19-1086; Rev 1; 11/05
8-Pin µP Supervisory Circuits
with ±1.5% Reset Accuracy
The MAX801/MAX808 microprocessor (µP) supervisory
circuits monitor and control the activities of +5V µPs by
providing backup-battery switchover, low-line indication, and µP reset. Additional features include a watchdog for the MAX801 and CMOS RAM write protection
for the MAX808.
The MAX801/MAX808 offer a choice of reset-threshold
voltage (denoted by suffix letter): 4.675V (L), 4.575V
(N), and 4.425V (M). These devices are available in
8-pin DIP and SO packages.
________________________Applications
Computers
Controllers
____________________________Features
♦ Precision Voltage Monitoring, ±1.5% Reset
Accuracy
♦ 200ms Power-OK/Reset Time Delay
♦ RESET Output (MAX808)
RESET and RESET Outputs (MAX801)
♦ Watchdog Timer (MAX801)
♦ On-Board Gating of Chip-Enable Signals (MAX808):
Memory Write-Cycle Completion
3ns CE Gate Propagation Delay
♦ 1µA Standby Current
♦ Power Switching:
250mA in VCC Mode
20mA in Battery-Backup Mode
♦ MaxCap™/SuperCap™ Compatible
♦ RESET Guaranteed Valid to VCC = 1V
♦ Low-Line Threshold 52mV Above Reset
Threshold
Intelligent Instruments
Critical µP Power Monitoring
MaxCap is a trademark of The Carborundum Corp.
SuperCap is a trademark of Baknor Industries.
Portable/Battery-Powered Equipment
Embedded Systems
______________Ordering Information
PART*
Pin Configurations appear at end of data sheet.
__________Typical Operating Circuit
+5V
0.1μF
0.1μF
OUT
VCC
BATT
LOWLINE
POWER FOR
CMOS RAM
μP
POWER
NMI
0.1μF
RESET
RESET
µP SYSTEM
MAX808
CE IN
CE OUT
GND
FROM I/O SYSTEM OR
ADDRESS DECODER
TEMP RANGE
PIN-PACKAGE
MAX801_CPA
0°C to +70°C
8 Plastic DIP
MAX801_CSA
MAX801_EPA
MAX801_ESA
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
8 SO
8 Plastic DIP
8 SO
MAX801_MJA
MAX808_CPA
MAX808_CSA
MAX808_EPA
-55°C to +125°C
0°C to +70°C
0°C to +70°C
-40°C to +85°C
8 CERDIP**
8 Plastic DIP
8 SO
8 Plastic DIP
MAX808_ESA
-40°C to +85°C
8 SO
MAX808_MJA
-55°C to +125°C
8 CERDIP**
*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.
**Contact factory for availability and processing to MIL-STD-883.
Devices in PDIP, SO and µMAX 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.
RESET THRESHOLD (V)
SUFFIX
MIN
TYP
MAX
L
4.60
4.675
4.75
N
4.50
4.575
4.65
M
4.35
4.425
4.50
TO CMOS RAM
________________________________________________________________ Maxim Integrated Products
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.
