MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
Microprocessor Supervisory Circuits
________________________________________________________________ Maxim Integrated Products 1
1
2
3
4
8
7
6
5
VBATT
RESET (RESET)
WDI
PFO
PFI
GND
VCC
VOUT
DIP/SO
TOP VIEW
MAX690A
MAX692A
MAX802L
MAX802M
MAX805L
( ) ARE FOR MAX805L ONLY.
_________________Pin Configurations
VCC
NMI
I/O LINE GND
BUS
VCCCMOS
RAM GND
UNREGULATED DC REGULATED +5V
RESET
PFO
WDI
VOUT
GND
VBATT
VCC
PFI
R1
R2
O.1µF
3.6V
LITHIUM
BATTERY
MAX690A
MAX802L
µP
RESET
__________Typical Operating Circuit
19-4333; Rev 4; 12/05
__________________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 signifi-
cantly 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 guaran-
tee 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.
_______________________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 Battery-
Backup Mode
♦Voltage Monitor for Power-Fail or Low-Battery
Warning
♦Power-Fail Accuracy Guaranteed to ±2%
(MAX802L/M)
♦Guaranteed RREESSEETTAssertion to VCC = 1V
♦8-Pin SO and DIP Packages
______________Ordering Information
PART TEMP. RANGE PIN-PACKAGE
MAX690ACPA 8Plastic DIP
MAX690ACSA 0°C to +70°C 8 SO
MAX690AC/D 0°C to +70°C Dice*
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**
Ordering Information continued on last page.
*Dice are specified at TA= +25°C
**Contact factory for availability and processing to MIL-STD-883.
Devices in PDIP and SO packages are available in both leaded
and lead-free packaging. Specify lead free by adding the +
symbol at the end of the part number when ordering. Lead free
not available for CERDIP package.
0°C to +70°C
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
Microprocessor Supervisory Circuits
2_______________________________________________________________________________________
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
ELECTRICAL CHARACTERISTICS
(VCC = 4.75V to 5.5V for MAX690A/MAX802L/MAX805L, VCC = 4.5V to 5.5V for MAX692A/MAX802M, VBATT = 2.8V,
TA= TMIN to TMAX, unless otherwise noted.)
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.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Operating Voltage Range,
VCC, VBATT (Note 2)
MAX69_AC, MAX802_C 1.0 5.5
VMAX805LC
MAX69_AE/M, MAX80_ _E 1.2 5.5
Supply Current (Excluding IOUT) ISUPPLY
MAX69_AC, MAX802_C 200 350 µA
MAX69_AE/M, MAX802_E, MAX805LE/M 200 500
ISUPPLY in Battery-Backup Mode
(Excluding IOUT)
VCC = 0V,
VBATT = 2.8V
0.05 1.0
5.0
5.5V > VCC>
VBATT +0.2V
-0.1 0.02 µA
-1.0 0.02
VOUT Output IOUT = 5mA VCC - 0.05 VCC - 0.025 V
ABSOLUTE MAXIMUM RATINGS
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.
VBATT Standby Current (Note 3)
VCC - 0.5 VCC - 0.25
VOUT in Battery-Backup Mode IOUT = 250µA, VCC < VBATT - 0.2V VBATT - 0.1 VBATT - 0.02 V
IOUT = 50mA
Battery Switch Threshold, VCC
to VBATT
VCC < VRT
20 mV
-20
µA
TA= +25°C
TA= +25°C
TA= TMIN to TMAX
Power-up
Power-down
Battery Switchover Hysteresis 40 mV
Reset Threshold VRT
MAX690A, MAX802L, MAX805L
V
MAX692A, MAX802M 4.25 4.40 4.50
MAX802L, TA= +25°C, VCC falling 4.55 4.70
MAX802M, TA= +25°C, VCC falling 4.30 4.45
Reset Threshold Hysteresis 40 mV
Reset Pulse Width tRS 140 200 280 ms
RESET Output Voltage
ISOURCE = 800µA VCC - 1.5
V
ISINK = 3.2mA 0.4
0.3
0.3
TA= TMIN to TMAX
MAX69_AC, MAX802_C, VCC = 1.0V
ISINK = 50µA
MAX69_AE/M, MAX802_E,
VCC = 1.2V, ISINK = 100µA
1.1 5.5
4.50 4.65 4.75
V
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
Microprocessor Supervisory Circuits
_______________________________________________________________________________________ 3
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
RESET Output Voltage
MAX805LC, ISOURCE = 4µA, VCC = 1.1V 0.8
MAX805LE/M, ISOURCE = 4µA, VCC = 1.2V 0.9
MAX805L, ISOURCE = 800µA VCC - 1.5
MAX805L, ISINK = 3.2mA 0.4
Watchdog Timeout tWD 1.00 1.60 2.25 sec
PFO Output Voltage ISOURCE = 800µA VCC - 1.5 V
ISINK = 3.2mA 0.4
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.
