Maxim MAX805LCSA, MAX805LESA, MAX805LMJA, MAX805LC-D, MAX802MEPA Datasheet

19-4333; Rev 3; 9/93

RESET

Microprocessor Supervisory Circuits

__________________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.

_______________________Applications

Battery-Powered Computers and Controllers
Intelligent Instruments
Automotive Systems
Critical µP Power Monitoring

__________Typical Operating Circuit

UNREGULATED DC REGULATED +5V

V

O.1µF

R

1

3.6V

R

2

LITHIUM
BATTERY

PFI

V

MAX690A
MAX802L

V

BATT

CC

RESET

PFO
WDI

V

OUT

GND

CC

RESET
NMI

I/O LINE

BUS

V

CC

CMOS

RAM

µP

GND

GND

___________________________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
♦ 8-Pin SO and DIP Packages

Assertion to VCC= 1V

______________Ordering Information

PART TEMP. RANGE PIN-PACKAGE

MAX690ACPA

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 T
**Contact factory for availability and processing to MIL-STD-883.

0°C to +70°C

= +25°C

A

8 Plastic DIP

_________________Pin Configurations

TOP VIEW

V

OUT

1

PFI

MAX690A

2

CC

MAX692A
MAX802L

3

MAX802M

4

MAX805L

DIP/SO

V

GND

( ) ARE FOR MAX805L ONLY.

8

BATT

V
RESET (RESET)

7

WDI

6
5

PFO

MAX690A/MAX692A/MAX802L/MAX802M/MAX805L

________________________________________________________________

Maxim Integrated Products

Call toll free 1-800-998-8800 for free samples or literature.

1

Microprocessor Supervisory Circuits

ABSOLUTE MAXIMUM RATINGS

Terminal Voltage (with respect to GND)

............................................... -0.3V to 6.0V

V

CC

.............................................. -0.3V to 6.0V

V

BATT

All Other Inputs (Note 1) ................ -0.3V to (V

Input Current

...................................................... 200mA

V

CC

..................................................... 50mA

V

BATT

GND ...................................................... 20mA

Output Current

................... Short-Circuit Protected for up to 10sec

V

OUT

All Other Outputs ........................................ 20mA

CC

+ 0.3V)

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

(VCC= 4.75V to 5.5V for MAX690A/MAX802L/MAX805L, VCC= 4.5V to 5.5V for MAX692A/MAX802M, V

= T

to T

T

A

MIN

Operating Voltage Range,

, V

V

CC

BATT

Supply Current (Excluding I

I

SUPPLY

(Excluding I

V

Standby Current (Note 3)

BATT

V

Output

OUT

V

in Battery-Backup Mode I

OUT

Battery Switch Threshold, V
to V

BATT

Battery Switchover Hysteresis 40 mV

Reset Threshold

Reset Threshold Hysteresis 40 mV
Reset Pulse Width

MAX690A/MAX692A/MAX802L/MAX802M/MAX805L

RESET Output Voltage

, unless otherwise noted.)

MAX

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

MAX69_AC, MAX802_C 1.0 5.5

(Note 2)

OUT

in Battery-Backup Mode

)

