Maxim MAX818MCPA 5v microprocessor supervisory circuit Datasheet

19-0494; Rev 1; 3/96
+5V Microprocessor Supervisory Circuits
The MAX817/MAX818/MAX819 microprocessor (µP)
supervisory circuits simplify power-supply monitoring,
battery control, and chip-enable gating in µP systems
by reducing the number of components required.
These devices are designed for use in +5V-powered
systems. Low supply current (11µA typical) and small
package size make these devices ideal for portable
applications. The MAX817/MAX818/MAX819 are specifically designed to ignore fast transients on VCC. Other
supervisory functions include active-low reset, backupbattery switchover, watchdog input, battery freshness
seal, and chip-enable gating. The Selector Guide below
lists the specific functions available from each device.
These devices offer two pretrimmed reset threshold voltages for ±5% or ±10% power supplies: 4.65V for the L
versions and 4.40V for the M versions. The MAX817/
MAX818/MAX819 are available in space-saving µMAX
packages, as well as 8-pin DIP/SO.
_____________________Selector Guide
MAX817
L/M
MAX818
L/M
MAX819
L/M
Active-Low Reset
✔
✔
✔
Backup-Battery Switchover
✔
✔
✔
Power-Fail Comparator
✔
✔
Watchdog Input
✔
—
✔
Battery Freshness Seal
✔
✔
—
✔
Manual Reset Input
—
Chip-Enable Gating
—
—
✔
—
FEATURE
Pin-Package
____________________________Features
♦ Precision Supply-Voltage Monitor:
4.65V (MAX81_L)
4.40V (MAX81_M)
♦ 11µA Quiescent Supply Current
♦ 200ms Reset Time Delay
♦ Watchdog Timer with 1.6sec Timeout
(MAX817/MAX818)
♦ Battery-Backup Power Switching; Battery Voltage
Can Exceed VCC
♦ Battery Freshness Seal
♦ On-Board, 3ns Gating of Chip-Enable Signals
(MAX818)
♦ Uncommitted Voltage Monitor for Power-Fail or
Low-Battery Warning (MAX817/MAX819)
♦ Manual Reset Input (MAX819)
______________Ordering Information
PART†
—
PIN-PACKAGE
0°C to +70°C
8 Plastic DIP
MAX817_CSA
MAX817_CUA
0°C to +70°C
0°C to +70°C
8 SO
8 µMAX
Ordering Information continued on last page.
†These parts offer a choice of reset threshold voltage. From the
table below, select the suffix corresponding to the desired
threshold and insert it into the blank to complete the part number.
✔
SUFFIX
RESET THRESHOLD (V)
L
4.65
M
4.40
8-DIP/SO/ 8-DIP/SO/ 8-DIP/SO/
µMAX
µMAX
µMAX
Low-Power, PinMAX690A/
Compatible Upgrades for: MAX692A
TEMP. RANGE
MAX817_CPA
MAX703/
MAX704
________________________Applications
_________________Pin Configurations
TOP VIEW
Battery-Powered Computers and Controllers
Embedded Controllers
Intelligent Instruments
Critical µP Monitoring
Portable Equipment
Typical Operating Circuit appears at end of data sheet.
*Patents Pending
OUT 1
8
BATT
7
RESET
GND 3
6
WDI
PFI 4
5
PFO
VCC 2
MAX817
DIP/SO/µMAX
Pin Configurations continued at end of data sheet.
________________________________________________________________ Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
MAX817L/M, MAX818L/M, MAX819L/M*
_______________General Description
MAX817L/M, MAX818L/M, MAX819L/M*
+5V Microprocessor Supervisory Circuits
ABSOLUTE MAXIMUM RATINGS
Input Voltage
VCC, BATT ..........................................................-0.3V to +6.0V
All Other Pins (Note 1).............................-0.3V to (VCC + 0.3V)
Input Current
VCC Peak ..............................................................................1A
VCC Continuous .............................................................250mA
BATT Peak .....................................................................250mA
BATT Continuous .............................................................50mA
GND .................................................................................25mA
Output Current
OUT................................................................................250mA
All Other Outputs .............................................................25mA
OUT Short-Circuit Duration.................................................10sec
Continuous Power Dissipation (TA = +70°C)
Plastic DIP (derate 9.09mW/°C above +70°C) .............727mW
SO (derate 5.88mW/°C above +70°C) ..........................471mW
µMAX (derate 4.10mW/°C above +70°C) .....................330mW
Operating Temperature Ranges
MAX81_ _C_A ......................................................0°C to +70°C
