Maxim MAX690 Microprocessor supervisory circuit Datasheet

19-4333; Rev 3; 9/93
Microprocessor Supervisory Circuits
_______________________Applications
Battery-Powered Computers and Controllers
Intelligent Instruments
Automotive Systems
Critical µP Power Monitoring
___________________________Features
♦ Precision Supply-Voltage Monitor:
4.65V for MAX690A/MAX802L/MAX805L
4.40V for MAX692A/MAX802M
♦ Reset Time Delay – 200ms
♦ Watchdog Timer – 1.6sec Timeout
♦ Battery-Backup Power Switching
♦ 200µA Quiescent Supply Current
♦ 50nA Quiescent Supply Current in BatteryBackup Mode
♦ Voltage Monitor for Power-Fail or Low-Battery
Warning
♦ Power-Fail Accuracy Guaranteed to ±2%
(MAX802L/M)
♦ Guaranteed RESET Assertion to VCC = 1V
♦ 8-Pin SO and DIP Packages
______________Ordering Information
PART
TEMP. RANGE
PIN-PACKAGE
MAX690ACPA
0°C to +70°C
8 Plastic DIP
MAX690ACSA
0°C to +70°C
8 SO
MAX690AC/D
0°C to +70°C
MAX690AEPA
-40°C to +85°C
8 Plastic DIP
MAX690AESA
-40°C to +85°C
8 SO
MAX690AMJA
-55°C to +125°C
8 CERDIP**
Dice*
Ordering Information continued on last page.
* Dice are specified at TA = +25°C
**Contact factory for availability and processing to MIL-STD-883.
_________________Pin Configurations
__________Typical Operating Circuit
TOP VIEW
UNREGULATED DC REGULATED +5V
R1
O.1µF
VCC
VCC
PFI
R2
PFO
WDI
VBATT
VOUT
GND
NMI
µP
I/O LINE GND
VCC
MAX690A
MAX802L
V OUT
1
V CC
2
RESET
BUS
3.6V
LITHIUM
BATTERY
RESET
GND 3
PFI 4
MAX690A
MAX692A
MAX802L
MAX802M
MAX805L
8
V BATT
7
RESET (RESET)
6
WDI
5
PFO
DIP/SO
CMOS
RAM
GND
( ) ARE FOR MAX805L ONLY.
________________________________________________________________ Maxim Integrated Products
Call toll free 1-800-998-8800 for free samples or literature.
1
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
__________________General Description
The MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
reduce the complexity and number of components
required for power-supply monitoring and battery-control
functions in microprocessor (µP) systems. They significantly improve system reliability and accuracy compared
to separate ICs or discrete components.
These parts provide four functions:
1) A reset output during power-up, power-down, and
brownout conditions.
2) Battery-backup switching for CMOS RAM, CMOS
µP, or other low-power logic.
3) A reset pulse if the optional watchdog timer has not
been toggled within 1.6sec.
4) A 1.25V threshold detector for power-fail warning or
low-battery detection, or to monitor a power supply
other than +5V.
The parts differ in their reset-voltage threshold levels
and reset outputs. The MAX690A/MAX802L/MAX805L
generate a reset pulse when the supply voltage drops
below 4.65V, and the MAX692A/MAX802M generate a
reset below 4.40V. The MAX802L/MAX802M guarantee power-fail accuracies to ±2%. The MAX805L is the
same as the MAX690A except that RESET is provided
instead of RESET.
All parts are available in 8-pin DIP and SO packages.
The MAX690A/MAX802L are pin compatible with the
MAX690 and MAX694. The MAX692A/MAX802M are
pin compatible with the MAX692.