OUT

) I

SUPPLY

MAX69_AE/M, MAX80_ _E 1.2 5.5
MAX69_AC, MAX802_C 200 350
MAX69_AE/M, MAX802_E, MAX805LE/M 200 500
VCC= 0V,

V

BATT

5.5V > VCC>
V

BATT

I

= 5mA VCC- 0.05 VCC- 0.025

OUT

I

= 50mA

OUT

= 250µA, VCC< V

OUT

CC

VCC< V

MAX690A, MAX802L, MAX805L
MAX692A, MAX802M 4.25 4.40 4.50

V

RT

MAX802L, TA= +25°C, VCCfalling
MAX802M, TA= +25°C, VCCfalling

t

RS

I

SOURCE

I

= 3.2mA

SINK

MAX69_AC, MAX802_C, VCC= 1.0V

= 50µA

I

SINK

MAX69_AE/M, MAX802_E,

= 1.2V, I

V

CC

Rate of Rise, V
Continuous Power Dissipation

, V

CC

................................ 100V/µs

BATT

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

1.1 5.5

= 2.8V

+0.2V

TA= +25°C
TA= T

MIN

TA= +25°C
TA= T

MIN

to T

to T

MAX

MAX

0.05 1.0

-0.1 0.02

-1.0 0.02

VCC- 0.5 VCC- 0.25

- 0.2V V

BATT

RT

Power-up
Power-down

BATT

- 0.1 V

BATT

20

-20

4.50 4.65 4.75

4.55 4.70

4.30 4.45

140 200 280 ms

= 800µA VCC- 1.5

= 100µA

SINK

- 0.02

5.0

0.4

0.3

0.3

BATT

= 2.8V,

VMAX805LC

µA

µA

µA

V
V

mV

V

V

2 _______________________________________________________________________________________

Microprocessor Supervisory Circuits

ELECTRICAL CHARACTERISTICS (continued)

(VCC= 4.75V to 5.5V for MAX690A/MAX802L/MAX805L, VCC= 4.5V to 5.5V for MAX692A/MAX802M, V

= T

to T

T

A

MIN

, unless otherwise noted.)

MAX

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

RESET Output Voltage

Watchdog Timeout
WDI Pulse Width

WDI Input Threshold (Note 4)

WDI Input Current

PFI Input Threshold

MAX805LC, I
MAX805LE/M, I
MAX805L, I
MAX805L, I

t

WD

t

VIL= 0.4V, VIH= (0.8) (VCC)

WP

VCC= 5V
WDI = V

CC

WDI = 0V -150 -50
MAX69_A, MAX805L, VCC= 5V
MAX802_C/E, VCC= 5V

= 4µA, VCC= 1.1V

SOURCE

SOURCE
SINK

= 4µA, VCC= 1.2V

SOURCE

= 800µA VCC- 1.5

= 3.2mA

Logic low
Logic high

0.8

0.9

1.00 1.60 2.25 sec
50 ns

3.5
50 150

1.20 1.25 1.30

1.225 1.250 1.275

PFI Input Current -25 0.01 25 nA

I

= 800µA VCC- 1.5

PFO Output Voltage

Note 2: Either VCCor V
Note 3: "-" = battery-charging current, "+" = battery-discharging current.

can go to 0V, if the other is greater than 2.0V.

BATT

Note 4: WDI is guaranteed to be in an intermediate, non-logic level state if WDI is floating and V

WDI is internally biased to 35% of V

SOURCE

I

= 3.2mA

SINK

with an input impedance of 50kΩ.

CC

is in the operating voltage range.

CC

BATT

0.4

0.8

0.4

MAX690A/MAX692A/MAX802L/MAX802M/MAX805L

= 2.8V,

V

V

µA

V

V

_______________________________________________________________________________________ 3

Microprocessor Supervisory Circuits

__________________________________________Typical Operating Characteristics

5.00

4.95

4.90

(V)

OUT

V

4.85

4.80

4.75

RESET

+5V
V

+5V

0

CC

V

CC

OUTPUT VOLTAGE

vs. LOAD CURRENT

VCC = +5V

= +2.8V

V

BATT

= +25°C

T

A

SLOPE = 5Ω

20

10 30 50

I

OUT



MAX690A

RESET RESPONSE TIME

V

CC

10k

RESET

30pF

GND

2µs/div

40

(mA)

TA = +25°C

RESET

+4V

1V/div

0V

2.80

2.78

2.76

(V)

OUT

V

2.74

2.72

2.70

+1.30V

SLOPE = 80Ω

0

0.2 0.6 1.0

POWER-FAIL COMPARATOR

+5V

PFO

PFI

+1.25V

PFI

OUTPUT VOLTAGE

vs. LOAD CURRENT

0.4
I

(mA)