1
MAX801L/M/N, MAX808L/M/N
_______________General Description
MAX801L/M/N, MAX808L/M/N
8-Pin µP Supervisory Circuits
with ±1.5% Reset Accuracy
ABSOLUTE MAXIMUM RATINGS
Input Voltage (with respect to GND)
VCC .......................................................................-0.3V to +6V
VBATT ....................................................................-0.3V to +6V
All Other Pins........................................-0.3V to (VOUT + 0.3V)
Input Current
VCC Peak ..........................................................................1.0A
VCC Continuous ............................................................500mA
IBATT Peak.....................................................................250mA
IBATT Continuous ............................................................50mA
GND ................................................................................50mA
All Other Inputs ...............................................................50mA
Output Current
OUT Peak..........................................................................1.0A
OUT Continuous............................................................500mA
All Other Outputs ............................................................50mA
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
CERDIP (derate 8.00mW/°C above +70°C) .................640mW
Operating Temperature Ranges
MAX801_C_A/MAX808_C_A...............................0°C to +70°C
MAX801_E_A/MAX808_E_A ............................-40°C to +85°C
MAX801_MJA/MAX808_MJA.........................-55°C to +125°C
Storage Temperature Range .............................-65°C to +160°C
Lead Temperature (soldering, 10sec) .............................+300°C
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 = 4.6V to 5.5V for the MAX80_L, VCC = 4.5V to 5.5V for the MAX80_N, VCC = 4.35V to 5.5V for the MAX80_M; VBATT = 2.8V;
TA = TMIN to TMAX. Typical values are at VCC = 5V and TA = +25°C, unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
Operating Voltage Range
VCC, BATT (Note 1)
MIN
TYP
MAX
UNITS
0
X
5.5
V
IOUT = 25mA
VOUT in Normal Operating
Mode
VCC to OUT
On-Resistance
VCC = 4.5V
VCC - 0.02
IOUT = 250mA, MAX80_C/E
VCC - 0.38
IOUT = 250mA, MAX80_M
VCC - 0.45
VCC = 3V, VBATT = 2.8V, IOUT = 100mA
VCC - 0.25
MAX80_C/E
BATT to OUT
On-Resistance
VCC = 0V
1.5
1.8
VCC = 3V, IOUT = 100mA
VCC = 0V
V
VCC - 0.12
1.0
VCC = 4.5V,
IOUT = 250mA MAX80_M
VOUT in Battery-Backup
Mode
VCC - 0.25
1.2
VBATT = 4.5V, IOUT = 20mA
VBATT - 0.16
VBATT = 2.8V, IOUT = 10mA
VBATT - 0.25 VBATT - 0.12
VBATT = 2.0V, IOUT = 5mA
VBATT - 0.20 VBATT - 0.08
2.5
V
VBATT = 4.5V, IOUT = 20mA
8
VBATT = 2.8V, IOUT = 10mA
12
25
VBATT = 2.0V, IOUT = 5mA
16
40
Supply Current in Normal
Operating Mode
(excludes IOUT)
MAX801
68
110
MAX808
48
90
Supply Current in BatteryBackup Mode (excludes
IOUT) (Note 2)
TA = +25°C
VCC = 0V,
VBATT = 2.8V TA = TMIN
to TMAX
BATT Standby Current
(Note 3)
VBATT + 0.2V TA = +25°C
≤ VCC
TA = TMIN to TMAX
Battery-Switchover
Threshold
VBATT = 2.8V
Battery-Switchover
Hysteresis
2
Ω
Ω
µA
Power-up
Power-down
0.4
MAX80_C/E
1
5
MAX80_M
µA
50
-0.1
0.1
-1.0
1.0
VBATT + 0.05
VBATT
50
_______________________________________________________________________________________
µA
V
mV
8-Pin µP Supervisory Circuits
with ±1.5% Reset Accuracy
(VCC = 4.6V to 5.5V for the MAX80_L, VCC = 4.5V to 5.5V for the MAX80_N, VCC = 4.35V to 5.5V for the MAX80_M; VBATT = 2.8V;
TA = TMIN to TMAX. Typical values are at VCC = 5V and TA = +25°C, unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
RESET AND LOW-LINE
Reset Threshold
VRST
VCC rising
and falling
MAX80_L
4.600
4.675
4.750
