0V
-0.3V to (V
CC
+ 0.3V)
20mA
20mA
Continuous Power Dissipation (T
A
= 70°C)
SOT23 (derate 4mW above 70°C)
Operating Temperature Range
Storage Temperature Range
Lead Temperature (soldering, 10s)
300mW
-40°C to +105°C
-65°C to +150°C
+300°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 Threshold
Reset Output Voltage
Reset Timeout Period
Reset Timeout Period
Pushbutton Detect
Pushbutton Release
Symbol
V
CC
I
CC
V
TH
V
TH
V
TH
V
OL
t
RST
t
RST
PBV
DET
PBV
REL
FM1233AC,
FM1233AD
FM1233ABA
V
CC
< 3.3V
FM1233AC
FM1233AD
FM1233ABA
Conditions
Min
1.2
Typ
3.3
3
Max
5.5
6
2.97
2.86
3.19
0.4
Units
V
µA
V
V
V
V
ms
ms
V
V
2.70
2.58
2.97
I
SINK
= 1.2mA
V
CC
= V
TH
(min)
140
17
V
CC
= 3.3V
Note 2
0.8
2.88
2.72
3.08
256
32
560
64
2.0
0.3
1.5
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. E
www.fairchildsemi.com
FM1233A 3-Pin µC 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. E
www.fairchildsemi.com
FM1233A 3-Pin µC Supervisor Circuit
Pin Descriptions
Pin Number
1
2
3
Name
GND
RESET
V
CC
GROUND
Function
RESET remains LOW while V
CC
is below V
TH
, and for at least
the minimum Reset Timeout period after V
CC
rises above V
TH
.
Circuit Timing (Ex: FM1233A)
V
CC
3.3V
V
TH
1V
GND
RESET
3.3V
V
TH
1V
GND
140ms
(min)
When operating properly with 3V V
CC
(for example), RESET will
also be about 3V. 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 after the minimum Reset Timeout period.
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. E
www.fairchildsemi.com
FM1233A 3-Pin µC 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 pro-
vides 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 100K
Ω
pull-down resistor between RESET and
V
SS
.
Connecting an External Reset to the FM1233A
VCC
3
FM1233
Controller
1
GND
100pF
2
RESET
Bi-Directional Reset
The FM1233A permits a pushbutton (or a signal) to initiate a
reset externally. Once this external reset is detected on the
reset pin, FM1233 also actively asserts the reset signal to main-
tain the Reset Timeout period when the external reset is not
present .
FM1233 internally debounces glitches seen during a typical
push button closure. For proper operation, the external switch
should be paralleled by an external capacitor of 100pF to
0.01µF.
PUSH
BUTTON
5
FM1233A Rev. E
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Embedded System
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