MAX81_ _E_A ...................................................-40°C to +85°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 (up to 12V VIN) 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 MAX81_L, VCC = +4.5V to +5.5V for MAX81_M, VBATT = 2.8V, TA = TMIN to TMAX, unless otherwise
noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
Operating Voltage Range, VCC,
VBATT (Note 2)
Supply Current (excluding IOUT)
TYP
0
ISUPPLY
As applicable; CE IN = 0V,
WDI and MR unconnected
Supply Current in BatteryBackup Mode (excluding IOUT)
VCC = 0V
BATT Standby Current (Note 3)
5.5V > VCC > (VBATT + 0.2V)
BATT Leakage Current,
Freshness Seal Enabled
VCC = 0V, VOUT = 0V
5.5
V
11
45
MAX81_ _E
11
60
TA = +25°C
0.05
1.0
TA = TMIN to
TMAX
5.0
TA = +25°C
-0.10
0.02
TA = TMIN to
TMAX
-1.00
0.02
1
IOUT = 5mA
VCC 0.05
VCC 0.025
IOUT = 50mA
VCC 0.5
VCC 0.25
VCC to OUT On-Resistance
5
BATT to OUT On-Resistance
100
VOUT in Battery-Backup Mode
IOUT = 250µA, VCC < (VBATT - 0.2V)
Battery Switch Threshold
(VCC - VBATT)
VCC < VRST
2
UNITS
MAX81_ _C
VOUT Output
Battery Switchover Hysteresis
MAX
VBATT - VBATT 0.1
0.02
Power-up
20
Power-down
-20
40
_______________________________________________________________________________________
µA
µA
µA
µA
V
10
Ω
Ω
V
mV
mV
+5V Microprocessor Supervisory Circuits
(VCC = +4.75V to +5.5V for MAX81_L, VCC = +4.5V to +5.5V for MAX81_M, VBATT = 2.8V, TA = TMIN to TMAX, unless otherwise
noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
MAX81_L
4.50
4.65
4.75
MAX81_M
4.25
4.40
4.50
140
200
UNITS
RESET AND WATCHDOG TIMER
Reset Threshold
VRST
Reset Threshold Hysteresis
Reset Timeout Period
25
tRP
VOH
RESET Output Voltage
VOL
VCC to RESET Delay
tWD
WDI Pulse Width
tWDI
VIL
VIH
0.4
MAX81_ _C, VCC = 1V, VCC falling,
VBATT = 0V, ISINK = 50µA
0.3
MAX81_ _E, VCC = 1.2V, VCC falling,
VBATT = 0V, ISINK = 100µA
0.3
100
1.00
VIL = 0.4V, VIH = 0.8VCC
VCC = 5V
WDI = GND, time average
ms
VCC - 1.5
1.60
V
µs
2.25
50
sec
ns
0.8
3.5
WDI = VCC, time average
WDI Input Current (Note 5)
mV
280
VCC < VRST(MIN), ISINK = 3.2mA
From VRST, VCC falling at 10V/ms
Watchdog Timeout Period
WDI Input Threshold (Note 4)
VCC > VRST(MAX), ISOURCE = 800µA
V
120
-20
-15
1.20
1.25
160
V
µA
POWER-FAIL COMPARATOR (MAX817/MAX819 only)
PFI Input Threshold
VPFT
PFI Input Hysteresis
PFI Input Current
IPFI
PFO Output Voltage
1.30
4
-25
VOL
VPFI < 1.20V, ISINK = 3.2mA, VCC > 4.50V
VOH
VPFI > 1.30V, ISOURCE = 40µA, VCC > 4.5V
PFO Short-Circuit Current
0.01
25
0.4
VCC - 1.5
V P FO = 0V
250
V
mV
500
nA
V
µA
MANUAL RESET INPUT (MAX819 only)
MR Input Threshold
MR Pulse Width
VIL
0.8
VIH
2.0
1
V
µs
MR Pulse that Would Not Cause
a Reset
100
ns
MR to Reset Delay
120
ns
MR Pull-Up Resistance
45
63
85
kΩ
_______________________________________________________________________________________
3
MAX817L/M, MAX818L/M, MAX819L/M*
ELECTRICAL CHARACTERISTICS (continued)
MAX817L/M, MAX818L/M, MAX819L/M*
+5V Microprocessor Supervisory Circuits
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +4.75V to +5.5V for MAX81_L, VCC = +4.5V to +5.5V for MAX81_M, VBATT = 2.8V, TA = TMIN to TMAX, unless otherwise
noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
CHIP-ENABLE GATING (MAX818 only)
CE IN Leakage Current
Disable mode
±0.005
±1
µA
CE IN to CE OUT Resistance
(Note 6)
Enable mode
40
150
Ω
CE OUT Short-Circuit Current
(Reset Active)
Disable mode, CE OUT = 0V
0.75
2.0
mA
CE IN to CE OUT Propagation
Delay (Note 7)
50Ω source impedance driver, CLOAD = 50pF
3
8
ns
CE OUT Output
CE OUT Input Threshold
RESET to CE OUT Delay
VOH
VIH
VIL
IOUT = -100µA, VCC = 0V
IOUT = -1µA, VCC = 0V, VBATT = 2.8V
VCC = 5V
Power-down
0.1
VCC - 1V
V
2.7
0.8
3.5
15
V
µs
Either VCC or VBATT can go to 0V if the other is greater than 2.0V.
“-” = battery-charging current, “+” = battery-discharging current.
WDI is internally serviced within the watchdog timeout period if WDI is left unconnected.
WDI input is designed to be driven by a three-stated output device. To float WDI, the “high-impedance mode” of the output
device must have a maximum leakage current of 10µA and a maximum output capacitance of 200pF. The output device
must also be able to source and sink at least 200µA when active.
Note 6: The chip-enable resistance is tested with VCC = +4.75V for the MAX818L and VCC = +4.5V for the MAX818M.
