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Microprocessor Supervisory Circuits
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
__________________General Description
___________________________Features
The MAX690A/MAX692A/MAX802L/MAX802M/MAX805L ♦ Precision Supply-Voltage Monitor:
reduce the complexity and number of components 4.65V for MAX690A/MAX802L/MAX805L
required for power-supply monitoring and battery-control 4.40V for MAX692A/MAX802M
functions in microprocessor (µP) systems. They signifi- ♦ Reset Time Delay – 200ms
cantly improve system reliability and accuracy comparedto separate ICs or discrete components.
♦ Watchdog Timer – 1.6sec Timeout
♦ Battery-Backup Power Switching
1) A reset output during power-up, power-down, and ♦ 200µA Quiescent Supply Current
♦ 50nA Quiescent Supply Current in Battery-
2) Battery-backup switching for CMOS RAM, CMOS Backup Mode
♦ Voltage Monitor for Power-Fail or Low-Battery
3) A reset pulse if the optional watchdog timer has not ♦ Power-Fail Accuracy Guaranteed to ±2%
4) A 1.25V threshold detector for power-fail warning or (MAX802L/M)
low-battery detection, or to monitor a power supply ♦ Guaranteed RESET Assertion to V
♦ 8-Pin SO and DIP Packages
The parts differ in their reset-voltage threshold levelsand reset outputs. The MAX690A/MAX802L/MAX805Lgenerate a reset pulse when the supply voltage drops ______________Ordering Information
below 4.65V, and the MAX692A/MAX802M generate areset below 4.40V. The MAX802L/MAX802M guaran- TEMP. RANGE
PIN-PACKAGE
tee power-fail accuracies to ±2%. The MAX805L is thesame as the MAX690A except that RESET is provided MAX690ACPA
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 _______________________Applications
Ordering Information continued on last page.
Battery-Powered Computers and Controllers **Contact factory for availability and processing to MIL-STD-883. _________________Pin Configurations
__________Typical Operating Circuit
________________________________________________________________ Maxim Integrated Products
Call toll free 1-800-998-8800 for free samples or literature.
Microprocessor Supervisory Circuits
ABSOLUTE MAXIMUM RATINGS
Rate of Rise, VCC, VBATT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100V/µs VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to 6.0V VBATT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to 6.0V Plastic DIP (derate 9.09mW/°C above +70°C) . . . . . . . . 727mW All Other Inputs (Note 1) . . . . . . . . . . . . . . . . -0.3V to (VCC + 0.3V) SO (derate 5.88mW/°C above +70°C) . . . . . . . . . . . . . . . . 471mW CERDIP (derate 8.00mW/°C above +70°C) . . . . . . . . . . . 640mW VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200mA VBATT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50mA MAX69_AC_ _, MAX80_ _ C_ _ . . . . . . . . . . . . . . . . . . 0°C to +70°C GND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20mA MAX69_AE_ _, MAX80_ _ E_ _ . . . . . . . . . . . . . . . . -40°C to +85°C MAX69_AMJA, MAX805LMJA . . . . . . . . . . . . . . . . -55°C to +125°C VOUT . . . . . . . . . . . . . . . . . . . Short-Circuit Protected for up to 10sec Storage Temperature Range . . . . . . . . . . . . . . . . . . -65°C to +160°C All Other Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20mA Lead Temperature (soldering, 10sec) . . . . . . . . . . . . . . . . . . +300°C Note 1: The input voltage limits on PFI and WDI may be exceeded if the current into these pins is limited to less than 10mA.
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functionaloperation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure toabsolute 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, VBATT = 2.8V,
TA = TMIN to TMAX, unless otherwise noted.)
PARAMETER
CONDITIONS
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
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Microprocessor Supervisory Circuits
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
ELECTRICAL CHARACTERISTICS (continued)
(VCC = 4.75V to 5.5V for MAX690A/MAX802L/MAX805L, VCC = 4.5V to 5.5V for MAX692A/MAX802M, VBATT = 2.8V,
TA = TMIN to TMAX, unless otherwise noted.)
