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MAX793RCSE

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
MAX793/MAX794/MAX795
Adding Hysteresis to the Power-Fail
Comparator (MAX793/MAX794)
The power-fail comparator has a typical input hystere-
sis of 10mV. This is sufficient for most applications
where a power-supply line is being monitored through
an external voltage divider (see the section
Monitoring
an Additional Power Supply
).
If additional noise margin is desired, connect a resistor
between PFO and PFI as shown in Figure 16a. Select
the ratio of R1 and R2 such that PFI sees V
PFT
when
V
IN
falls to its trip point (V
TRIP
). R3 adds the additional
hysteresis and should typically be more than 10 times
the value of R1 or R2. The hysteresis window extends
both above (V
H
) and below (V
L
) the original trip point
(V
TRIP
).
Connecting an ordinary signal diode in series with R3,
as shown in Figure 16b, causes the lower trip point (V
L
)
to coincide with the trip point without hysteresis (V
TRIP
),
so the entire hysteresis window occurs above V
TRIP
.
This method provides additional noise margin without
compromising the accuracy of the power-fail threshold
when the monitored voltage is falling. It is useful for
accurately detecting when a voltage falls past a thresh-
old. The current through R1 and R2 should be at least
1µA to ensure that the 25nA (max over temperature)
PFI input current does not shift the trip point. R3 should
be larger than 82kΩ so it does not load down the PFO
pin. Capacitor C1 is optional, and adds noise rejection.
3.0V/3.3V Adjustable Microprocessor
Supervisory Circuits
16 ______________________________________________________________________________________
MAX793
MAX794
MAX795
3.0V OR 3.3V
TO CMOS RAM
BODY DIODE
GND
MAX793
MAX794
MAX795
BATT ONV
CC
OUT
S
D
G
PMOS FET
BATT ONV
CC
OUT
GND
(b)(a)
Figure 14. Driving an External Transistor with BATT ON
MAX793
MAX794
OUT
TO STATIC
RAM
BATT
V
CC
GND
1N4148
RESET
TO µP
0.47F
3.0V OR 3.3V
MAX793
MAX794
OUT
TO STATIC
RAM
BATT
V
CC
V
CC
GND
1N4148
RESET
TO µP
0.47F
3.0V OR
3.3V
+5V
(b)(a)
V
CC
Figure 15. Using a Super Cap as a Backup Source
Monitoring an Additional Power Supply
These µP supervisors can monitor either positive or
negative supplies using a resistor voltage divider to
PFI. PFO can be used to generate an interrupt to the µP
or to cause reset to assert (Figure 12).
Interfacing to µPs with
Bidirectional Reset Pins
Since the RESET output is open drain, the MAX793/
MAX794/MAX795 interface easily with µPs that have
bidirectional reset pins, such as the Motorola 68HC11.
Connecting the RESET output of the µP supervisor
directly to the RESET input of the microcontroller with a
single pullup resistor allows either device to assert
reset (Figure 17).
Negative-Going V
CC
Transients
These supervisors are relatively immune to short-dura-
tion negative-going V
CC
transients (glitches) while issu-
ing resets to the µP during power-up, power-down, and
brownout conditions. Therefore, resetting the µP when
V
CC
experiences only small glitches is usually not rec-
ommended.
MAX793/MAX794/MAX795
3.0V/3.3V Adjustable Microprocessor
Supervisory Circuits
______________________________________________________________________________________ 17
MAX793
MAX794
V
CC
GND
0V
TO µP
V
L
= R1
V
PFT
PFI
PFO
R1
R2 R3
*OPTIONAL
C1*
V
IN
V
TRIP
V
IN
PFO
0V
V
H
V
L
R1
+ R2
R2
V
H
= (V
PFT
+ V
PFH
) (R1)
V
TRIP
= V
PFT
+
R1
1
+
R2
1
R3
1
–
R3
V
CC
V
PFT
= 1.237V
V
PFH
= 10mV
WHERE
V
CC
GND
TO µP
PFI
PFO
R1
R2 R3
*OPTIONAL
C1*
V
IN
R1
+ R2
)
R2
V
H
= R1 (V
PFT
+ V
PFH
)
V
TRIP
= V
PFT
(
+
R1
1
+
R2
1
R3
1
–
R3
V
D
V
PFT
= 1.237V
V
PFH
= 10mV
V
D
= DIODE FORWARD VOLTAGE DROP
V
L
= V
TRIP
WHERE
MAX793
MAX794
0V
PFO
0V
V
H
V
IN
V
TRIP
(b)(a)
(
)
( )
( )
+
R1
1
+
R2
1
R3
1
( )
Figure 16. Adding Hysteresis to the Power-Fail Comparator: (a) Symmetrical Hysteresis, (b) Hysteresis Only on Rising V
IN
MAX793
MAX794
MAX795
V
CC
GND
V
CC
N
RESET
GENERATOR
GND
V
CC
RESET
RESET
µP
Figure 17. Interfacing to µPs with Bidirectional Reset I/O
MAX793/MAX794/MAX795
Figure 18 shows maximum transient duration vs. reset-
comparator overdrive, for which reset pulses are not
generated. The graph was produced using negative-
going V
CC
pulses, starting at 3.3V and ending below
the reset threshold by the magnitude indicated (reset
comparator overdrive). The graph shows the maximum
pulse width a negative-going V
CC
transient can typically
have without causing a reset pulse to be issued. As
the amplitude of the transient increases (i.e., goes far-
ther below the reset threshold), the maximum allowable
pulse width decreases. Typically, a V
CC
transient that
goes 40mV below the reset threshold and lasts for 10µs
or less does not cause a reset pulse to be issued.
A 0.1µF bypass capacitor mounted close to the V
CC
pin provides additional transient immunity.
Watchdog Software Considerations
There is a way to help the watchdog timer monitor soft-
ware execution more closely, which involves setting
and resetting the watchdog input at different points in
the program rather than pulsing the watchdog input
high-low-high or low-high-low. This technique avoids a
stuck loop, in which the watchdog timer would continue
to be reset within the loop, keeping the watchdog from
timing out. Figure 19 shows an example of a flow dia-
gram where the I/O driving the watchdog input is set
high at the beginning of the program, set low at the
beginning of every subroutine or loop, then set high
again when the program returns to the beginning. If the
program should hang in any subroutine, the problem
would quickly be corrected, since the I/O is continually
set low and the watchdog timer is allowed to time out,
causing a reset or interrupt to be issued.
3.0V/3.3V Adjustable Microprocessor
Supervisory Circuits
18 ______________________________________________________________________________________
100
0
10 20 30 100
10
20
30
80
90
MAX793-FIG 18
RESET COMPARATOR OVERDRIVE, V
RST
- V
CC
(mV)
MAXIMUM PULSE DURATION (µs)
40 50 60 70 80 90
60
40
50
70
Figure 18. Maximum Transient Duration without Causing a
Reset Pulse vs. Reset Comparator Overdrive
Figure 19. Watchdog Flow Diagram
START
SET WDI
HIGH
RETURN
PROGRAM
CODE
Subroutine or
Program Loop
SET WDI LOW
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

MAX793RCSE

Mfr. #:
Buy MAX793RCSE
Manufacturer:
Maxim Integrated
Description:
Supervisory Circuits 3.0V/3.3V Adjustable Microprocessor Supervisory Circuits
Lifecycle:
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