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
Microprocessor Supervisory Circuits
ABSOLUTE MAXIMUM RATINGS
Terminal Voltage (with respect to GND)
VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to 6.0V
VBATT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to 6.0V
All Other Inputs (Note 1) . . . . . . . . . . . . . . . . -0.3V to (VCC + 0.3V)
Input Current
VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200mA
VBATT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50mA
GND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20mA
Output Current
VOUT . . . . . . . . . . . . . . . . . . . Short-Circuit Protected for up to 10sec
All Other Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20mA
Rate of Rise, VCC, VBATT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100V/µs
Continuous Power Dissipation
Plastic DIP (derate 9.09mW/°C above +70°C) . . . . . . . . 727mW
SO (derate 5.88mW/°C above +70°C) . . . . . . . . . . . . . . . . 471mW
CERDIP (derate 8.00mW/°C above +70°C) . . . . . . . . . . . 640mW
Operating Temperature Ranges:
MAX69_AC_ _, MAX80_ _ C_ _ . . . . . . . . . . . . . . . . . . 0°C to +70°C
MAX69_AE_ _, MAX80_ _ E_ _ . . . . . . . . . . . . . . . . -40°C to +85°C
MAX69_AMJA, MAX805LMJA. . . . . . . . . . . . . . . . -55°C to +125°C
Storage Temperature Range . . . . . . . . . . . . . . . . . . -65°C to +160°C
Lead Temperature (soldering, 10sec) . . . . . . . . . . . . . . . . . . +300°C
Note 1: The input voltage limits on PFI and WDI may be exceeded if the current into these pins is limited to less than 10mA.
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V CC = 4.75V to 5.5V for MAX690A/MAX802L/MAX805L, V CC = 4.5V to 5.5V for MAX692A/MAX802M, V BATT = 2.8V,
TA = TMIN to TMAX, unless otherwise noted.)
PARAMETER
SYMBOL
Operating Voltage Range,
VCC, VBATT (Note 2)
Supply Current (Excluding IOUT)
ISUPPLY
ISUPPLY in Battery-Backup Mode
(Excluding IOUT)
CONDITIONS
MAX69_AC, MAX802_C
MIN
1.0
MAX805LC
MAX69_AE/M, MAX80_ _E
VOUT Output
VOUT in Battery-Backup Mode
Battery Switch Threshold, VCC
to VBATT
MAX
5.5
UNITS
1.1
5.5
V
1.2
5.5
MAX69_AC, MAX802_C
200
350
MAX69_AE/M, MAX802_E, MAX805LE/M
TA = +25°C
VCC = 0V,
VBATT = 2.8V
TA = TMIN to TMAX
200
500
5.5V > VCC>
VBATT +0.2V
VBATT Standby Current (Note 3)
TA = +25°C
TA = TMIN to TMAX
0.05
-0.1
IOUT = 5mA
IOUT = 50mA
VCC - 0.5
IOUT = 250µA, VCC < VBATT - 0.2V
VBATT - 0.1 VBATT - 0.02
VCC < VRT
2
VCC - 0.25
Power-up
20
Power-down
-20
40
VRT
4.65
4.75
MAX692A, MAX802M
MAX802L, TA = +25°C, VCC falling
4.25
4.40
4.50
MAX802M, TA = +25°C, VCC falling
4.30
4.70
140
VCC - 1.5
µA
V
V
V
4.45
40
tRS
µA
mV
4.50
4.55
µA
mV
MAX690A, MAX802L, MAX805L
ISOURCE = 800µA
RESET Output Voltage
0.02
-1.0
0.02
VCC - 0.05 VCC - 0.025
Reset Threshold Hysteresis
Reset Pulse Width
1.0
5.0
Battery Switchover Hysteresis
Reset Threshold
TYP
200
mV
280
ISINK = 3.2mA
0.4
MAX69_AC, MAX802_C, VCC = 1.0V
ISINK = 50µA
0.3
MAX69_AE/M, MAX802_E,
VCC = 1.2V, ISINK = 100µA
0.3
_______________________________________________________________________________________
ms
V
Microprocessor Supervisory Circuits
(V CC = 4.75V to 5.5V for MAX690A/MAX802L/MAX805L, V CC = 4.5V to 5.5V for MAX692A/MAX802M, V BATT = 2.8V,
TA = TMIN to TMAX, unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MAX805LC, ISOURCE = 4µA, VCC = 1.1V
MAX805LE/M, ISOURCE = 4µA, VCC = 1.2V
RESET Output Voltage
MAX805L, ISOURCE = 800µA
MIN
TYP
0.9
VCC - 1.5
tWD
WDI Pulse Width
tWP
WDI Input Threshold (Note 4)
0.4
1.00
VIL = 0.4V, VIH = (0.8) (VCC)
VCC = 5V
PFI Input Threshold
3.5
50
150
-150
1.20
1.25
1.30
MAX802_C/E, VCC = 5V
1.225
1.250
1.275
-25
VCC - 1.5
0.01
25
ISOURCE = 800µA
ISINK = 3.2mA
sec
ns
0.8
WDI = 0V
MAX69_A, MAX805L, VCC = 5V
PFI Input Current
PFO Output Voltage
2.25
50
WDI = VCC
WDI Input Current
1.60
Logic low
Logic high
UNITS
V
MAX805L, ISINK = 3.2mA
Watchdog Timeout
MAX
0.8
-50
0.4
V
µA
V
nA
V
Note 2: Either VCC or VBATT can go to 0V, if the other is greater than 2.0V.