PARAMETER
CONDITIONS
Note 2: Either VCC or VBATT can go to 0V, if the other is greater than 2.0V.
Note 3: "-" = battery-charging current, "+" = battery-discharging current.
Note 4: WDI is guaranteed to be in an intermediate, non-logic level state if WDI is floating and VCC is in the operating voltage range.
WDI is internally biased to 35% of VCC with an input impedance of 50kΩ.
_______________________________________________________________________________________
Microprocessor Supervisory Circuits
__________________________________________Typical Operating Characteristics
OUTPUT VOLTAGE
OUTPUT VOLTAGE
RESET OUTPUT VOLTAGE
vs. LOAD CURRENT
vs. LOAD CURRENT
vs. SUPPLY VOLTAGE
POWER-FAIL COMPARATOR
POWER-FAIL COMPARATOR
RESET RESPONSE TIME
RESPONSE TIME
RESPONSE TIME
RESET OUTPUT VOLTAGE
vs. SUPPLY VOLTAGE
RESET RESPONSE TIME
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
_______________________________________________________________________________________
Microprocessor Supervisory Circuits
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
______________________________________________________________Pin Description
MAX690A/MAX692A
FUNCTION
MAX802L/MAX802M
Supply Output for CMOS RAM. When VCC is above the reset threshold,V OUT connects to VCC through a P-channel MOSFET switch. When VCC is below the reset threshold, the higher of VCC or VBATT will beconnected to VOUT.
Power-Fail Comparator Input. When PFI is less than 1.25V, PFO goes low. Connect PFI to GND or VCC when not used.
Power-Fail Output. When PFI is less than 1.25V, PFO goes low; Watchdog Input. If WDI remains high or low for 1.6sec, the internalwatchdog timer runs out and reset is triggered. If WDI is left floating or connected to a high-impedance three-state buffer, the watchdogfeature is disabled. The internal watchdog timer clears whenever resetis asserted, WDI is three-stated, or WDI sees a rising or falling edge.
Reset Output. Whenever RESET is triggered, it pulses low for 200ms. Itstays low when VCC is below the reset threshold (4.65V in the MAX690A/MAX802L and 4.4V in the MAX692A/MAX802M) and remainslow for 200ms after VCC rises above the reset threshold. A watchdogtimeout also triggers RESET.
Active-High Reset Output is the inverse of RESET. When RESET is asserted, the RESET output voltage = VCC or VBATT, whichever is higher.
Backup-Battery Input. When VCC falls below the reset threshold, VBATTwill be switched to V OUT if VBATT is 20mV greater than VCC. When VCC rises to 20mV above VBATT, VOUT will be reconnected to VCC. The 40mVhysteresis prevents repeated switching if VCC falls slowly.
_______________________________________________________________________________________
Microprocessor Supervisory Circuits
_______________Detailed Description
ed, the watchdog timer remains clear. When RESETcomes high, the watchdog resumes timing and must be Reset Output
serviced within 1.6sec. If WDI is tied high or low, a A microprocessor’s (µP’s) reset input starts the µP in a RESET pulse is triggered every 1.8sec (tWD plus tRS).
known state. When the µP is in an unknown state, it should The MAX805L active-high RESET output is the inverse be held in reset. The MAX690A/MAX692A/MAX802L/ of the MAX690A/MAX692A/MAX802L/MAX802M RESET MAX802M assert reset during power-up and prevent code output, and is guaranteed to be valid with V execution errors during power-down or brownout condi- 1.1V. Some µPs, such as Intel’s 80C51, require an On power-up, once VCC reaches 1V, RESET is guaran-teed to be a logic low. As V Watchdog Input
low. When VCC exceeds the reset threshold, an internal The watchdog circuit monitors the µP’s activity. If the timer keeps RESET low for a time equal to the reset µP does not toggle the watchdog input (WDI) within pulse width; after this interval, RESET goes high (Figure 1.6sec, a reset pulse is triggered. The internal 1.6sec 2). If a brownout condition occurs (if VCC dips below timer is cleared by either a reset pulse or by open cir- the reset threshold), RESET is triggered. Each time cuiting the WDI input. As long as reset is asserted or RESET is triggered, it stays low for the reset pulse width the WDI input is open circuited, the timer remains interval. Any time VCC goes below the reset threshold, cleared and does not count. As soon as reset is the internal timer restarts the pulse. If a brownout con- released or WDI is driven high or low, the timer starts dition interrupts a previously initiated reset pulse, the counting. It can detect pulses as short as 50ns.