MAX80_N
4.500
4.575
4.650
MAX80_M
4.350
4.425
4.500
Reset-Threshold Hysteresis
LOWLINE to RESET
Threshold Voltage
13
VLR
VCC falling
30
V
mV
52
70
MAX80_L
4.73
4.81
MAX80_N
4.63
4.71
MAX80_M
4.48
4.56
mV
LOWLINE Threshold,
VCC Rising
VLL
VCC to RESET Delay
tRD
VCC falling at 1mV/µs
17
µs
VCC to LOWLINE Delay
tLL
VCC falling at 1mV/µs
17
µs
RESET Active Timeout
Period
tRP
VCC rising
140
ISINK = 50µA, VCC = 1.0V, MAX80_C
VBATT = 0V,
VCC = 1.2V, MAX80_E/M
VCC falling
RESET Output Voltage
ISOURCE = 0.1mA
ISC
0.3
RESET Output ShortCircuit Current (MAX801)
VCC - 1.5
ISC
40
Output source current
1.6
mA
0.4
55
Output source current, VCC = 4.25V
15
ISINK = 3.2mA, VCC = 4.25V
ISC
V
0.4
VCC - 1.5
Output sink current
ISOURCE = 5mA, VCC = 4.25V
ms
VCC - 0.1
Output sink current, VCC = 4.25V
ISOURCE = 5mA, VCC = 4.25V
LOWLINE Output Voltage
LOWLINE Output
Short-Circuit Current
0.1
ISINK = 3.2mA
RESET Output Voltage
(MAX801)
280
0.3
ISINK = 3.2mA, VCC = 4.25V
RESET Output
Short-Circuit Current
200
V
mA
0.4
VCC - 1.5
Output sink current, VCC = 4.25V
40
Output source current
20
V
V
mA
WATCHDOG TIMER (MAX801)
Watchdog Timeout Period
tWD
Minimum Watchdog Input
Pulse Width
WDI Threshold Voltage
(Note 4)
WDI Input Current
1.12
VIL = 0.8V, VIH = 0.75V x VCC
VIH
1.6
2.24
100
ns
0.75 x VCC
VIL
0.8
RESET deasserted, WDI = 0V
RESET deasserted, WDI = VCC
-50
sec
-10
16
50
V
µA
_______________________________________________________________________________________
3
MAX801L/M/N, MAX808L/M/N
ELECTRICAL CHARACTERISTICS (continued)
ELECTRICAL CHARACTERISTICS (continued)
(VCC = 4.6V to 5.5V for the MAX80_L, VCC = 4.5V to 5.5V for the MAX80_N, VCC = 4.35V to 5.5V for the MAX80_M; VBATT = 2.8V;
TA = TMIN to TMAX. Typical values are at VCC = 5V and TA = +25°C, unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
±0.00002
±1
µA
150
Ω
CHIP-ENABLE GATING (MAX808)
CE IN Leakage Current
VCC = 4.25V
CE IN to CE OUT
Resistance (Note 5)
Enabled mode, VCC = VRST(max)
75
CE OUT Short-Circuit
Current (RESET Active)
VCC = 4.25V, CE OUT = 0V
15
CE IN to CE OUT
Propagation Delay (Note 6)
VCC = 5V, CLOAD = 50pF,
50Ω source-impedance driver
3
CE OUT Output Voltage
High (RESET Active)
VCC = 4.25V, IOUT = 2mA
VCC = 0V, IOUT = 10µA
RESET to CE OUT Delay
(Note 7)
mA
8
3.5
VBATT - 0.1
VCC falling, CE IN = 0V
ns
V
VBATT
18
µs
Note 1: Either VCC or VBATT can go to 0V if the other is greater than 2V.
Note 2: The supply current drawn by the MAX80_ from the battery (excluding IOUT) typically goes to 15µA when (VBATT - 0.1V) <
VCC < VBATT. In most applications, this is a brief period as VCC falls through this region (see Typical Operating
Characteristics).
Note 3: “+” = battery-discharging current, “-” = battery-charging current.
Note 4: WDI is internally connected to a voltage divider between VCC and GND. If unconnected, WDI is typically driven to 1.8V,
disabling the watchdog function.
Note 5: The chip-enable resistance is tested with V CE IN = VCC / 2 and I CE IN = 1mA.
Note 6: The chip-enable propagation delay is measured from the 50% point at CE IN to the 50% point at CE OUT.
Note 7: If CE IN goes high, CE OUT goes high immediately and stays high until reset is deasserted and CE IN is low.
__________________________________________Typical Operating Characteristics
(VCC = 5V, VBATT = 2.8V, no load, TA = +25°C, unless otherwise noted.)
65
60
55
50
MAX808
45
2.0
1.5
1.0
0.5
-55 -35 -15
5
25
45 65 85 105 125
TEMPERATURE (°C)
MAX801/808-03
5
4
3
2
1
0
40
4
2.5
PROPAGATION DELAY (ns)
MAX801
6
MAX801/808-02
70
3.0
BATTERY SUPPLY CURRENT (μA)
MAX801/808-01
75
MAX808
CHIP-ENABLE PROPAGATION DELAY
vs. TEMPERATURE
BATTERY SUPPLY CURRENT vs.