V C E IN = V C E OUT = VCC/2.
Note 7: The chip-enable propagation delay is measured from the 50% point at CE IN to the 50% point at CE OUT.
Note 2:
Note 3:
Note 4:
Note 5:
4
_______________________________________________________________________________________
+5V Microprocessor Supervisory Circuits
12
10
8
-20
0
20
40
60
80
VBATT = 5.0V
120
100
80
VBATT = 2.8V
60
VBATT = 2.0V
40
20
100
VCE IN = 4V
80
70
60
VCE IN = 3V
50
40
VCE IN = 2V
30
20
10
0
-40
-20
0
20
40
60
80
-40
100
-20
0
20
40
60
TEMPERATURE (°C)
TEMPERATURE (°C)
BATT TO OUT ON-RESISTANCE
vs. TEMPERATURE
VCC TO OUT ON-RESISTANCE
vs. TEMPERATURE
RESET TIMEOUT PERIOD
vs. TEMPERATURE
VBATT = 2.0V
200
150
VBATT = 2.8V
100
VBATT = 5.0V
50
0
5
4
0
20
40
60
80
100
80
100
210
200
190
180
3
-20
100
MAX817/18/19-06
MAX817/18/19-05
6
80
220
RESET TIMEOUT PERIOD (ms)
250
7
VCC TO OUT ON-RESISTANCE (Ω)
VCC = 0V
-40
90
TEMPERATURE (°C)
MAX817/18/19-04
300
-40
-20
0
20
40
60
80
-40
100
-20
0
20
40
60
TEMPERATURE (°C)
TEMPERATURE (°C)
VCC TO RESET PROPAGATION DELAY
vs. TEMPERATURE
WATCHDOG TIMEOUT PERIOD
vs. TEMPERATURE
BATTERY FRESHNESS SEAL
LEAKAGE CURRENT vs. TEMPERATURE
300
1V/ms
200
10V/ms
100
0
-40
-20
0
20
40
60
TEMPERATURE (°C)
80
100
20
1.65
1.60
1.55
MAX817/18/19-09
400
1.70
MAX817/18/19-08
VCC FALLING AT:
0.25V/ms
WATCHDOG TIMEOUT PERIOD (sec)
500
MAX817/18/19-07
TEMPERATURE (°C)
LEAKAGE CURRENT (nA)
BATT TO OUT ON-RESISTANCE (Ω)
140
100
0
-40
VCC TO RESET PROPAGATION DELAY (ms)
VCC = 0V
CE IN TO CE OUT ON-RESISTANCE (Ω)
MAX817/18/19-01
14
160
BATTERY SUPPLY CURRENT (nA)
SUPPLY CURRENT (µA)
16
CE IN TO CE OUT ON-RESISTANCE
vs. TEMPERATURE
MAX817/18/19-02
BATTERY SUPPLY CURRENT
(BACKUP MODE) vs. TEMPERATURE
MAX817/18/19-03
SUPPLY CURRENT
vs. TEMPERATURE (NO LOAD)
15
10
5
0
1.50
-40
-20
0
20
40
60
TEMPERATURE (°C)
80
100
-40
-20
0
20
40
60
80
100
TEMPERATURE (°C)
_______________________________________________________________________________________
5
MAX817L/M, MAX818L/M, MAX819L/M*
__________________________________________Typical Operating Characteristics
(VCC = +5V, VBATT = 3.0V, TA = +25°C, unless otherwise noted.)
____________________________Typical Operating Characteristics (continued)
(VCC = +5V, VBATT = 3.0V, TA = +25°C, unless otherwise noted.)
4.5
MAX81_M
4.3
-40
1000
800
RESET OCCURS
ABOVE CURVE
600
400
200
0
-20
0
20
40
60
6
5
4
3
2
1
0
1
80
MAX817/18/19-12
MAX817/18/19-11
1200
7
10
100
1000
10,000
0
1
2
4
3
5
6
TEMPERATURE (°C)
RESET COMPARATOR OVERDRIVE, VTH-VCC (mV)
VCC (V)
CE IN TO CE OUT PROPAGATION DELAY
vs. TEMPERATURE
MAX817/MAX819 PFI THRESHOLD
vs. TEMPERATURE
MAX817/MAX819 PFI TO PFO PROPAGATION
DELAY vs. TEMPERATURE
5
tPD-
4
3
tPD+
1.250
1.248
1.246
2
1.244
1
1.242
0
0
20
40
60
TEMPERATURE (°C)
80
100
32
31
30
29
28
1.240
-20
MAX817/18/19-15
1.252
33
PROPAGATION DELAY (µs)
6
MAX817/18/19-14
1.254
MAX817/18/19-13
7
-40
6
1400
8
BATTERY SUPPLY CURRENT (µA)
4.6
1600
THRESHOLD (V)
RESET THRESHOLD (V)
MAX81_L
MAXIMUM TRANSIENT DURATION (µs)
MAX817/18/19-10
4.7
4.4
BATTERY SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAXIMUM TRANSIENT DURATION
vs. RESET THRESHOLD OVERDRIVE
RESET THRESHOLD
vs. TEMPERATURE
CE IN TO CE OUT PROPAGATION DELAY (ns)
MAX817L/M, MAX818L/M, MAX819L/M*
+5V Microprocessor Supervisory Circuits
-40
-20
0
20
40
60
TEMPERATURE (°C)
80
100
-40
-20
0
20
40
60
TEMPERATURE (°C)
_______________________________________________________________________________________
80
100
+5V Microprocessor Supervisory Circuits
PIN
NAME
FUNCTION
MAX817
MAX818
MAX819
1
1
1
OUT
Supply Output for CMOS RAM. When VCC rises above the reset threshold
or above VBATT, OUT is connected to VCC through an internal P-channel
MOSFET switch. When VCC falls below VBATT, BATT connects to OUT.