Note 3: "-" = battery-charging current, "+" = battery-discharging current.
Note 4: WDI is guaranteed to be in an intermediate, non-logic level state if WDI is floating and VCC is in the operating voltage range.
WDI is internally biased to 35% of VCC with an input impedance of 50kΩ.
_______________________________________________________________________________________
3
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
ELECTRICAL CHARACTERISTICS (continued)
__________________________________________Typical Operating Characteristics
OUTPUT VOLTAGE
vs. LOAD CURRENT
5.00
2.78
VOUT (V)
4.90
4.85
VBATT = OV
TA = +25°C
VCC = 0V
VBATT = +2.8V
TA = +25°C
VCC
2.76
+5V
0V
VCC
VCC
2.74
SLOPE = 80Ω
GND
0V
2.70
0
10
20
40
30
0
50
0.4
0.2
IOUT (mA)
0.6
0.8
1.0
500ms/div
IOUT (mA)
MAX690A
RESET RESPONSE TIME
POWER-FAIL COMPARATOR
RESPONSE TIME
+5V
VCC
POWER-FAIL COMPARATOR
RESPONSE TIME
+5V
PFO
VCC
RESET
GND
PFI
PFO
PFO
1k
30pF
RESET
30pF
+1.30V
PFI
+0V
0V
PFI
+1.2V
2µs/div
+1.20V
400ns/div
MAX805L
RESET OUTPUT VOLTAGE
vs. SUPPLY VOLTAGE
400ns/div
MAX805L
RESET RESPONSE TIME
+5V
VCC
1V/div
+5V
10k
+5V
VCC
0V
+4V
VCC
RESET
GND
10k
330pF
+4V
330
pF
1V/div
RESET
0V
0V
500ms/div
4
+3V
+1.3V
VCC = +5V
TA = +25°C
0V
VCC
VBATT
RESET
GND
1k
+1.25V
30pF
+1.25V
10k
0V
VCC = +5V
TA = +25°C
PFO
+4V
TA = +25°C
VCC
+5V
PFI
+5V
+5V
RESET
330pF
RESET
RESET
2.72
4.75
2k
1V/div
SLOPE = 5Ω
4.80
1V/div
VOUT (V)
+5V
2.80
VCC = +5V
VBATT = +2.8V
TA = +25°C
4.95
RESET
MAX690A
RESET OUTPUT VOLTAGE
vs. SUPPLY VOLTAGE
OUTPUT VOLTAGE
vs. LOAD CURRENT
1V/div
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
Microprocessor Supervisory Circuits
2µs/div
_______________________________________________________________________________________
Microprocessor Supervisory Circuits
PIN
NAME
FUNCTION
1
VOUT
Supply Output for CMOS RAM. When VCC is above the reset threshold,
VOUT connects to VCC through a P-channel MOSFET switch. When VCC
is below the reset threshold, the higher of VCC or VBATT will be
connected to VOUT.
2
2
VCC
+5V Supply Input
3
3
GND
Ground
4
4
PFI
Power-Fail Comparator Input. When PFI is less than 1.25V, PFO goes
low. Connect PFI to GND or VCC when not used.
5
5
PFO
Power-Fail Output. When PFI is less than 1.25V, PFO goes low;
otherwise PFO stays high.
WDI
Watchdog Input. If WDI remains high or low for 1.6sec, the internal
watchdog timer runs out and reset is triggered. If WDI is left floating or
connected to a high-impedance three-state buffer, the watchdog
feature is disabled. The internal watchdog timer clears whenever reset
is asserted, WDI is three-stated, or WDI sees a rising or falling edge.
RESET
Reset Output. Whenever RESET is triggered, it pulses low for 200ms. It
stays low when VCC is below the reset threshold (4.65V in the
MAX690A/MAX802L and 4.4V in the MAX692A/MAX802M) and remains
low for 200ms after VCC rises above the reset threshold. A watchdog
timeout also triggers RESET.