OUT



RESPONSE TIME

+5V

1k

PFO

30pF

VCC = +5V

= +25°C

T

A

400ns/div

VCC = 0V
V

BATT

= +25°C

T

A

= +2.8V

0.8

+0V

+1.2V

V

CC

0V

RESET

0V

PFO

PFI

+1.20V

RESET OUTPUT VOLTAGE

vs. SUPPLY VOLTAGE

V

CC

POWER-FAIL COMPARATOR

PFI

+1.25V

0V

MAX690A

V

= OV

BATT

TA = +25°C

V

CC

RESET

GND

500ms/div



RESPONSE TIME

+5V

PFO

30pF 1k

400ns/div

2k

RESET

330pF

VCC = +5V
T

= +25°C

A

+5V
1V/div

+5V
1V/div

+3V

+1.3V



MAX805L

RESET OUTPUT VOLTAGE

vs. SUPPLY VOLTAGE

V

CC

V

0V

MAX690A/MAX692A/MAX802L/MAX802M/MAX805L

RESET

0V

V

BATT 

RESET 



GND

CC 





500ms/div

10k

330

pF

+5V

1V/div

+5V

1V/div

+5V
V

CC

RESET

0V

0V

RESET RESPONSE TIME

V

CC

GND

MAX805L

10k



+4V

330pF

+4V

2µs/div

4 _______________________________________________________________________________________

Microprocessor Supervisory Circuits

______________________________________________________________Pin Description

PIN

MAX690A/MAX692A
MAX802L/MAX802M

MAX805L

1 1

2 2
3 3 GND Ground

4 4 PFI

5 5

6 6 WDI

7

NAME FUNCTION

Supply Output for CMOS RAM. When VCCis above the reset threshold,

connects to VCCthrough a P-channel MOSFET switch. When V

V

V

OUT

V

CC

OUT

is below the reset threshold, the higher of VCCor V
connected to V

OUT

.

+5V Supply Input

Power-Fail Comparator Input. When PFI is less than 1.25V, PFO goes

when not used.

CC

PFO

low. Connect PFI to GND or V

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.

Reset Output. Whenever RESET is triggered, it pulses low for 200ms. It

RESET

stays low when V
MAX690A/MAX802L and 4.4V in the MAX692A/MAX802M) and remains
low for 200ms after V
timeout also triggers RESET.

is below the reset threshold (4.65V in the

CC

rises above the reset threshold. A watchdog

CC

BATT

will be

CC

MAX690A/MAX692A/MAX802L/MAX802M/MAX805L

– 7 RESET

8 8

V

BATT

_______________________________________________________________________________________ 5

Active-High Reset Output is the inverse of RESET. When RESET is

or V

asserted, the RESET output voltage = V
higher.

CC

, whichever is

BATT

Backup-Battery Input. When VCCfalls below the reset threshold, V
will be switched to V
rises to 20mV above V
hysteresis prevents repeated switching if V

OUT

BATT

if V

is 20mV greater than VCC. When V

BATT

, V

will be reconnected to VCC. The 40mV

OUT

falls slowly.

CC

BATT

CC

Microprocessor Supervisory Circuits

V

BATT

V

CC

WDI

PFI

( ) ARE FOR MAX805L ONLY.

Figure 1. Block Diagram

_______________Detailed Description

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 VCCreaches 1V, RESET is guaran-

BATTERY-SWITCHOVER

CIRCUITRY

3.5V

1.25V

0.8V

1.25V

WATCHDOG

TIMER

GND

RESET

GENERATOR

MAX690A
MAX692A
MAX802L

MAX802M

MAX805L

Reset Output

V

OUT

RESET
(RESET)

PFO



Figure 2. Timing Diagram

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 (t

The MAX805L active-high RESET output is the inverse
of the MAX690A/MAX692A/MAX802L/MAX802M RESET
output, and is guaranteed to be valid with VCCdown to

1.1V. Some µPs, such as Intel’s 80C51, require an
active-high reset pulse.

teed to be a logic low. As VCCrises, RESET remains
low. When VCCexceeds 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 VCCdips below
the reset threshold), RESET is triggered. Each time
RESET is triggered, it stays low for the reset pulse width
interval. Any time VCCgoes below the reset threshold,
the internal timer restarts the pulse. If a brownout con­dition interrupts a previously initiated reset pulse, the

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.
reset pulse continues for another 200ms. On power­down, once VCCgoes below the threshold, RESET is

MAX690A/MAX692A/MAX802L/MAX802M/MAX805L

guaranteed to be logic low until VCCdroops 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-

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

+5V

V

CC

0V

V

OUT

3.0V

0V

+5V

RESET

0V

+5V
(RESET)


3.0V

0V

+5V

PFO

0V

( ) ARE FOR MAX805L ONLY.


t

RS

= PFI = 3.0V

V

BATT

= 0mA

I

OUT

plus tRS).