2
2
2
VCC
Input Supply Voltage, +5V input.
3
3
3
GND
Ground. 0V reference for all signals.
4
—
4
PFI
—
4
—
CE IN
5
—
6
—
—
5
6
—
5
—
—
6
Power-Fail Comparator Input. When VPFI is below VPFT or when VCC is below
VBATT, PFO goes low; otherwise, PFO remains high (see Power-Fail Comparator
section). Connect to ground if unused.
Chip-Enable Input. The input to the chip-enable gating circuit. Connect to
ground if unused.
PFO
Power-Fail Comparator Output. When PFI is less than VPFT or when VCC is
below VBATT, PFO goes low; otherwise PFO remains high. PFO is also used to
enable the battery freshness seal (see Battery Freshness Seal and Power-Fail
Comparator sections).
CE OUT
Chip-Enable Output. CE OUT goes low only if CE IN is low while reset is not
asserted. If CE IN is low when reset is asserted, CE OUT will remain low for
15µs or until CE IN goes high, whichever occurs first. CE OUT is pulled up to
OUT in battery-backup mode. CE OUT is also used to enable the battery
freshness seal (see Battery Freshness Seal section).
WDI
Watchdog Input. If WDI remains either high or low for longer than the watchdog timeout period, the internal watchdog timer runs out and a reset is triggered. If WDI is left unconnected or is 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. The WDI input is designed to be driven by a three-statedoutput device with a maximum high-impedance leakage current of 10µA and a
maximum output capacitance of 200pF. The output device must also be capable of sinking and sourcing 200µA when active.
MR
Manual Reset Input. A logic low on MR asserts reset. Reset remains asserted
for as long as MR is held low and for 200ms after MR returns high. The activelow input has an internal 63kΩ pull-up resistor. It can be driven from a TTL- or
CMOS-logic line or shorted to ground with a switch. Leave open, or connect to
VCC if unused.
7
7
7
RESET
Active-Low Reset Output. Pulses low for 200ms when triggered and remains
low whenever VCC is below the reset threshold or when MR is a logic low. It
remains low for 200ms after VCC rises above the reset threshold, the watchdog
triggers a reset, or MR goes low to high.
8
8
8
BATT
Backup-Battery Input. When VCC falls below VBATT, OUT switches from VCC to
BATT. When VCC rises above VBATT, OUT reconnects to VCC.
_______________________________________________________________________________________
7
MAX817L/M, MAX818L/M, MAX819L/M*
______________________________________________________________Pin Description
MAX817L/M, MAX818L/M, MAX819L/M*
+5V Microprocessor Supervisory Circuits
BATT
OUT
BATTERY SWITCHOVER
CIRCUITRY
VCC
MAX817
MAX818
MAX819
RESET
GENERATOR
RESET
1.25V
THIS PIN
FOR MAX819
ONLY.
MR
BATTERY
FRESHNESS
SEAL CIRCUITRY
WATCHDOG
TIMER
WDI
THIS SECTION
FOR MAX817/
MAX818 ONLY.
THIS SECTION PFI
FOR MAX817/
MAX819 ONLY.
PFO
1.25V
CHIP-ENABLE
OUTPUT
CONTROL
THIS SECTION
FOR MAX818
ONLY.
CE IN
CE OUT
GND
Figure 1. Functional Diagram
8
_______________________________________________________________________________________
+5V Microprocessor Supervisory Circuits
General Timing Characteristics
Designed for 5V systems, the MAX817/MAX818/
MAX819 provide a number of microprocessor (µP)
supervisory functions (see the Selector Guide on the
first page). Figure 2 shows the typical timing relationships of the various outputs during power-up and
power-down with typical VCC rise and fall times.
RESET Output
A µP’s reset input starts the µP in a known state. The
MAX817/MAX818/MAX819 µP supervisory circuits
assert a reset to prevent code-execution errors during
power-up, power-down, and brownout conditions.
RESET is guaranteed to be a logic low for 0V < VCC <
VRST if VBATT is greater than 1V. Without a backup battery (V BATT = GND) RESET is guaranteed valid for
VCC ≥ 1V. Once VCC exceeds the reset threshold an
internal timer keeps RESET low for the reset timeout
period, t RP . After this interval RESET returns high
(Figure 2).
If a brownout condition occurs (VCC drops below the
reset threshold), RESET goes low. Each time RESET is
asserted it stays low for at least the reset timeout period. Any time VCC goes below the reset threshold the
internal timer clears. The reset timer starts when VCC
returns above the reset threshold. RESET both sources
and sinks current.