ELECTRICAL CHARACTERISTICS (continued)
(VCC = 4.75V to 5.5V for MAX690A/MAX802L/MAX805L, VCC = 4.5V to 5.5V for MAX692A/MAX802M, VBATT = 2.8V,
TA= TMIN to TMAX, unless otherwise noted.)
WDI = 0V -150 -50
V
WDI Input Current WDI = VCC 50 150
3.5
µA
WDI Input Threshold (Note 4) VCC = 5V 0.8 V
WDI Pulse Width tWP VIL = 0.4V, VIH = (0.8) (VCC)50 ns
Logic low
Logic high
MAX69_A, MAX805L, VCC = 5V 1.20 1.25 1.30
PFI Input Threshold MAX802_C/E, VCC = 5V 1.225 1.250 1.275
PFI Input Current -25 0.01 25 nA
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
Microprocessor Supervisory Circuits
4_______________________________________________________________________________________
__________________________________________Typical Operating Characteristics
MAX805L
RESET OUTPUT VOLTAGE
vs. SUPPLY VOLTAGE
500ms/div
0V
VCC
+5V
1V/div
RESET
0V
+5V
1V/div
10k
330
pF
VCC
RESET
GND
VBATT
MAX805L
RESET RESPONSE TIME
2µs/div
0V
VCC +4V
0V
+4V
+5V
GND
RESET
VCC
330pF
10k
4.75
0
OUTPUT VOLTAGE
vs. LOAD CURRENT
4.80
5.00
IOUT (mA)
VOUT (V)
40
4.90
4.85
10 30 50
4.95
20
VCC = +5V
VBATT = +2.8V
TA = +25°C
SLOPE = 5Ω
2.70
0
OUTPUT VOLTAGE
vs. LOAD CURRENT
2.72
2.80
IOUT (mA)
VOUT (V)
0.8
2.76
2.74
0.2 0.6 1.0
2.78
0.4
VCC = 0V
VBATT = +2.8V
TA = +25°C
SLOPE = 80Ω
MAX690A
RESET OUTPUT VOLTAGE
vs. SUPPLY VOLTAGE
500ms/div
VCC
0V
RESET
0V
+5V
1V/div
+5V
1V/div
VCC
GND
RESET
VCC 2k
RESET
330pF
TA = +25°C
VBATT = OV
MAX690A
RESET RESPONSE TIME
2µs/div
VCC
+5V
RESET
+5V
+4V
1V/div
0V
TA = +25°C
VCC
GND
RESET
VCC 10k
RESET
30pF
POWER-FAIL COMPARATOR
RESPONSE TIME
400ns/div
PFI
+1.30V
PFO
+5V
+0V
+1.2V
+5V
PFO
30pF
1k
+1.25V
PFI
VCC = +5V
TA = +25°C
POWER-FAIL COMPARATOR
RESPONSE TIME
400ns/div
PFI
+1.20V
PFO
0V
+3V
+1.3V
VCC = +5V
TA = +25°C
+5V
PFO
30pF 1k
+1.25V
PFI
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
Microprocessor Supervisory Circuits
_______________________________________________________________________________________ 5
______________________________________________________________Pin Description
NAME FUNCTION
MAX805L
1 1 VOUT
2 2 VCC +5V Supply Input
3 3 GND Ground
4 4 PFI
5 5 PFO
6 6 WDI
7RESET
8 8 VBATT
Power-Fail Comparator Input. When PFI is less than 1.25V, PFO goes
low. Connect PFI to GND or VCC when not used.
Power-Fail Output. When PFI is less than 1.25V, PFO goes low;
otherwise PFO stays high.
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.
PIN
MAX690A/MAX692A
MAX802L/MAX802M
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.
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.
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.
– 7 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
_______________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 condi-
tions.
On power-up, once VCC reaches 1V, RESET is guaran-
teed 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 con-
dition interrupts a previously initiated reset pulse, the
reset pulse continues for another 200ms. On power-
down, 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 assert-
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 cir-
cuiting 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 refer-
ence. If PFI is less than 1.25V, PFO goes low. The
power-fail comparator is intended for use as an under-
voltage detector to signal a failing power supply; it
need not be dedicated to this function though, as it is
Microprocessor Supervisory Circuits
6_______________________________________________________________________________________
1.25V
0.8V
WATCHDOG
TIMER
PFO
RESET
(RESET)
VOUT
RESET
GENERATOR
BATTERY-SWITCHOVER
CIRCUITRY
1.25V
3.5V
VBATT
VCC
PFI
WDI
MAX690A
MAX692A
MAX802L
MAX802M
MAX805L
( ) ARE FOR MAX805L ONLY. GND
Figure 1. Block Diagram
tRS
+5V
0V
VCC
0V
+5V
0V
RESET
+5V
0V
(RESET)
3.0V
3.0V
VOUT
+5V
0V
PFO
VBATT = PFI = 3.0V
IOUT = 0mA
( ) ARE FOR MAX805L ONLY.