TEMPERATURE (BATTERY-BACKUP MODE)
VCC SUPPLY CURRENT vs. TEMPERATURE
(NORMAL OPERATING MODE)
VCC SUPPLY CURRENT (μA)
MAX801L/M/N, MAX808L/M/N
8-Pin µP Supervisory Circuits
with ±1.5% Reset Accuracy
0
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (°C)
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (°C)
_______________________________________________________________________________________
8-Pin µP Supervisory Circuits
with ±1.5% Reset Accuracy
4
2
0
50
MAX801/808-05
VBATT = 2.0V
20
15
VBATT = 2.8V
10
VBATT = 4.5V
1.5
IOUT = 250mA
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
-60 -40 -20 0
100
-60 -40 -20 0
20 40 60 80 100 120 140
20 40 60 80 100 120 140
CLOAD (pF)
TEMPERATURE (°C)
TEMPERATURE (°C)
RESET THRESHOLD
vs. TEMPERATURE
RESET TIMEOUT PERIOD
vs. TEMPERATURE (VCC RISING)
LOWLINE to RESET THRESHOLD
vs. TEMPERATURE (VCC FALLING)
4.60
MAX80_N
4.55
4.50
4.45
240
220
200
180
160
MAX80_M
4.40
-60 -40 -20 0
140
20 40 60 80 100 120 140
MAX801/808-09
260
80
LOWLINE TO RESET THRESHOLD (mV)
MAX80_L
MAX801/808-08
4.65
280
RESET TIMEOUT PERIOD (ms)
MAX801/808-07
4.70
70
60
50
40
30
20
10
0
-60 -40 -20 0
20 40 60 80 100 120 140
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (°C)
TEMPERATURE (°C)
LOWLINE THRESHOLD
vs. TEMPERATURE (VCC RISING)
LOWLINE COMPARATOR PROPAGATION
DELAY vs. TEMPERATURE (VCC FALLING)
RESET COMPARATOR PROPAGATION
DELAY vs. TEMPERATURE (VCC FALLING)
4.70
MAX80_N
4.65
4.60
4.55
MAX80_M
4.50
VCC FALLING AT 1mV/µs
35
30
25
20
15
10
5
4.45
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (°C)
VCC FALLING AT 1mV/µs
35
30
25
20
15
10
5
0
4.40
40
PROPAGATION DELAY (µs)
MAX80_L
4.75
40
MAX801/808-10
4.80
MAX801/808-12
TEMPERATURE (°C)
PROPAGATION DELAY (µs)
RESET THRESHOLD (V)
25
1.6
5
0
LOWLINE THRESHOLD (V)
VCC = 0V
IOUT = 10mA
VCC TO VOUT ON-RESISTANCE (Ω)
6
30
MAX801/808-11
PROPAGATION DELAY (ns)
50Ω DRIVER
VBATT TO VOUT ON-RESISTANCE (Ω)
MAX801/808-04
8
VCC to OUT ON-RESISTANCE
vs. TEMPERATURE
BATT to OUT ON-RESISTANCE
vs. TEMPERATURE
MAX801/808-06
MAX808
CHIP-ENABLE PROPAGATION DELAY
vs. CE OUT LOAD CAPACITANCE
0
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (°C)
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (°C)
_______________________________________________________________________________________
5
MAX801L/M/N, MAX808L/M/N
____________________________Typical Operating Characteristics (continued)
(VCC = 5V, VBATT = 2.8V, no load, TA = +25°C, unless otherwise noted.)
____________________________Typical Operating Characteristics (continued)
(VCC = 5V, VBATT = 2.8V, no load, TA = +25°C, unless otherwise noted.)
BATTERY CURRENT
vs. INPUT SUPPLY VOLTAGE
12
10
8
6
4
MAX801/808-14
1000
VBATT TO VOUT VOLTAGE (mV)
14
BATTERY CURRENT (µA)
BATT to OUT VOLTAGE vs.
OUTPUT CURRENT
MAX801/808-13
16
VCC = 0V
SLOPE = 12Ω
100
2
0
10
2.6
2.7
2.8
2.9
3.0
1
10
100
VCC (V)
IOUT (mA)
VCC to OUT VOLTAGE vs.
OUTPUT CURRENT
MAXIMUM TRANSIENT DURATION vs.