Manual Reset Input (MAX819)
Many µP-based products require manual reset capability, allowing the operator, a test technician, or external
logic circuitry to initiate a reset. On the MAX819, a logic
low on MR asserts reset. Reset remains asserted while
MR is low, and for tRP (200ms) after it returns high.
During the reset timeout period (tRP), MR’s state is
ignored if the battery freshness seal is enabled. MR has
an internal 63kΩ pull-up resistor, so it can be left open
if not used. This input can be driven with TTL/CMOSlogic levels or with open-drain/collector outputs.
Connect a normally open momentary switch from MR to
GND to create a manual reset function; external
debounce circuitry is not required. If MR is driven from
long cables or the device is used in a noisy environment, connect a 0.1µF capacitor from MR to GND to
provide additional noise immunity.
Note that MR must be high or open to enable the battery freshness seal. Once the battery freshness seal is
enabled its operation is unaffected by MR.
Battery Freshness Seal
The MAX817/MAX818/MAX819 battery freshness seal
disconnects the backup battery from internal circuitry
and OUT until it is needed. This allows an OEM to
ensure that the backup battery connected to BATT will
be fresh when the final product is put to use. To enable
the freshness seal on the MAX817 and MAX819:
1) Connect a battery to BATT.
2) Ground PFO.
3) Bring V CC above the reset threshold and hold it
there until reset is deasserted following the reset
timeout period.
4) Bring VCC down again (Figure 3).
Use the same procedure for the MAX818, but ground
CE OUT instead of PFO. Once the battery freshness
seal is enabled (disconnecting the backup battery from
internal circuitry and anything connected to OUT), it
remains enabled until VCC is brought above VRST.
VBATT
VCC
VOUT
VRST
VRST
VRST
VRST
VBATT
VCC
VBATT
tRP
VRESET
PFO FOLLOWS PFI
VPFO*
VCE OUT**
VBATT
CE OUT FOLLOWS CE IN
*MAX817/MAX819 ONLY.
** MAX818 ONLY.
Figure 2. Power-Up and Power-Down Timing
RESET TO
CE OUT
DELAY**
RESET
tRP
CE OUT (MAX818)
(EXTERNALLY HELD AT 0V)
CE OUT STATE LATCHED
AT 1/2 tRP AND 3/4 tRP,
FRESHNESS SEAL ENABLED
PFO (MAX817/MAX819)
(EXTERNALLY HELD AT 0V)
PFO STATE LATCHED
AT 1/2 tRP AND 3/4 tRP,
FRESHNESS SEAL ENABLED
Figure 3. Battery Freshness Seal Timing
_______________________________________________________________________________________
9
MAX817L/M, MAX818L/M, MAX819L/M*
_______________Detailed Description
MAX817L/M, MAX818L/M, MAX819L/M*
+5V Microprocessor Supervisory Circuits
On the MAX819, MR must be high or open to enable
the battery freshness seal. Once the battery freshness
seal is enabled its operation is unaffected by MR.
Watchdog Input (MAX817/MAX818)
In the MAX817/MAX818, the watchdog circuit monitors
the µP’s activity. If the µP does not toggle the watchdog
input (WDI) within tWD (1.6sec), reset asserts. The internal 1.6sec timer is cleared by either a reset pulse or by
toggling WDI, which can detect pulses as short as
50ns. The timer remains cleared and does not count for
as long as reset is asserted. As soon as reset is
released, the timer starts counting (Figure 4).
To disable the watchdog function, leave WDI unconnected or three-state the driver connected to WDI. The
watchdog input is internally driven low during the first
7/8 of the watchdog timeout period, then momentarily
pulses high, resetting the watchdog counter. When
WDI is left open-circuited, this internal driver clears the
1.6sec timer every 1.4sec. When WDI is three-stated or
left unconnected, the maximum allowable leakage current is 10µA and the maximum allowable load capacitance is 200pF.
VCC
tRP
tWD
RESET
WDI
Figure 4. Watchdog Timing
BATTERY
SWITCHOVER
CIRCUITRY
MAX817
MAX818
BATTERY
FRESHNESS
SEAL CIRCUITRY
RESET
GENERATOR
Chip-Enable Gating (MAX818)
Internal gating of the chip-enable (CE) signal prevents
erroneous data from corrupting CMOS RAM in the
event of an undervoltage condition. The MAX818 uses
a series transmission gate from CE IN to CE OUT
(Figure 5). During normal operation (reset not asserted), the CE transmission gate is enabled and passes
all CE transitions. When reset is asserted, this path
becomes disabled, preventing erroneous data from
corrupting the CMOS RAM. The short CE propagation
delay from CE IN to CE OUT enables the MAX818 to be
used with most µPs. If CE IN is low when reset asserts,
CE OUT remains low for typically 15µs to permit the
current write cycle to complete.
OUT
CHIP-ENABLE
OUTPUT
CONTROL
P
CE IN
CE OUT
N
Figure 5. Chip-Enable Transmission Gate
Chip-Enable Input (MAX818)
The CE transmission gate is disabled and CE IN is high
impedance (disabled mode) while reset is asserted.
