FM1233B 3-Pin µC Supervisor Circuit
February 2003
FM1233B
3-Pin µC Supervisor Circuit
General Description
The FM1233B 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 5V 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
approximately 250 ms to allow the system clock and other cir-
cuits to stabilize. The reset output of FM1233B is of open-drain
active low type.
The FM1233B 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 FM1233B.
Features
s
Precision monitoring of 5V and lower voltage microprocessor
systems
s
V
TH
values of 4.62V, 4.38V and 4.12V
s
Automatic restart of microprocessor after power failure
s
140ms (min) power-on RESET delay (typ.: 256ms)
s
s
s
s
s
s
Internal 5k
Ω
pull-up resistor
Other reset choices available: 32 to 128ms
Operating Temperature -40°C to +105°C
Monitors external pushbutton override
Internal switch debounce circuitry
SOT23-3 package
Typical Operating Circuit
VCC
3
FM1233B
Controller
1
GND
Connection Diagram
2
RESET
GND
1
FM1233B
3
VCC
RESET
2
SOT23-3 Package
©2002 Fairchild Semiconductor Corporation
1
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FM1233B Rev. D
FM1233B 3-Pin µC Supervisor Circuit
Absolute Maximum Ratings
Votage on any pin relative to GND
V
CC
ESD Rating:
Human body model
>2kV
RESET
-0.3V to (V
CC
+ 0.3V)
Machine Model
>200 V
Input Current
20mA
Continuous Power Dissipation (T
A
= 70°C)
SOT23 (derate 4mW above 70°C)
300mW
Output Current (RESET)
20mA
Operating Temperature Range
-40°C to +105°C
Storage Temperature Range
-65°C to +150°C
Lead Temperature (soldering, 10s)
+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.
-0.3V to +6.0V
Electrical Characteristics (V
CC
= 5V; 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
Pushbutton Detect
Pushbutton Release
Symbol
V
CC
I
CC
V
TH
V
TH
V
TH
V
OL
t
RST
PBV
DET
PBV
REL
V
CC
< 5V
FM1233BF
FM1233BD
FM1233BE
FM1233B
FM1233B
FM1233B
FM1233B
Conditions
Min
1.2
Typ
3
Max
5.5
6
4.86
4.55
4.32
0.4
Units
V
µA
V
V
V
V
ms
V
V
4.40
4.16
3.91
I
sink
= 5mA
V
CC
= V
TH
(min)
140
V
CC
= 5V
Note 2
0.8
4.62
4.38
4.12
256
560
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
= 5V. It is recommended to connect 100pF capacitor between the Reset pin and Ground pin if pushbutton reset is implemented.
2
FM1233B Rev. D
www.fairchildsemi.com
FM1233B 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 140ms after V
CC
rises above V
TH
.
Circuit Timing
V
CC
3.3V
V
TH
1V
GND
RESET
3.3V
V
TH
1V
GND
140ms
(min)
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.
3
FM1233B Rev. D
www.fairchildsemi.com
FM1233B 3-Pin µC Supervisor Circuit
General Description
The FM1233B features a highly accurate voltage reference to
which V
CC
is compared. Once V
CC
is below the specified
threshold, 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 FM1233B 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 FM1233B 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 FM1233B 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 FM1233B
V
CC
3
FM1233
Controller
1
GND
100pF
2
RESET
PUSH
BUTTON
Bi-Directional Reset
The FM1233B 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.
4
FM1233B Rev. D
www.fairchildsemi.com
FM1233B 3-Pin µC Supervisor Circuit
Ordering Information
Part
Number
FM1233BFS3X
FM1233BDS3X
FM1233BES3X
Top
Marking
3BF
3BD
3BE
RESET
Threshold (V)
4.62
4.38
4.12
Output Type
Open-Drain, active LOW
Open-Drain, active LOW
Open-Drain, active LOW
Package
Type
3-Pin, SOT23
3-Pin, SOT23
3-Pin, SOT23
Packing
Method
3000 units in T&R
3000 units in T&R
3000 units in T&R
Note 3
: Devices listed above feature 250ms typical reset pulse width. Consult Fairchild Sales for other reset pulse width options.
5
FM1233B Rev. D
www.fairchildsemi.com
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