Active-High Reset Output is the inverse of RESET. When RESET is
asserted, the RESET output voltage = VCC or VBATT, whichever is
higher.
MAX690A/MAX692A
MAX802L/MAX802M
MAX805L
1
6
6
7
–
7
RESET
8
8
VBATT
Backup-Battery Input. When VCC falls below the reset threshold, VBATT
will be switched to VOUT if VBATT is 20mV greater than VCC. When VCC
rises to 20mV above VBATT, VOUT will be reconnected to VCC. The 40mV
hysteresis prevents repeated switching if VCC falls slowly.
_______________________________________________________________________________________
5
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
______________________________________________________________Pin Description
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
Microprocessor Supervisory Circuits
VBATT
+5V
BATTERY-SWITCHOVER
CIRCUITRY
VOUT
VCC
0V
VCC
RESET
GENERATOR
3.5V
RESET
(RESET)
1.25V
0V
+5V
tRS
3.0V
RESET
0V
+5V
(RESET)
WATCHDOG
TIMER
WDI
VOUT
3.0V
0V
MAX690A
MAX692A
MAX802L
MAX802M
MAX805L
0.8V
PFI
+5V
PFO
PFO
0V
1.25V
( ) ARE FOR MAX805L ONLY.
GND
( ) ARE FOR MAX805L ONLY.
Figure 1. Block Diagram
Figure 2. Timing Diagram
_______________Detailed Description
Reset Output
A microprocessor’s (µP’s) reset input starts the µP in a
known state. When the µP is in an unknown state, it should
be held in reset. The MAX690A/MAX692A/MAX802L/
MAX802M assert reset during power-up and prevent code
execution errors during power-down or brownout conditions.
On power-up, once VCC reaches 1V, RESET is guaranteed to be a logic low. As VCC rises, RESET remains
low. When VCC exceeds the reset threshold, an internal
timer keeps RESET low for a time equal to the reset
pulse width; after this interval, RESET goes high (Figure
2). If a brownout condition occurs (if VCC dips below
the reset threshold), RESET is triggered. Each time
RESET is triggered, it stays low for the reset pulse width
interval. Any time VCC goes below the reset threshold,
the internal timer restarts the pulse. If a brownout condition interrupts a previously initiated reset pulse, the
reset pulse continues for another 200ms. On powerdown, once VCC goes below the threshold, RESET is
guaranteed to be logic low until VCC droops below 1V.
RESET is also triggered by a watchdog timeout. If a
high or low is continuously applied to the WDI pin for
1.6sec, RESET pulses low. As long as RESET is assert6
VBATT = PFI = 3.0V
IOUT = 0mA
ed, the watchdog timer remains clear. When RESET
comes high, the watchdog resumes timing and must be
serviced within 1.6sec. If WDI is tied high or low, a
RESET pulse is triggered every 1.8sec (tWD plus tRS).
The MAX805L active-high RESET output is the inverse
of the MAX690A/MAX692A/MAX802L/MAX802M RESET
output, and is guaranteed to be valid with VCC down to
1.1V. Some µPs, such as Intel’s 80C51, require an
active-high reset pulse.
Watchdog Input
The watchdog circuit monitors the µP’s activity. If the
µP does not toggle the watchdog input (WDI) within
1.6sec, a reset pulse is triggered. The internal 1.6sec
timer is cleared by either a reset pulse or by open circuiting the WDI input. As long as reset is asserted or
the WDI input is open circuited, the timer remains
cleared and does not count. As soon as reset is
released or WDI is driven high or low, the timer starts
counting. It can detect pulses as short as 50ns.
Power-Fail Comparator
The PFI input is compared to an internal 1.25V reference. If PFI is less than 1.25V, PFO goes low. The
power-fail comparator is intended for use as an undervoltage detector to signal a failing power supply; it
_______________________________________________________________________________________
Microprocessor Supervisory Circuits
VCC
+5V
SW1
SW2
SW4
SW3
D1
VOUT
VCC
TO STATIC RAM
D2
MAX690A
MAX802L
VBATT MAX805L RESET
(RESET)
SUBSTRATE
MAX690A MAX802L MAX805L
MAX692A MAX802M
D3
TO µP
+
0.1F
VOUT
GND
CONDITION
SW1/SW2
SW3/SW4
VCC > Reset Threshold
Open
Closed
VCC < Reset Threshold and
VCC > VBATT
Open
Closed
VCC < Reset Threshold and
VCC < VBATT
Closed
Open
( ) ARE FOR MAX805L ONLY.