reset pulse continues for another 200ms. On power- MAX690A/MAX692A/MAX802L/MAX802M/MAX805L down, once V
Power-Fail Comparator
The PFI input is compared to an internal 1.25V refer- ence. If PFI is less than 1.25V, PFO goes low. The RESET is also triggered by a watchdog timeout. If ahigh or low is continuously applied to the WDI pin for power-fail comparator is intended for use as an under- voltage detector to signal a failing power supply; it RESET pulses low. As long as RESET is assert- _______________________________________________________________________________________
Microprocessor Supervisory Circuits
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
CONDITION
Figure 4. Using a SuperCap as a Backup Power Source with a MAX690A/MAX802L/MAX805L and a +5V ±5% Supply RESET THRESHOLD = 4.65V IN MAX690A/MAX802L/MAX805L.
RESET THRESHOLD = 4.4V IN MAX692A/MAX802M Figure 3. Backup-Battery Switchover Block Diagram When VCC exceeds the reset threshold, it is connected to need not be dedicated to this function though, as it is the MAX690A/MAX692A/MAX802L/MAX802M/MAX805L completely separate from the rest of the circuitry. The substrate, regardless of the voltage applied to VBATT external voltage divider drives PFI to sense the unregu- (Figure 3). During this time, the diode (D1) between VBATT lated DC input to the +5V regulator (see Typical and the substrate will conduct current from VBATT to VCC if Operating Circuit). The voltage-divider ratio can bechosen such that the voltage at PFI falls below 1.25V Table 1. Input and Output Status in Battery-Backup Mode
just before the +5V regulator drops out. PFO then trig-gers an interrupt which signals the µP to prepare for To conserve backup-battery power, the power-faildetector comparator is turned off and PFO is forced Connected to VBATT through an internal 80Ω Backup-Battery Switchover
In the event of a brownout or power failure, it may be necessary to preserve the contents of RAM. With a cally switch RAM to backup power when VCC fails.
As long as VCC exceeds the reset threshold, VOUT connectsto VCC through a 5Ω PMOS power switch. Once VCC falls below the reset threshold, VCC or VBATT (whichever is high-er) switches to VOUT. Unlike the MAX690/MAX692, the MAX690A/MAX692A/MAX802L/MAX802M/MAX805L don’talways connect V VCC. VBATT connects to VOUT (through an 80Ω switch) only when VCC is below the reset threshold and VBATT is greater
_______________________________________________________________________________________
Microprocessor Supervisory Circuits
Figure 5. Using a SuperCap™ as a Backup Power Source withthe MAX692A/MAX802M and a +5V ±10% Supply BATT connects to VOUT, backup mode is activat- ed and the internal circuitry is powered from the battery drawn from VBATT is typically 30µA. When VCC drops to parator shuts off and the supply current falls to less __________Applications Information
Figure 6. Adding Hysteresis to the Power-Fail Comparator Using a SuperCap™ as a Backup
Power Source
SuperCaps are capacitors with extremely high capaci- Allowable Backup Power-Source
tance values, on the order of 0.1F. Figure 4 shows a Batteries
SuperCap used as a backup power source. Do not allowthe SuperCap’s voltage to exceed the maximum reset Lithium batteries work very well as backup batteries due to threshold by more than 0.6V. In Figure 4’s circuit, the very low self-discharge rates and high energy density.