RESET THRESHOLD OVERDRIVE
SLOPE = 1.0Ω
100
10
1000
MAX801/808-16
MAX801/808-15
1000
MAXIMUM TRANSIENT DURATION (µs)
2.5
VCC TO VOUT VOLTAGE (mV)
MAX801L/M/N, MAX808L/M/N
8-Pin µP Supervisory Circuits
with ±1.5% Reset Accuracy
RESET OCCURS
100
10
1
1
1
10
100
1000
1
10
100
1000
RESET THRESHOLD OVERDRIVE (mV)
IOUT (mA)
______________________________________________________________Pin Description
PIN
6
NAME
FUNCTION
MAX801
MAX808
1
1
VCC
2
2
LOWLINE
Low-Line Comparator Output. This CMOS-logic output goes low when VCC falls to 52mV
above the reset threshold. Use LOWLINE to generate an NMI, initiating an orderly shutdown routine when VCC is falling. LOWLINE swings between VCC and GND.
Active-Low Reset Output. RESET is triggered and stays low when VCC is below the reset
threshold (or during a watchdog timeout for the MAX801). It remains low 200ms after
VCC rises above the reset threshold (or 200ms after the watchdog timeout occurs).
RESET has a strong pull-down but a relatively weak pull-up, and can be wire-OR connected to logic gates. Valid for VCC ≥ 1V. RESET swings between VCC and GND.
3
3
RESET
4
4
GND
Input Supply Voltage, nominally +5V. Bypass with a 0.1µF capacitor to GND.
Ground
_______________________________________________________________________________________
8-Pin µP Supervisory Circuits
with ±1.5% Reset Accuracy
PIN
NAME
FUNCTION
MAX801
MAX808
5
—
RESET
—
5
CE OUT
6
—
WDI
—
6
CE IN
Chip-Enable Input
Active-High Reset Output. RESET is the inverse of RESET. It is a CMOS output that
sources and sinks current. RESET swings between VCC and GND.
Chip-Enable Output. Output to the chip-enable gating circuit. CE OUT is pulled up to
the higher of VCC or VBATT when the chip-enable gate is disabled.
Watchdog Input. If WDI remains high or low longer than the watchdog timeout period
(typically 1.6sec), RESET will be asserted for 200ms. Leave unconnected to disable the
watchdog function.
7
7
BATT
Backup-Battery Input. When VCC falls below the reset threshold and VBATT, OUT switches from VCC to BATT. VBATT may exceed VCC. The battery can be removed while the
MAX801/MAX808 is powered up, provided BATT is bypassed with a 0.1µF capacitor to
GND. If no battery is used, connect BATT to ground and VCC to OUT.
8
8
OUT
Output Supply Voltage to CMOS RAM. When VCC exceeds the reset threshold or VBATT,
OUT connects to VCC. When VCC falls below the reset threshold and VBATT, OUT connects to BATT. Bypass OUT with a 0.1µF capacitor to GND.
VCC
OUT
BATT
BATTERY-BACKUP
COMPARATOR
MAX801
MAX808
LOWLINE
RESET
COMPARATOR
MAX801 ONLY
WATCHDOG
TRANSITION
DETECTOR
WDI
LOW-LINE
COMPARATOR
RESET (MAX801 ONLY)
STATE
MACHINE
OSCILLATOR
RESET
2.275V
GND
THE HIGHER
OF VCC
OR VBATT
MAX808 ONLY
P
P
CE IN
CE OUT
N
Figure 1. Functional Diagram
_______________________________________________________________________________________
7
MAX801L/M/N, MAX808L/M/N
_________________________________________________Pin Description (continued)
MAX801L/M/N, MAX808L/M/N
8-Pin µP Supervisory Circuits
with ±1.5% Reset Accuracy
VRST VLL
VRST + VLR VRST
VCC
VCC
VLOWLINE
VLOWLINE
VRESET
tRP
VRESET
VRESET
(MAX801)
tRP
VRESET
(MAX801)
VCE OUT
(MAX808)
VCE OUT
(MAX808)
VBATT
SHOWN FOR VCC = 0V to 5V, VBATT = 2.8V, CE IN = GND
tLL
tRD
tRD
tRCE
VBATT
SHOWN FOR VCC = 5V to 0V, VBATT = 2.8V, CE IN = GND
Figure 2a. Timing Diagram, VCC Rising
Figure 2b. Timing Diagram, VCC Falling
_______________Detailed Description
The RESET output is active low, and is implemented with
a strong pull-down/relatively weak pull-up structure. It is
guaranteed to be a logic low for 0V < VCC < VRST, provided VBATT is greater than 2V. Without a backup battery, RESET is guaranteed valid for VCC ≥ 1V.