During a power-down sequence when VCC passes the
reset threshold, the CE transmission gate disables and
CE IN immediately becomes high impedance if the voltage at CE IN is high. If CE IN is low when reset asserts,
the CE transmission gate will disable 15µs after reset
asserts (Figure 6). This permits the current write cycle
to complete during power-down.
VRST
VRST
VRST
VCC
VCE OUT
VBATT
VBATT
tRP
15µs
tRP
VRESET
VCE IN
Figure 6. Chip-Enable Timing
10
______________________________________________________________________________________
VRST
+5V Microprocessor Supervisory Circuits
Chip-Enable Output (MAX818)
When the CE transmission gate is enabled, the impedance of CE OUT is equivalent to a 40Ω resistor in series
with the source driving CE IN. In the disabled mode,
the transmission gate is off and an active pull-up connects CE OUT to OUT (Figure 5). This pull-up turns off
when the transmission gate is enabled.
+5V
Power-Fail Comparator
(MAX817/MAX819)
The MAX817/MAX819 PFI input is compared to an internal reference. If PFI is less than the power-fail threshold
(VPFT), PFO goes low. The power-fail comparator is
intended for use as an undervoltage detector to signal a
failing power supply (Figure 8). However, the comparator
does not need to be dedicated to this function because it
is completely separate from the rest of the circuitry.
The power-fail comparator turns off and PFO goes low
when VCC falls below VBATT. During the reset timeout
period (tRP), PFO is forced high, regardless of the state
of VPFI (see Battery Freshness Seal section). If the comparator is unused, connect PFI to ground and leave PFO
unconnected. PFO can be connected to MR on the
MAX819 so that a low voltage on PFI will generate a
reset (Figure 9). In this configuration, when the monitored
voltage causes PFI to fall below VPFT, PFO pulls MR low,
causing a reset to be asserted. Reset remains asserted
as long as PFO holds MR low, and for tRP (200ms) after
PFO pulls MR high when the monitored supply is above
the programmed threshold. When PFO is connected to
MR, it is not possible to enable the battery freshness
seal. Enabling the battery freshness seal requires MR to
be high or open. Once the battery freshness seal is
enabled, it is no longer affected by PFO’s connection to
MR.
VIN
+5V
REGULATOR
POWER-FAIL-WARNING TRIP VOLTAGE
R1 + R2
VWARN = 1.25
R2
VCC
(
BATT
VCC
MAX818
CE IN
50Ω
50Ω
)
MAX817
MAX819
R1
CE OUT
RESET
RESET
PFI
GND
50pF
CL*
PFO
NMI
µP
R2
1.25V
* CL INCLUDES LOAD CAPACITANCE, STRAY CAPACITANCE,
AND SCOPE-PROBE CAPACITANCE.
Figure 7. CE Propagation Delay Test Circuit
Figure 8. Using the Power-Fail Comparator to Generate a
Power-Fail Warning
______________________________________________________________________________________
11
MAX817L/M, MAX818L/M, MAX819L/M*
Any time a reset is generated, the CE transmission gate
remains disabled and CE IN remains high impedance
(regardless of CE IN activity) for the reset timeout period. When the CE transmission gate is enabled, the
impedance of CE IN appears as a 40Ω resistor in series
with the load at CE OUT. The propagation delay
through the CE transmission gate depends on VCC, the
source impedance of the drive connected to CE IN,
and the loading on CE OUT (see Typical Operating
Characteristics). The CE propagation delay is production tested from the 50% point on CE IN to the 50%
point on CE OUT using a 50Ω driver and a 50pF load
capacitance (Figure 7). For minimum propagation
delay, minimize the capacitive load at CE OUT and use
a low-output-impedance driver.
MAX817L/M, MAX818L/M, MAX819L/M*
+5V Microprocessor Supervisory Circuits
Backup-Battery Switchover
In a brownout or power failure, it may be necessary to
preserve the contents of RAM. With a backup battery
installed at BATT, the MAX817/MAX818/MAX819 automatically switch RAM to backup power when VCC falls.
These devices require two conditions before switching
to battery-backup mode: 1) VCC must be below the
reset threshold, and 2) V CC must be below V BATT .
Table 1 lists the status of the inputs and outputs in battery-backup mode.
As long as VCC exceeds the reset threshold, OUT connects to VCC through a 5Ω PMOS power switch. Once
V CC falls below the reset threshold, V CC or V BATT
(whichever is higher) switches to OUT. When VCC falls
below VRST and VBATT, BATT switches to OUT through
an 80Ω switch.
When VCC exceeds the reset threshold, it is connected to
the substrate, regardless of the voltage applied to BATT
(Figure 10). During this time, the diode (D1) between
BATT and the substrate will conduct current from BATT
to VCC if VBATT is 0.6V greater than VCC. When BATT
connects to OUT, backup mode is activated and the
internal circuitry is powered from the battery (Table 1).
When VCC is just below VBATT, the current draw from
BATT is typically 6µA. 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
The MAX817/MAX818/MAX819 are protected for typical
short-circuit conditions of 10sec or less. Shorting OUT
to ground for longer than 10sec destroys the device.
Decouple VCC, OUT, and BATT to ground by placing
0.1µF capacitors as close to the device as possible.
Table 1. Input and Output Status in
Battery-Backup Mode
SIGNAL
STATUS
BATT
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 - 0.2V.