WD

Watchdog Input

Power-Fail Comparator

6 _______________________________________________________________________________________

Microprocessor Supervisory Circuits

V

BATT

SW2

SW1

D1

D2

SUBSTRATE

MAX690A
MAX692A

V

CC

V

CC

V

CC

VCC < Reset Threshold and
V

CC

RESET THRESHOLD = 4.65V IN MAX690A/MAX802L/MAX805L.
RESET THRESHOLD = 4.4V IN MAX692A/MAX802M

MAX805L

MAX802L

MAX802M

> Reset Threshold Open

< Reset Threshold and
> V

BATT

< V

BATT

D3

V

OUT

SW1/SW2

Open

Closed

SW3

SW3/SW4CONDITION

Closed

Closed

Open

SW4

V

CC

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

Operating Circuit

). The voltage-divider ratio can be

Typical

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 V

connects to V

BATT

OUT

.

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 V
cally switch RAM to backup power when VCCfails.

As long as VCCexceeds the reset threshold, V
to VCCthrough a 5Ω PMOS power switch. Once VCCfalls
below the reset threshold, VCCor V
er) switches to V
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L don’t
always connect V
VCC. V
when VCCis below the reset threshold and V

connects to V

BATT

. Unlike the MAX690/MAX692, the

OUT

to V

BATT

OUT

, the devices automati-

BATT

connects

OUT

(whichever is high-

BATT

when V

OUT

(through an 80Ω switch) only

is greater than

BATT

BATT

is greater

+5V

MAX690A
MAX802L
MAX805L

GND

V

RESET

(RESET)

OUT

TO STATIC RAM

TO µP

+

0.1F

( ) ARE FOR MAX805L ONLY.

V

CC

V

BATT

Figure 4. Using a SuperCap as a Backup Power Source with a
MAX690A/MAX802L/MAX805L and a +5V ±5% Supply

than V

.

CC

When VCCexceeds the reset threshold, it is connected to
the MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
substrate, regardless of the voltage applied to V
(Figure 3). During this time, the diode (D1) between V
and the substrate will conduct current from V

Table 1. Input and Output Status in Battery-Backup Mode

SIGNAL STATUS

V

CC

V

OUT

V

BATT

PFI Power-fail comparator is disabled.

PFO

RESET

RESET Logic high (MAX805L only)

WDI Watchdog timer is disabled

Disconnected from V

Connected to V
PMOS switch

Connected to V
battery is less than 1µA, as long as

< V

V

CC

BATT

OUT

through an internal 80Ω

BATT

. Current drawn from the

OUT

- 1V.

Logic low
Logic low

BATT

BATT

BATT

to VCCif

MAX690A/MAX692A/MAX802L/MAX802M/MAX805L

_______________________________________________________________________________________ 7

Microprocessor Supervisory Circuits

+5V

MAX692A

MAX802M

GND

V

OUT

RESET

TO STATIC RAM

TO µP



100k

0.1F

V

CC

V

BATT

+

Figure 5. Using a SuperCap™ as a Backup Power Source with
the MAX692A/MAX802M and a +5V ±10% Supply

V

is 0.6V or greater than VCC.

BATT

When V
ed and the internal circuitry is powered from the battery
(Table 1). When VCCis just below V
drawn from V
more than 1V below V
parator shuts off and the supply current falls to less

connects to V

BATT

BATT

, backup mode is activat-

OUT

, the current

is typically 30µA. When VCCdrops to

, the internal switchover com-

BATT

BATT

than 1µA.

__________Applications Information

V

IN

R

1

R

C1*



R



R2R



R2|| R

R1R2|| R

+

5 1.25

R

3

TO µP

2

+

3

=

3






2




3

1.25
R

2

+5V

PFO

0V

R

2

0V

V

TRIP

VIH1.25/

=

VL1.25

−+−

R

1

1.25/

=




Figure 6. Adding Hysteresis to the Power-Fail Comparator

PFI

PFO

+5V

V

CC

MAX690
MAX692A
MAX802L

MAX802M

MAX805L

GND

*OPTIONAL

V

V

L

H

V



TRIP

V

IN

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, V
may not exceed 4.75V + 0.6V = 5.35V.