The RESET output is the inverse of the RESET output; it
both sources and sinks current and cannot be wire-OR
connected.
The MAX801/MAX808 microprocessor (µP) supervisory
circuits provide power-supply monitoring and backupbattery switchover in µP systems. The MAX801 also
provides program-execution watchdog functions
(Figure 1). Use of BiCMOS technology results in an
improved, 1.5% reset-threshold precision while keeping
supply currents typically at 68µA (48µA for the
MAX808). The MAX801/MAX808 are intended for battery-powered applications that require high resetthreshold precision, allowing a wide power-supply
operating range while preventing the system from operating below its specified voltage range.
RESET and RESET Outputs
The MAX801/MAX808’s RESET output ensures that the
µP powers up in a known state, and prevents codeexecution errors during power-down and brownout
conditions. It does this by resetting the µP, terminating
program execution when V CC dips below the reset
threshold. Each time RESET is asserted, it stays low for
at least the 200ms reset timeout period (set by an internal timer) to ensure the µP has adequate time to return
to an initial state. The internal timer restarts any time
VCC goes below the reset threshold (VRST) before the
reset timeout period is completed. The watchdog timer
on the MAX801 can also initiate a reset (see the
MAX801 Watchdog Timer section).
8
Low-Line Comparator
The low-line comparator monitors VCC with a threshold
voltage typically 52mV above the reset threshold, with
13mV of hysteresis. Use LOWLINE to provide a nonmaskable interrupt (NMI) to the µP when power begins
to fall, initiating an orderly software shutdown routine. In
most battery-operated portable systems, reserve energy in the battery provides ample time to complete the
shutdown routine once the low-line warning is encountered and before reset asserts. If the system must contend with a more rapid VCC fall time (such as when the
main battery is disconnected, when a DC-DC converter
shuts down, or when a high-side switch is opened during normal operation), use capacitance on the VCC line
to provide time to execute the shutdown routine (Figure
3). First calculate the worst-case time required for the
system to perform its shutdown routine. Then, with
worst-case shutdown time, worst-case load current,
and minimum low-line to reset threshold (VLR(min) ),
_______________________________________________________________________________________
8-Pin µP Supervisory Circuits
with ±1.5% Reset Accuracy
VCC
LOWLINE
TO μP NMI
P
VCC
MAX801L/M/N, MAX808L/M/N
4.5V to 5.5V
REGULATOR
MAX801
MAX808
CHOLD
CONTROL
CIRCUITRY
MAX801
MAX808
OUT
0.1μF
CHOLD > ILOAD x tSHDN
VLR
GND
BATT
P
P
Figure 3. Using LOWLINE to Provide a Power-Fail Warning to
the µP
Figure 4. VCC and BATT to OUT Switch
calculate the amount of capacitance required to allow
the shutdown routine to complete before reset is
asserted:
CHOLD = (ILOAD x tSHDN) / (VLR(min))
where tSHDN is the time required for the system to complete the shutdown routine (including the VCC to lowline propagation delay), I LOAD is the current being
drained from the capacitor, and VLR is the low-line to
reset threshold.
Table 1. Input and Output Status in
Battery-Backup Mode
PIN
NAME
STATUS
MAX801 MAX808
Battery switchover
comparator monitors VCC
for active switchover.
1
1
VCC
Output Supply Voltage
2
2
LOWLINE
Logic low
The output supply (OUT) transfers power from VCC or
BATT to the µP, RAM, and other external circuitry. At
the maximum source current of 250mA, VOUT will typically be 220mV below VCC. Decouple OUT with a 0.1µF
capacitor to ground.
3
3
RESET
Logic low
4
4
GND
5
—
RESET
Logic high; the open-circuit
voltage is equal to VCC.
—
5
CE OUT
Logic high. The open-circuit
output voltage is equal to
VBATT (MAX808).
6
—
WDI
—
6
CE IN
High impedance (MAX808)
7
7
BATT
Supply current is 1µA max for
VBATT ≤ 2.8V.
8
8
OUT
OUT is connected to BATT
through two internal PMOS
switches in series.