V R ESET
VWDI
VCC
SW2
SW1
D1
D2
SW3
SW4
SUBSTRATE
Logic low
Watchdog timer is disabled.
V C E OUT
V C E IN
Logic high. The open-circuit voltage is equal
to VOUT.
High impedance
D3
MAX817
MAX818
MAX819
OUT
V1
ADDITIONAL SUPPLY RESET VOLTAGE
R1 + R2
V2 (RESET) = 1.25
R2
(
VCC
V2
MAX819
R1
RESET
PFI
R2
)
RESET
MR
PFO
µP
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
CONDITION
RESET THRESHOLD = 4.65V IN MAX81_L
RESET THRESHOLD = 4.4V IN MAX81_M
Figure 9. Monitoring an Additional Supply by Connecting
PFO to MR.
12
Figure 10. Backup-Battery-Switchover Block Diagram
______________________________________________________________________________________
+5V Microprocessor Supervisory Circuits
Using a SuperCap™ as a
Backup Power Source
SuperCaps are capacitors with extremely high capacitance values (on the order of 0.47F) for their size. Since
BATT has the same operating voltage range as VCC, and
the battery switchover threshold voltages are typically
±30mV centered at VBATT, a SuperCap and simple
charging circuit can be used as a backup power source.
Figure 11 shows a SuperCap used as a backup source.
If VCC is above the reset threshold and VBATT is 0.5V
above VCC, current flows to OUT and VCC from BATT
until the voltage at BATT is less than 0.5V above VCC.
For example, if a SuperCap is connected to BATT
through a diode to VCC, and VCC quickly changes from
5.4V to 4.9V, the capacitor discharges through OUT
and VCC until VBATT reaches 5.1V typical. Leakage current through the SuperCap charging diode and the
internal power diode eventually discharges the
SuperCap to VCC. Also, if VCC and VBATT start from
0.1V above the reset threshold and power is lost at
VCC, the SuperCap on BATT discharges through VCC
until VBATT reaches the reset threshold. Battery-backup
mode is then initiated and the current through V CC
goes to zero.
Operation Without a
Backup Power Source
The MAX817/MAX818/MAX819 were designed for battery-backed applications. If a backup battery is not
used, connect V CC to OUT, and connect BATT to
ground.
Replacing the Backup Battery
The backup power source can be removed while VCC
remains valid, without danger of triggering a reset
pulse, if BATT is decoupled with a 0.1µF capacitor to
ground. As long as VCC stays above the reset threshold, battery-backup mode cannot be entered.
Adding Hysteresis to the Power-Fail
Comparator (MAX817/MAX819)
The power-fail comparator has a typical input hysteresis of 4mV. This is sufficient for most applications where
a power-supply line is being monitored through an
external voltage divider (see Monitoring an Additional
Supply).
For additional noise margin, connect a resistor between
PFO and PFI, as shown in Figure 12. Select the ratio of
R1 and R2 such that PFI sees VPFT when VIN falls to the
+5V
VIN
VCC
R1
PFI
R2
MAX817
MAX819
R3
+5V
C1*
PFO
OUT
VCC
BATT
MAX817
MAX818
MAX819 RESET
GND
TO STATIC RAM
TO µP
TO µP
PFO
0V
100k
0.1F
GND
0V
SuperCap is a trademark of Baknor Industries.
VL
R2
VTRIP = 1.25V R1 + R2
(
VH = 1.25V
Figure 11. Using a SuperCap™ as a Backup Power Source
with a +5V ±10% Supply
*OPTIONAL
+5V
(
)
R2 || R3
R1 + R2 || R3
VL - 1.25
5 - 1.25
=
+
R1
R3
VTRIP
VIN
VH
)
1.25
R2
Figure 12. Adding Hysteresis to the Power-Fail Comparator
______________________________________________________________________________________
13
MAX817L/M, MAX818L/M, MAX819L/M*
Watchdog Input Current
The MAX817/MAX818 WDI inputs are internally driven
through a buffer and series resistor from the watchdog
counter (Figure 1). When WDI is left unconnected, the
watchdog timer is serviced within the watchdog timeout
period by a low-high-low pulse from the counter chain.
For minimum watchdog input current (minimum overall
power consumption), leave WDI low for the majority of the
watchdog timeout period, pulsing it low-high-low once
within 7/8 of the watchdog timeout period to reset the
watchdog timer. If instead WDI is externally driven high for
the majority of the timeout period, up to 150µA can flow
into WDI.
MAX817L/M, MAX818L/M, MAX819L/M*
+5V Microprocessor Supervisory Circuits
desired trip point (VTRIP). Resistor R3 adds hysteresis.
It will typically be an order of magnitude greater than R1
or R2. The current through R1 and R2 should be at least
1µA to ensure that the 25nA (max) PFI input leakage
current does not shift the trip point. R3 should be larger
than 200kΩ to prevent it from loading down the PFO pin.
Capacitor C1 adds additional noise rejection.
+5V
VCC
R1
MAX817
MAX819
PFI
PFO
R2
Monitoring an Additional Supply
(MAX817/MAX819)
The MAX817/MAX819 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 trigger a reset (Figures 9 and 13).
