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
Internal 5k
Ω
pull-up resistor
s
Other reset choices available: 32 to 128ms
s
Operating Temperature -40°C to +105°C
s
Monitors external pushbutton override
s
Internal switch debounce circuitry
s
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
www.fairchildsemi.com
FM1233B Rev. D
FM1233B 3-Pin µC Supervisor Circuit
Absolute Maximum Ratings
Votage 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
ESD Rating:
Human body model
Machine Model
Continuous Power Dissipation (T
A
= 70°C)
SOT23 (derate 4mW above 70°C)
Operating Temperature Range
>2kV
>200 V
300mW
-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.
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
Embedded System
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