Battery-Backup Mode
Battery-backup mode preserves the contents of RAM in
the event of a brownout or power failure. With a backup
battery installed at BATT, the MAX801/MAX808 automatically switches RAM to backup power when VCC falls.
Two conditions are required for switchover to batterybackup mode: 1) VCC must be below the reset threshold;
2) VCC must be below VBATT. Table 1 lists the status of
inputs and outputs during battery-backup mode.
BATT is designed to conduct up to 20mA to OUT during battery backup. The PMOS switch on-resistance is
approximately 12Ω. Figure 4 shows the two series pass
elements (between the BATT input and OUT) that
facilitate UL recognition. VBATT can exceed VCC during
normal operation without causing a reset.
Ground—0V reference for
all signals
WDI is ignored and goes
high impedance.
_______________________________________________________________________________________
9
MAX801L/M/N, MAX808L/M/N
8-Pin µP Supervisory Circuits
with ±1.5% Reset Accuracy
MAX801 Watchdog Timer
The watchdog monitors the µP’s activity. If the µP does
not toggle the watchdog input (WDI) within 1.6sec,
reset asserts for the reset timeout period. The internal
1.6sec timer is cleared when reset asserts or when a
transition (low-to-high or high-to-low) occurs at WDI
while reset is not asserted. The timer remains cleared
and does not count as long as reset is asserted. It
starts counting as soon as reset is released (Figure 5).
Supply current is typically reduced by 10µA when WDI
is at a valid logic level. To disable the watchdog function, leave WDI unconnected. An internal voltage
divider sets WDI to about mid-supply, disabling the
watchdog timer/counter.
VCC
tRP
RESET
tRP
tWD
WDI
MAX808 Chip-Enable Gating
The MAX808 provides internal gating of chip-enable
(CE) signals to prevent erroneous data from corrupting
CMOS RAM in the event of a power failure. During normal operation, the CE 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 MAX808 uses a series
transmission gate from the chip-enable input (CE IN) to
the chip-enable output (CE OUT) (Figure 1). The 8ns
max chip-enable propagation from CE IN to CE OUT
enables the MAX808 to be used with most µPs.
The MAX808 also features write-cycle-completion circuitry. If VCC falls below the reset threshold while the
µP is writing to RAM, the MAX808 holds the CE gate
enabled for 18µs to allow the µP to complete the write
instruction. If the write cycle has not completed by the
end of the 18µs period, the CE transmission gate turns
off and CE OUT goes high. If the µP completes the
write instruction during the 18µs period, the CE gate
turns off (high impedance) and CE OUT goes high as
soon as the µP pulls CE IN high. CE OUT remains high,
even if CE IN falls low for any reason (Figure 6).
Chip-Enable Input
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. CE IN becomes high impedance 18µs after
reset asserts, provided CE IN is still low. If the µP completes the write instruction during the 18µs period, the
CE gate turns off. CE IN becomes high impedance as
soon as the µP pulls CE IN high. CE IN remains high
impedance even if the signal at CE IN falls low (Figure
6). During a power-up sequence, CE IN remains high
impedance (regardless of CE IN activity) until reset is
deasserted following the reset timeout period.
10
Figure 5. Watchdog Timing
VCC
RESET
THRESHOLD
CE IN
CE OUT
18μs
17μs
18μs
17μs
RESET
Figure 6. Chip-Enable Timing
In high-impedance mode, the leakage currents into this
input are ±1µA max over temperature. In low-impedance mode, the impedance of CE IN appears as a 75Ω
resistor in series with the load at CE OUT.
The propagation delay through the CE transmission
gate depends on both the source impedance of the
drive to CE IN and the capacitive loading on CE OUT
(see the Chip-Enable Propagation Delay vs. CE OUT
Load Capacitance graph in the 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 50pF of load
capacitance (Figure 7). For minimum propagation
delay, minimize the capacitive load at CE OUT and use
a low-output-impedance driver.
______________________________________________________________________________________
8-Pin µP Supervisory Circuits
with ±1.5% Reset Accuracy
+5V
VCC
BATT
MAX808
CE IN
VCC
1N4148
0.47F
CE OUT
50pF CLOAD
50Ω DRIVER
OUT
GND
MAX801
MAX808
GND
Figure 7. MAX808 CE Gate Test Circuit
Figure 8. Using the MAX801/MAX808 with a SuperCap
Chip-Enable Output
In enabled mode, CE OUT’s impedance is equivalent to
75Ω in series with the source driving CE IN. In disabled
mode, the 75Ω transmission gate is off and CE OUT is
actively pulled to the higher of V CC or V BATT . The
source turns off when the transmission gate is enabled.
circuit as a backup source (Figure 8). Since VBATT can
exceed VCC while VCC is above the reset threshold, no
special precautions are needed when using these µP
supervisors with a SuperCap.