SuperCap rapidly charges to within a diode drop of V Single lithium batteries with open-circuit voltages of 3.0V to However, after a long time, the diode leakage current will 3.6V are ideal. Any battery with an open-circuit voltage less than the minimum reset threshold plus 0.3V can be SuperCap with the MAX690A/MAX802L/MAX805L, VCCmay not exceed 4.75V + 0.6V = 5.35V.
Table 2. Allowable Backup-Battery Voltages
Use the SuperCap circuit of Figure 5 with a MAX692A (see Using a SuperCap as a Backup Power Source section for use with a SuperCap) o r M A X 8 0 2 M a n d a ± 1 0 % s u p p l y . T h i s c i r c u i tensures that the SuperCap only charges to V MAXIMUM BACKUP-BATTERY
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
VOLTAGE (V)
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.
_______________________________________________________________________________________
Microprocessor Supervisory Circuits
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
connected directly to the VBATT input of the ICs where battery-backup mode is entered whenever MAX690A/MAX692A/MAX802L/MAX802M/MAX805L with VBATT gets close to VCC, an unconnected VBATT pin no additional circuitry (see the Typical Operating Circuit).
However, batteries with open-circuit voltages that are greater cannot be used for backup, as current is sourced
into the substrate through the diode (D1 in Figure 3) when
Adding Hysteresis to the
Power-Fail Comparator
Hysteresis adds a noise margin to the power-fail com- Operation
parator and prevents repeated triggering of PFO when Without a Backup Power Source
VIN is close to its trip point. Figure 6 shows how to add If a backup power source is not used, ground VBATT hysteresis to the power-fail comparator. Select the and connect VOUT to VCC. Since there is no need to ratio of R1 and R2 such that PFI sees 1.25V when VIN switch over to any backup power source, VOUT does falls to its trip point (VTRIP). R3 adds the hysteresis. It not need to be switched. A direct connection to VCC will typically be an order of magnitude greater than R1 eliminates any voltage drops across the switch which or R2 (about 10 times either R1 or R2). The current through R1 and R2 should be at least 1µA to ensurethat the 25nA (max) PFI input current does not shift the Replacing the Backup Battery
trip point. R3 should be larger than 10kΩ so it does not The backup battery can be removed while VCC remains load down the PFO pin. Capacitor C1 adds additional valid, without danger of triggering RESET/RESET. As long as VCC stays above the reset threshold, battery-backup mode cannot be entered. In other switchover Monitoring a Negative Voltage
The power-fail comparator can be used to monitor anegative supply rail using the circuit of Figure 7. When the negative rail is good (a negative voltage of largemagnitude), PFO is low. When the negative rail is degraded (a negative voltage of lesser magnitude),PFO goes high. This circuit’s accuracy is affected bythe PFI threshold tolerance, the V Interfacing to µPs with
Bidirectional Reset Pins
µPs with bidirectional reset pins, such as the Motorola68HC11 series, can contend with the put. If, for example, the RESET output is driven highand the µP wants to pull it low, indeterminate logic lev-els may result. To correct this, connect a 4.7kΩ resis- tor between the RESET output and the µP reset I/O, asin Figure 8. Buffer the RESET output to other system _______________________________________________________________________________________
Microprocessor Supervisory Circuits
BUFFERED RESET TO OTHER SYSTEM COMPONENTS Figure 8. Interfacing to µPs with Bidirectional Reset I/O MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
______________________________________________________________________________________
Microprocessor Supervisory Circuits
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
________________________µP Supervisory Circuits
Minimum Nominal
Watchdog
Timeout Backup- CE -
Manual- Watch-
Battery-
Threshold Width
Protect parator
______________________________________________________________________________________
Microprocessor Supervisory Circuits
__Ordering Information (continued)
____________________Chip Topography
TEMP. RANGE
PIN-PACKAGE
VOUT VBATT
MAX692ACPA
RESET (RESET)
0.061"
(1.55mm)
MAX802LCPA
MAX802MCPA
0.078"
(1.98mm)
MAX805LCPA
* Dice are specified at TA = +25°C.
**Contact factory for availability and processing to MIL-STD-883. 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 areimplied. 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
is a registered trademark of Maxim Integrated Products.

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