GND
V+5V
Interfacing to µPs with
Bidirectional Reset Pins
µPs with bidirectional reset pins, such as the Motorola
68HC11 series, can contend with the MAX817/MAX818/
MAX819 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 14. Buffer the RESET
output to other system components.
PFO
0V
VTRIP
0V
V5 - 1.25
1.25 - VTRIP
=
R1
R2
NOTE: VTRIP IS NEGATIVE
Figure 13. Monitoring a Negative Voltage
Negative-Going VCC Transients
These supervisors are relatively immune to short-duration, negative-going VCC transients (glitches) while
issuing a reset to the µP during power-up, power-down,
and brownout conditions. Therefore, resetting the µP
when VCC experiences only small glitches is usually not
desirable.
The Typical Operating Characteristics show a graph of
Maximum Transient Duration vs. Reset Threshold
Overdrive for which reset pulses are not generated. The
graph was produced using negative-going VCC pulses,
starting at 3.3V and ending below the reset threshold by
the magnitude indicated (reset threshold overdrive). The
graph shows the maximum pulse width that a negativegoing VCC transient can typically have without triggering
a reset pulse. As the amplitude of the transient increases
(i.e., goes farther below the reset threshold), the maximum allowable pulse width decreases. Typically, a VCC
transient that goes 100mV below the reset threshold and
lasts for 135µs will not trigger a reset pulse.
A 0.1µF bypass capacitor mounted close to the VCC
pin provides additional transient immunity.
BUFFERED RESET TO OTHER SYSTEM COMPONENTS
VCC
VCC
MAX817
MAX818
MAX819
RESET
4.7k
RESET
GND
GND
Figure 14. Interfacing to µPs with Bidirectional Reset I/O
14
______________________________________________________________________________________
+5V Microprocessor Supervisory Circuits
To help the watchdog timer monitor software execution
more closely, set and reset 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 15 shows an example of a flow diagram
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, triggering a reset or
an interrupt. As described in the Watchdog Input Current
section, this scheme results in higher average WDI input
current than does the method of leaving WDI low for the
majority of the timeout period and periodically pulsing it
low-high-low.
__________Typical Operating Circuit
START
SET
WDI
LOW
SUBROUTINE
OR PROGRAM LOOP,
SET WDI
HIGH
RETURN
END
Figure 15. Watchdog Flow Diagram
____Pin Configurations (continued)
TOP VIEW
+5V
VCC
BATT
0.1µF
REALTIME
CLOCK
CMOS
RAM
0.1µF
OUT 1
8
BATT
7
RESET
GND 3
6
WDI
CE IN 4
5
CE OUT
8
BATT
7
RESET
GND 3
6
MR
PFI 4
5
PFO
VCC 2
MAX818
OUT
MAX817
MAX818
MAX819
0.1µF
DIP/SO/µMAX
A0–A15
RESET
RESET
WDI**
I/O
µP
CE OUT*
GND
CE IN*
*CE IN AND CE OUT APPLY TO MAX818 ONLY.
**WDI APPLIES TO MAX817/MAX818 ONLY.
ADDRESS
DECODE
OUT 1
VCC 2
MAX819
DIP/SO/µMAX
______________________________________________________________________________________
15
MAX817L/M, MAX818L/M, MAX819L/M*
Watchdog Software Considerations
(MAX817/MAX818)
MAX817L/M, MAX818L/M, MAX819L/M*
+5V Microprocessor Supervisory Circuits
_Ordering Information (continued)
PART†
TEMP. RANGE
PIN-PACKAGE
MAX817_EPA
-40°C to +85°C
8 Plastic DIP
MAX817_ESA
MAX818_CPA
MAX818_CSA
-40°C to +85°C
0°C to +70°C
0°C to +70°C
8 SO
8 Plastic DIP
8 SO
MAX818_CUA
MAX818_EPA
MAX818_ESA
MAX819_CPA
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
0°C to +70°C
8 µMAX
8 Plastic DIP
8 SO
8 Plastic DIP
MAX819_CSA
MAX819_CUA
MAX819_EPA
MAX819_ESA
0°C to +70°C
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
8 SO
8 µMAX
8 Plastic DIP
8 SO
___________________Chip Information
TRANSISTOR COUNT: 719
†These parts offer a choice of reset threshold voltage. From the
table below, select the suffix corresponding to the desired
threshold and insert it into the blank to complete the part number.
SUFFIX
RESET THRESHOLD (V)
L
4.65
M
4.40
________________________________________________________Package Information
DIM
C
α
A
0.101mm
0.004 in
e
B
A1
L
A
A1
B
C
D
E
e
H
L
α
INCHES
MAX
MIN
0.044
0.036
0.008
0.004
0.014
0.010
0.007
0.005
0.120
0.116
0.120
0.116
0.0256
0.198
0.188
0.026
0.016
6°
0°
MILLIMETERS
MIN
MAX
0.91
1.11
0.10
0.20
0.25
0.36
0.13
0.18
2.95
3.05
2.95
3.05
0.65
4.78
5.03
0.41
0.66
0°
6°
21-0036D
E
H
8-PIN µMAX
MICROMAX SMALL-OUTLINE
PACKAGE
D
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.
16 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600
© 1996 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.