__________Applications Information
The MAX801/MAX808 are not short-circuit protected.
Shorting OUT to ground, other than power-up transients
such as charging a decoupling capacitor, may destroy
the device. If long leads connect to the IC’s inputs,
ensure that these lines are free from ringing and other
conditions that would forward bias the IC’s protection
diodes. Bypass OUT, V CC , and BATT with 0.1µF
capacitors to GND.
The MAX801/MAX808 operate in two distinct modes:
1) Normal Operating Mode, with all circuitry powered
up. Typical supply current from VCC is 68µA (48µA
for the MAX808), while only leakage currents flow
from the battery.
2) Battery-Backup Mode, where VCC is below VBATT
and VRST. The supply current from the battery is typically less than 1µA.
Using SuperCaps™ or MaxCaps™
with the MAX801/MAX808
BATT has the same operating voltage range as VCC, and
the battery-switchover threshold voltage is typically
VBATT when VCC is decreasing or VBATT + 0.05V when
V CC is increasing. This hysteresis allows use of a
SuperCap (e.g., around 0.47F) and a simple charging
Backup-Battery Replacement
The backup battery can be disconnected while VCC is
above the reset threshold, provided BATT is bypassed
with a 0.1µF capacitor to ground. No precautions are
necessary to avoid spurious reset pulses.
Negative-Going VCC Transients
While issuing resets to the µP during power-up, powerdown, and brownout conditions, these supervisors are
relatively immune to short-duration, negative-going VCC
transients (glitches). It is usually undesirable to reset
the µP when VCC experiences only small glitches.
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 5V and ending below the reset
threshold by the magnitude indicated (reset comparator overdrive). The graph shows the maximum pulse
width that a negative-going VCC transient may typically
have without causing a reset pulse to be issued. 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 40mV below the reset threshold and lasts for 3µs
or less will not cause a reset pulse to be issued. A
0.1µF bypass capacitor mounted close to the VCC pin
provides additional transient immunity.
______________________________________________________________________________________
11
MAX801L/M/N, MAX808L/M/N
VRST(max)
MAX801L/M/N, MAX808L/M/N
8-Pin µP Supervisory Circuits
with ±1.5% Reset Accuracy
Watchdog Software Considerations
To help the watchdog timer keep a closer watch on
software execution, you can set and reset the watchdog input at different points in the program, rather than
“pulsing” the watchdog input high-low-high or low-highlow. This technique avoids a “stuck” loop, where the
watchdog timer continues to be reset within the loop,
keeping the watchdog from timing out.
Figure 9 shows a sample flow diagram where the I/O
driving the watchdog input is set high at the beginning
of the program, low at the beginning of every subroutine or loop, then high again when the program returns
to the beginning. If the program should “hang” in any
subroutine, the I/O would be continually set low and the
watchdog timer would be allowed to time out, causing a
reset or interrupt to be issued.
Maximum VCC Fall Time
The VCC fall time is limited by the propagation delay of
the battery switchover comparator and should not
exceed 0.03V/µs. A standard rule for filter capacitance
on most regulators is around 100µF per Ampere of current. When the power supply is shut off or the main battery is disconnected, the associated initial VCC fall rate
is just the inverse, or 1A/100µF = 0.01V/µs.
_________________Pin Configurations
START
SET
WDI
LOW
SUBROUTINE
OR PROGRAM LOOP,
SET WDI
HIGH
RETURN
END
Figure 9. Watchdog Flow Diagram
___________________Chip Information
TRANSISTOR COUNT: 922
TOP VIEW
VCC 1
8
OUT
7
BATT
RESET 3
6
WDI
GND 4
5
RESET
8
OUT
7
BATT
RESET 3
6
CE IN
GND 4
5
CE OUT
LOWLINE 2
MAX801
DIP/SO
VCC 1
LOWLINE 2
MAX808
DIP/SO
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.
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