RESET THRESHOLD = 4.65V IN MAX690A/MAX802L/MAX805L.
RESET THRESHOLD = 4.4V IN MAX692A/MAX802M
Figure 3. Backup-Battery Switchover Block Diagram
need not be dedicated to this function though, as it is
completely separate from the rest of the circuitry. The
external voltage divider drives PFI to sense the unregulated DC input to the +5V regulator (see Typical
Operating Circuit). The voltage-divider ratio can be
chosen such that the voltage at PFI falls below 1.25V
just before the +5V regulator drops out. PFO then triggers an interrupt which signals the µP to prepare for
power-down.
To conserve backup-battery power, the power-fail
detector comparator is turned off and PFO is forced
low when VBATT connects to VOUT.
Figure 4. Using a SuperCap as a Backup Power Source with a
MAX690A/MAX802L/MAX805L and a +5V ±5% Supply
than VCC.
When VCC exceeds the reset threshold, it is connected to
the MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
substrate, regardless of the voltage applied to VBATT
(Figure 3). During this time, the diode (D1) between VBATT
and the substrate will conduct current from VBATT to VCC if
Table 1. Input and Output Status in Battery-Backup Mode
SIGNAL
VCC
Disconnected from VOUT
VOUT
Connected to VBATT through an internal 80Ω
PMOS switch
VBATT
Connected to VOUT. Current drawn from the
battery is less than 1µA, as long as
VCC < VBATT - 1V.
Backup-Battery Switchover
In the event of a brownout or power failure, it may be
necessary to preserve the contents of RAM. With a
backup battery installed at VBATT, the devices automatically switch RAM to backup power when VCC fails.
As long as VCC exceeds the reset threshold, VOUT connects
to VCC through a 5Ω PMOS power switch. Once VCC falls
below the reset threshold, VCC or VBATT (whichever is higher) switches to VOUT. Unlike the MAX690/MAX692, the
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L don’t
always connect VBATT to VOUT when VBATT is greater than
VCC. VBATT connects to VOUT (through an 80Ω switch) only
when VCC is below the reset threshold and VBATT is greater
STATUS
PFI
Power-fail comparator is disabled.
PFO
Logic low
RESET
Logic low
RESET
Logic high (MAX805L only)
WDI
Watchdog timer is disabled
_______________________________________________________________________________________
7
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
VBATT
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
Microprocessor Supervisory Circuits
VIN
+5V
+5V
R1
VOUT
VCC
VCC
TO STATIC RAM
MAX692A
MAX802M
PFI
RESET
VBATT
TO µP
R3
R2
C1*
+
100k
0.1F
MAX690
MAX692A
MAX802L
MAX802M
MAX805L
PFO
GND
GND
TO µP
*OPTIONAL
+5V
Figure 5. Using a SuperCap™ as a Backup Power Source with
the MAX692A/MAX802M and a +5V ±10% Supply
PFO
0V
VBATT is 0.6V or greater than VCC.
When VBATT connects to VOUT, backup mode is activated and the internal circuitry is powered from the battery
(Table 1). When VCC is just below VBATT, the current
drawn from VBATT is typically 30µA. When VCC drops to
more than 1V below VBATT, the internal switchover comparator shuts off and the supply current falls to less
than 1µA.
__________Applications Information
VL
0V

VTRIP = 1.25/ 
 R2
R2
+ R2
VTRIP
VIN
VH



 R2 || R3 
VIH = 1.25/
 R1 + R2 || R3 
VL − 1.25 5 − 1.25
1.25
+
=
R1
R3
R2
Figure 6. Adding Hysteresis to the Power-Fail Comparator
Using a SuperCap™ as a Backup
Power Source
SuperCaps are capacitors with extremely high capacitance values, on the order of 0.1F. Figure 4 shows a
SuperCap used as a backup power source. Do not allow
the SuperCap’s voltage to exceed the maximum reset
threshold by more than 0.6V. In Figure 4’s circuit, the
SuperCap rapidly charges to within a diode drop of VCC.
However, after a long time, the diode leakage current will
pull the SuperCap voltage up to VCC. When using a
SuperCap with the MAX690A/MAX802L/MAX805L, VCC
may not exceed 4.75V + 0.6V = 5.35V.