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

CC

Table 2. Allowable Backup-Battery Voltages

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 VCCof 5.5V, the SuperCap

MAX690A/MAX692A/MAX802L/MAX802M/MAX805L

charges up to 5.0V, only 0.5V above the maximum
reset threshold—well within the requisite 0.6V.

™SuperCap is a trademark of Baknor Industries.

MAX802L/MAX805L

Allowable Backup Power-Source

(see

Using a SuperCap as a Backup Power Source

section for use with a SuperCap)

PART NO.

MAX690A/

MAX692A/
MAX802M

MAXIMUM BACKUP-BATTERY

VOLTAGE (V)

4.80

4.55

Batteries

8 _______________________________________________________________________________________

Microprocessor Supervisory Circuits

connected directly to the V
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L with
no additional circuitry (see the

Typical Operating Circuit

BATT

input of the

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
VCCis close to the reset threshold.

Operation

Without a Backup Power Source

If a backup power source is not used, ground V
and connect V
switch over to any backup power source, V

to VCC. Since there is no need to

OUT

not need to be switched. A direct connection to V
eliminates any voltage drops across the switch which
may push V

below VCC.

OUT

OUT

BATT

does

CC

Replacing the Backup Battery

The backup battery can be removed while VCCremains
valid, without danger of triggering RESET/RESET. As
long as VCCstays above the reset threshold, battery­backup mode cannot be entered. In other switchover

+5V

+5V

PFO

0V

R

1

R

2

V-

V

CC

MAX690A

PFI

MAX692A
MAX802L
MAX802M
MAX805L

V



TRIP

V-

PFO

GND

0V

ICs where battery-backup mode is entered whenever
V

gets close to VCC, an unconnected V

BATT

).

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
VINis 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 V
falls to its trip point (V
will typically be an order of magnitude greater than R

). R3adds the hysteresis. It

TRIP

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 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 VCCline, 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 out-
put. 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

BATT

MAX690A/MAX692A/MAX802L/MAX802M/MAX805L

pin

IN

1

5 1.25

1.25 - V

−

NOTE: V

R

1

Figure 7. Monitoring a Negative Voltage

=

IS NEGATIVE

TRIP

TRIP

R

2

_______________________________________________________________________________________ 9

Microprocessor Supervisory Circuits

BUFFERED RESET TO OTHER SYSTEM COMPONENTS

V

CC

MAX690A
MAX692A

RESET

MAX802L

MAX802M

GND

Figure 8. Interfacing to µPs with Bidirectional Reset I/O

4.7k



RESET

V

GND

CC

MAX690A/MAX692A/MAX802L/MAX802M/MAX805L

10 ______________________________________________________________________________________

Microprocessor Supervisory Circuits

CE

________________________

Nominal Reset Watchdog Power-

Reset Pulse Timeout Backup­Part 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 - - - ✔✔

Minimum Nominal

- Fail Manual- Watch- Low- Active- Battery-

µ

P Supervisory Circuits

MAX690A/MAX692A/MAX802L/MAX802M/MAX805L

______________________________________________________________________________________ 11

Microprocessor Supervisory Circuits

__Ordering Information (continued)

PART TEMP. RANGE PIN-PACKAGE

MAX692ACPA

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**
MAX802LCPA
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
MAX802MCPA
MAX802MCSA 0°C to +70°C 8 SO
MAX802MEPA -40°C to +85°C 8 Plastic DIP
MAX802MESA -40°C to +85°C 8 SO
MAX805LCPA
MAX805LCSA 0°C to +70°C 8 SO
MAX805LC/D 0°C to +70°C Dice*
MAX805LEPA -40°C to +85°C 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.

0°C to +70°C 8 Plastic DIP

0°C to +70°C 8 Plastic DIP

0°C to +70°C 8 Plastic DIP

0°C to +70°C 8 Plastic DIP

____________________Chip Topography

V

V

CC

GND

( ) ARE FOR MAX805L ONLY.
TRANSISTOR COUNT: 573;
SUBSTRATE MUST BE LEFT UNCONNECTED.

V

OUT

PFI

0.078"

(1.98mm)

BATT

PFO

RESET (RESET)

WDI

(1.55mm)

0.061"

MAX690A/MAX692A/MAX802L/MAX802M/MAX805L

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

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