Figure 2. Timing Diagram
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
Microprocessor Supervisory Circuits
_______________________________________________________________________________________ 7
SW1/SW2 SW3/SW4CONDITION
VCC > Reset Threshold Open Closed
Closed
Open
Open
Closed
VCC < Reset Threshold and
VCC > VBATT
VCC < Reset Threshold and
VCC < VBATT
RESET THRESHOLD = 4.65V IN MAX690A/MAX802L/MAX805L.
RESET THRESHOLD = 4.4V IN MAX692A/MAX802M
VOUT
D3
SUBSTRATE
D1 D2
SW2
SW1 SW4
SW3
VBATT VCC
MAX690A
MAX692A
MAX802L
MAX802M
MAX805L
Figure 3. Backup-Battery Switchover Block Diagram
VBATT
VCC VOUT
RESET
(RESET)
GND
TO STATIC RAM
TO µP
0.1F
MAX690A
MAX802L
MAX805L
+5V
+
( ) ARE FOR MAX805L ONLY.
Figure 4. Using a SuperCap as a Backup Power Source with a
MAX690A/MAX802L/MAX805L and a +5V ±5% Supply
completely separate from the rest of the circuitry. The
external voltage divider drives PFI to sense the unregu-
lated 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 trig-
gers 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.
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 automati-
cally switch RAM to backup power when VCC fails.
As long as VCC exceeds the reset threshold, VOUT connects
to VCC through a 5ΩPMOS power switch. Once VCC falls
below the reset threshold, VCC or VBATT (whichever is high-
er) 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 80Ωswitch) only
when VCC is below the reset threshold and VBATT is greater
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
VBATT is 0.6V or greater than VCC.
Table 1. Input and Output Status in Battery-Backup Mode
SIGNAL STATUS
VCC Disconnected from VOUT
VOUT
VBATT
PFI Power-fail comparator is disabled.
PFO Logic low
RESET Logic low
RESET Logic high (MAX805L only)
WDI Watchdog timer is disabled
Connected to VBATT through an internal 80Ω
PMOS switch
Connected to VOUT. Current drawn from the
battery is less than 1µA, as long as
VCC < VBATT - 1V.
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
When VBATT connects to VOUT, backup mode is activat-
ed 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 com-
parator shuts off and the supply current falls to less
than 1µA.
__________Applications Information
Using a SuperCap™ as a Backup
Power Source
SuperCaps are capacitors with extremely high capaci-
tance 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 VCC -
0.5V. At the maximum VCC of 5.5V, the SuperCap
charges up to 5.0V, only 0.5V above the maximum
reset threshold—well within the requisite 0.6V.
Microprocessor Supervisory Circuits
8_______________________________________________________________________________________
VBATT
VCC VOUT
RESET
GND
TO STATIC RAM
TO µP
0.1F
MAX692A
MAX802M
+5V
+
100k
Figure 5. Using a SuperCap™ as a Backup Power Source with
the MAX692A/MAX802M and a +5V ±10% Supply
PFO
VCC
GND
TO µP
MAX690
MAX692A
MAX802L
MAX802M
MAX805L
+5V
R2
R1
R3
PFI
*OPTIONAL
+5V
0V
PFO
VLVH
VTRIP
VIN
0V
VIN
C1*
Figure 6. Adding Hysteresis to the Power-Fail Comparator
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
connected directly to the VBATT input of the
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L with
Table 2. Allowable Backup-Battery Voltages
(see Using a SuperCap as a Backup Power Source
section for use with a SuperCap)
PART NO. MAXIMUM BACKUP-BATTERY
VOLTAGE (V)
MAX690A/
MAX802L/MAX805L 4.80
MAX692A/
MAX802M 4.55
VTRIP 1.25/ R2
R2R2
VIH 1.25/ R2|| R3
R1R2|| R3
V
L1.25
R1
5 1.25
R3
1.25
R2
=+
=+
−+−=
™SuperCap is a trademark of Baknor Industries.
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
Microprocessor Supervisory Circuits
_______________________________________________________________________________________ 9
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.
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, battery-
backup mode cannot be entered. In other switchover
ICs where battery-backup mode is entered whenever
VBATT gets close to VCC, an unconnected VBATT pin
PFO
VCC
GND
MAX690A
MAX692A
MAX802L
MAX802M
MAX805L
+5V
R2
R1
PFI
+5V
0V
PFO
VTRIP
V- 0V
V-
NOTE: VTRIP IS NEGATIVE
Figure 7. Monitoring a Negative Voltage
5 1.25
R1
1.25 - VTRIP
R2
−=
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 com-
parator 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 R1and R2such that PFI sees 1.25V when VIN
falls to its trip point (VTRIP). R3adds the hysteresis. It
will typically be an order of magnitude greater than R1
or R2(about 10 times either R1or R2). The current
through R1and R2should be at least 1µA to ensure
that the 25nA (max) PFI input current does not shift the
trip point. R3should be larger than 10kΩso 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 resis-
tors.