Use the SuperCap circuit of Figure 5 with a MAX692A
or MAX802M and a ±10% supply. This circuit
ensures that the SuperCap only charges to V CC 0.5V. At the maximum V CC of 5.5V, the SuperCap
charges up to 5.0V, only 0.5V above the maximum
reset threshold—well within the requisite 0.6V.
Allowable Backup Power-Source
Batteries
Lithium batteries work very well as backup batteries due to
very low self-discharge rates and high energy density.
Single lithium batteries with open-circuit voltages of 3.0V to
3.6V are ideal. Any battery with an open-circuit voltage
less than the minimum reset threshold plus 0.3V can be
Table 2. Allowable Backup-Battery Voltages
(see Using a SuperCap as a Backup Power Source
section for use with a SuperCap)
MAXIMUM BACKUP-BATTERY
PART NO.
VOLTAGE (V)
MAX690A/
4.80
MAX802L/MAX805L
MAX692A/
4.55
MAX802M
™SuperCap is a trademark of Baknor Industries.
8
_______________________________________________________________________________________
Microprocessor Supervisory Circuits
Operation
Without a Backup Power Source
If a backup power source is not used, ground VBATT
and connect VOUT to VCC. Since there is no need to
switch over to any backup power source, VOUT does
not need to be switched. A direct connection to VCC
eliminates any voltage drops across the switch which
may push VOUT below VCC.
Replacing the Backup Battery
The backup battery can be removed while VCC remains
valid, without danger of triggering RESET/RESET. As
long as VCC stays above the reset threshold, batterybackup mode cannot be entered. In other switchover
+5V
R1
VCC
PFI
R2
MAX690A
MAX692A
MAX802L
MAX802M
MAX805L
PFO
ICs where battery-backup mode is entered whenever
VBATT gets close to VCC, an unconnected VBATT pin
accumulates leakage charge and triggers
RESET/RESET in error.
Adding Hysteresis to the
Power-Fail Comparator
Hysteresis adds a noise margin to the power-fail comparator and prevents repeated triggering of PFO when
VIN is close to its trip point. Figure 6 shows how to add
hysteresis to the power-fail comparator. Select the
ratio of R1 and R2 such that PFI sees 1.25V when VIN
falls to its trip point (VTRIP). R3 adds the hysteresis. It
will typically be an order of magnitude greater than R1
or R 2 (about 10 times either R 1 or R 2). The current
through R1 and R2 should be at least 1µA to ensure
that the 25nA (max) PFI input current does not shift the
trip point. R3 should be larger than 10kΩ so it does not
load down the PFO pin. Capacitor C1 adds additional
noise rejection.
Monitoring a Negative Voltage
The power-fail comparator can be used to monitor a
negative supply rail using the circuit of Figure 7. When
the negative rail is good (a negative voltage of large
magnitude), PFO is low. When the negative rail is
degraded (a negative voltage of lesser magnitude),
PFO goes high. This circuit’s accuracy is affected by
the PFI threshold tolerance, the VCC line, and the resistors.
Interfacing to µPs with
Bidirectional Reset Pins
µPs with bidirectional reset pins, such as the Motorola
68HC11
series,
can
contend
with
the
MAX690A/MAX692A/MAX802L/MAX802M RESET output. If, for example, the RESET output is driven high
and the µP wants to pull it low, indeterminate logic levels may result. To correct this, connect a 4.7kΩ resistor between the RESET output and the µP reset I/O, as
in Figure 8. Buffer the RESET output to other system
components
GND
V-
+5V
PFO
0V
VTRIP
V-
5 − 1.25
0V
1.25 - VTRIP
=
NOTE:
R1 VTRIP IS NEGATIVE
R2
Figure 7. Monitoring a Negative Voltage
_______________________________________________________________________________________
9
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
connected directly to the V BATT input of the
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L with
no additional circuitry (see the Typical Operating Circuit).
However, batteries with open-circuit voltages that are
greater cannot be used for backup, as current is sourced
into the substrate through the diode (D1 in Figure 3) when
VCC is close to the reset threshold.
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
Microprocessor Supervisory Circuits
BUFFERED RESET TO OTHER SYSTEM COMPONENTS
VCC
MAX690A
MAX692A
MAX802L RESET
MAX802M
GND
VCC
4.7k
RESET
GND
Figure 8. Interfacing to µPs with Bidirectional Reset I/O
10
______________________________________________________________________________________
Microprocessor Supervisory Circuits
Part
Number
Minimum
Nominal
Reset
Reset
Pulse
Threshold Width
(V)
(ms)
Nominal
Watchdog
Timeout Backup- CE Period
Battery Write
(sec)
Switch Protect
MAX690A/692A
4.65/4.40
140
1.6
✔
MAX691A/693A
4.65/4.40
140/adj.
1.6/adj.
✔
MAX696
Adj.
35/adj.
1.6/adj.
✔
MAX697
Adj.
35/adj.
1.6/adj.
MAX700
4.65/adj.
200
-
MAX703/704
4.65/4.40
140
-
MAX705/706
4.65/4.40
140
MAX706P
2.63
140
MAX706R/S/T
2.63/2.93/
3.08
MAX707/708
MAX708R/S/T
MAX709L/M/
R/S/T
PowerFail
Comparator
Manual- Watch- LowReset
dog
Line
Input
Output Output
Active- BatteryHigh
On
Reset
Output
✔
✔/10ns
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
1.6
✔
✔
✔
1.6
✔
✔
✔
140
1.6
✔
✔
✔
4.65/4.40
140
-
✔
✔
✔
2.63/2.93/
3.08
140
-
✔
✔
✔
4.65/4.40/
140
2.63/2.93/3.08
✔
✔/10ns
✔
✔
✔
✔
✔
✔/10ns
✔
✔
✔
✔
✔
✔
✔
✔
MAX791
4.65
140
1
MAX792L/M/
R/S/T
4.65/4.40/
140
2.63/2.93/3.08
1
MAX800L/M
4.60/4.40
140
1.6/adj.
✔
MAX802L/M
4.60/4.40
140
1.6
✔
✔/±2%
MAX805L
4.65
140
1.6
✔
✔
MAX813L
4.65
140
1.6
MAX820L/M/
R/S/T
4.65/4.40/
140
2.63/2.93/3.08
1
MAX1232
4.37/4.62
250
0.15/0.60/1.2
MAX1259
-
-
-
✔/10ns
✔/10ns
✔/±2%
✔
✔
✔
✔
✔
✔
✔/±2%
✔
✔
✔
✔
✔
✔
✔
✔
✔
✔
______________________________________________________________________________________
11
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
________________________µ P Supervisory Circuits
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
Microprocessor Supervisory Circuits
__Ordering Information (continued)
PART
TEMP. RANGE
MAX692ACPA
0°C to +70°C
8 Plastic DIP
MAX692ACSA
0°C to +70°C
8 SO
MAX692AC/D
0°C to +70°C
MAX692AEPA
-40°C to +85°C
8 Plastic DIP
MAX692AESA
-40°C to +85°C
8 SO
MAX692AMJA
-55°C to +125°C
8 CERDIP**
MAX802LCPA
0°C to +70°C
8 Plastic DIP
MAX802LCSA
0°C to +70°C
8 SO
MAX802LEPA
-40°C to +85°C
MAX802LESA
-40°C to +85°C
MAX802MCPA
____________________Chip Topography
PIN-PACKAGE
V CC
V OUT V BATT
Dice*
RESET (RESET)
0.061"
WDI
(1.55mm)
GND
8 Plastic DIP
8 SO
0°C to +70°C
8 Plastic DIP
MAX802MCSA
0°C to +70°C
8 SO
MAX802MEPA
-40°C to +85°C
MAX802MESA
-40°C to +85°C
MAX805LCPA
0°C to +70°C
8 Plastic DIP
MAX805LCSA
0°C to +70°C
8 SO
MAX805LC/D
0°C to +70°C
MAX805LEPA
-40°C to +85°C
8 Plastic DIP
8 SO
PFI PFO
0.078"
(1.98mm)
( ) ARE FOR MAX805L ONLY.
TRANSISTOR COUNT: 573;
SUBSTRATE MUST BE LEFT UNCONNECTED.
Dice*
8 Plastic DIP
MAX805LESA
-40°C to +85°C
8 SO
MAX805LMJA
-55°C to +125°C
8 CERDIP**
* Dice are specified at TA = +25°C.
**Contact factory for availability and processing to MIL-STD-883.
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
12 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600
© 1993 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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