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 out-
put. If, for example, the RESET output is driven high
and the µP wants to pull it low, indeterminate logic lev-
els may result. To correct this, connect a 4.7kΩresis-
tor between the RESET output and the µP reset I/O, as
in Figure 8. Buffer the RESET output to other system
components
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
Microprocessor Supervisory Circuits
10 ______________________________________________________________________________________
MAX690A
MAX692A
MAX802L
MAX802M
BUFFERED RESET TO OTHER SYSTEM COMPONENTS
4.7k
VCC
GND
VCC
GND
RESET
RESET
Figure 8. Interfacing to µPs with Bidirectional Reset I/O
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
Microprocessor Supervisory Circuits
______________________________________________________________________________________ 11
Minimum Nominal
Nominal Reset Watchdog Power-
Reset Pulse Timeout Backup- CC
CCEE
EE -Fail Manual- Watch- Low- Active- Battery-
Part Threshold Width Period Battery Write Com- Reset dog Line High On
Number (V) (ms) (sec) Switch Protect parator Input Output Output Reset Output
MAX690A/692A 4.65/4.40 140 1.6 ✔✔
MAX691A/693A 4.65/4.40 140/adj. 1.6/adj. ✔✔/10ns ✔✔✔✔✔
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 1.6 ✔✔✔
MAX706P 2.63 140 1.6 ✔✔✔ ✔
MAX706R/S/T 2.63/2.93/ 140 1.6 ✔✔✔
3.08
MAX707/708 4.65/4.40 140 - ✔✔ ✔
MAX708R/S/T 2.63/2.93/ 140 - ✔✔ ✔
3.08
MAX709L/M/ 4.65/4.40/ 140 -
R/S/T 2.63/2.93/3.08
MAX791 4.65 140 1 ✔✔/10ns ✔✔✔✔ ✔✔
MAX792L/M/ 4.65/4.40/ 140 1 ✔/10ns ✔✔✔✔ ✔
R/S/T 2.63/2.93/3.08
MAX800L/M 4.60/4.40 140 1.6/adj. ✔✔/10ns ✔/±2% ✔✔ ✔✔
MAX802L/M 4.60/4.40 140 1.6 ✔✔/±2%
MAX805L 4.65 140 1.6 ✔✔ ✔
MAX813L 4.65 140 1.6 ✔✔✔ ✔
MAX820L/M/ 4.65/4.40/ 140 1 ✔/10ns ✔/±2% ✔✔✔ ✔
R/S/T 2.63/2.93/3.08
MAX1232 4.37/4.62 250 0.15/0.60/1.2 ✔✔
MAX1259 - - - ✔✔
________________________µP Supervisory Circuits
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
©2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
Microprocessor Supervisory Circuits
PART TEMP. RANGE PIN-PACKAGE
MAX692ACPA 0°C to +70°C 8 Plastic DIP
MAX692ACSA 0°C to +70°C 8 SO
MAX692AC/D 0°C to +70°C Dice*
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**
__Ordering Information (continued)
MAX802LCPA 0°C to +70°C 8 Plastic DIP
MAX802LCSA 0°C to +70°C 8 SO
MAX802LEPA -40°C to +85°C 8 Plastic DIP
MAX802LESA -40°C to +85°C 8 SO
MAX805LESA -40°C to +85°C 8 SO
MAX805LEPA -40°C to +85°C 8 Plastic DIP
MAX805LC/D 0°C to +70°C Dice*
MAX805LCSA 0°C to +70°C 8 SO
MAX805LCPA 0°C to +70°C 8 Plastic DIP
MAX802MESA -40°C to +85°C 8 SO
MAX802MEPA -40°C to +85°C 8 Plastic DIP
MAX802MCSA 0°C to +70°C 8 SO
MAX802MCPA 0°C to +70°C 8 Plastic DIP
*Dice are specified at TA= +25°C.
**Contact factory for availability and processing to MIL-STD-883.
Devices in PDIP and SO packages are available in both leaded
and lead-free packaging. Specify lead free by adding the +
symbol at the end of the part number when ordering. Lead free
not available for CERDIP package.
____________________Chip Topography
( ) ARE FOR MAX805L ONLY.
TRANSISTOR COUNT: 573;
SUBSTRATE MUST BE LEFT UNCONNECTED.
MAX805LMJA -55°C to +125°C 8 CERDIP**