MAX690MLP/883B ,Microprocessor Supervisory CircuitsElectrical CharacteristicsV = full operating range, V = 2.8V, T = +25°C, unless otherwise noted.)CC ..
MAX690RCSA ,3.0V or 3.3V microprocessor supervisory circuit. Reset threshold 2.625V. Active-low reset. Watchdog input. Backup-battery switch. Power-fail threshold accuracy +-4%. Power-fail comparator. Reset window +-75mVFeatures– — — — — –These microprocessor (µP) supervisory circuits reduce' RESET and RESET Outputsth ..
MAX690RCSA+ ,3.0V/3.3V Microprocessor Supervisory CircuitsElectrical Characteristics(V = 3.17V to 5.5V for the MAX690T/MAX704T/MAX80_T, V = 3.02V to 5.5V for ..
MAX690REPA ,3.0V or 3.3V microprocessor supervisory circuit. Reset threshold 2.625V. Active-low reset. Watchdog input. Backup-battery switch. Power-fail threshold accuracy +-4%. Power-fail comparator. Reset window +-75mVMAX690T/S/R, 704T/S/R, 802T/S/R, 804–806T/S/RReset WindowPower-Fail ComparatorPower-Fail Threshold ..
MAX690RESA ,3.0V or 3.3V microprocessor supervisory circuit. Reset threshold 2.625V. Active-low reset. Watchdog input. Backup-battery switch. Power-fail threshold accuracy +-4%. Power-fail comparator. Reset window +-75mVELECTRICAL CHARACTERISTICS(V = 3.17V to 5.5V for the MAX690T/MAX704T/MAX80_T, V = 3.02V to 5.5V for ..
MAX690RESA ,3.0V or 3.3V microprocessor supervisory circuit. Reset threshold 2.625V. Active-low reset. Watchdog input. Backup-battery switch. Power-fail threshold accuracy +-4%. Power-fail comparator. Reset window +-75mVMAX690T/S/R, 704T/S/R, 802T/S/R, 804–806T/S/RReset WindowPower-Fail ComparatorPower-Fail Threshold ..
MB3807A ,Power Management Switching IC (with flash memory power switching function)FUJITSU SEMICONDUCTORDS04-27602-1EDATA SHEETASSP Power SupplyBIPOLARPower Management Switching I ..
MB3813A ,Lithium Ion Battery Charger DC/DC Converter IC (High Precision with Constant-current Function)Applications (Secondary battery)Lithium Ion Battery Charger DC/DC Converter IC (High Precision with ..
MB3813A ,Lithium Ion Battery Charger DC/DC Converter IC (High Precision with Constant-current Function)Applications (Secondary battery)Lithium Ion Battery Charger DC/DC Converter IC (High Precision with ..
MB3813A ,Lithium Ion Battery Charger DC/DC Converter IC (High Precision with Constant-current Function)applications.Cell count Output voltage Part number3-cell 12.6 V/12.3 V MB3813A2-cell 8.4 V/8.2 V MB ..
MB3813APFV ,Lithium Ion Battery Charger DC/DC Converter IC (High Precision with Constant-current Function)Applications (Secondary battery)Lithium Ion Battery Charger DC/DC Converter IC (High Precision with ..
MB3817 ,Switching Regulator Controllerapplications as down-conversion or down/up-conversion (Zeta method).With fewer external components ..
MAX690CPA+-MAX690EPA+-MAX690MJA/883B-MAX690MLP/883B-MAX691CPE+-MAX691CWE+-MAX691CWE+T-MAX691EPE+-MAX691EWE+-MAX691EWE+T-MAX692MJA/883B-MAX693CWE+-MAX693CWE-T-MAX693EWE+-MAX693MJE/883B-MAX694CPA+-MAX695CWE+-MAX695CWE-T-MAX695EWE+
Microprocessor Supervisory Circuits
General DescriptionThe MAX690 family of supervisory circuits reduces the
complexity and number of components required for
power supply monitoring and battery control functions in
microprocessor systems. These include µP reset and
backup-battery switchover, watchdog timer, CMOS RAM
write protection, and power-failure warning. The MAX690
family significantly improves system reliability and accu-
racy compared to that obtainable with separate ICs or
discrete components.
The MAX690, MAX692, and MAX694 are supplied in
8-pin packages and provide four functions:●A reset output during power-up, power-down, and
brownout conditions.●Battery backup switching for CMOS RAM, CMOS
microprocessor or other low power logic.●A Reset pulse if the optional watchdog timer has not
been toggled within a specified time.●A 1.3V threshold detector for power fail warning, low
battery detection, or to monitor a power supply other
than +5V.
The MAX691, MAX693, and MAX695 are supplied in 16-pin
packages and perform all MAX690, MAX692, MAX694
functions, plus:●Write protection of CMOS RAM or EEPROM.●Adjustable reset and watchdog timeout periods.●Separate outputs for indicating a watchdog timeout,
backup-battery switchover, and low VCC.
Beneits and Features●Supervisory Function Integration Saves Board Space
while Fully Protecting Microprocessor-Based SystemsPrecision Voltage Monitor-4.65V (MAX690, MAX691, MAX694, MAX695)-4.40V (MAX692, MAX693)Power OK/Reset Time Delay-50ms, 200ms, or AdjustableWatchdog Timer-100ms, 1.6s, or AdjustableBattery Backup Power SwitchingVoltage Monitor for Power Fail or Low Battery
WarningMinimum External Component Count●Low Power Consumption in Battery Backup Mode
Extends Battery Life1µA Standby Current●Onboard Gating of Chip Enable Signals Protects
Against Erroneous Data Written to RAM During Low
VCC Events
Applications●Computers●Controllers●Intelligent Instruments●Automotive Systems●Critical µP Power Monitoring
Ordering information appears at end of data sheet.
MAX690
MAX692
MAX694123
VBATT
RESET
WDI
PFO4
VOUT
TOP VIEW
VCC
GND
PFI
MAX691
MAX693
MAX695123
RESET
RESET
WDO
CE IN4
VBATT
VOUT
VCC
GND56
CE OUT
WDI
PFO7
BATT ON
LOW LINE
OSC IN8PFIOSC SEL
MAX690VCC
VBATT
PFI
VOUT+5V
PFO
WDI
GND
MAX690 TYPICAL APPLICATION
RESET
POWER TO
CMOS RAM
POWER
SYSTEM
µP NMI
I/O
LINE
µP RESET
MAX690–MAX695Microprocessor Supervisory Circuits
Pin Conigurations
Typical Operating Circuit
Terminal Voltage (with respect to GND)
VCC ...................................................................-0.3V to +6.0V
VBATT ...............................................................-0.3V to +6.0VAll Other Inputs (Note 1) ...................-0.3V to (VOUT + 0.5V)
Input Current
VCC ...............................................................................200mA
VBATT .............................................................................50mA
GND ...............................................................................20mA
Output Current
VOUT .....................................................Short circuit protectedAll Other Outputs ............................................................20mA
Rate-of-Rise, VBATT, VCC ..............................................100V/µs
Operating Temperature RangeC suffix ................................................................0°C to +70°CE suffix ............................................................-40°C to +85°CM suffix .........................................................-55°C to +125°C
Power Dissipation8-Pin Plastic DIP(derate 5mW/°C above +70°C) ....................................400mV8-Pin CERDIP(derate 8mW/°C above +85°C) ....................................500mV16-Pin Plastic DIP(derate 7mW/°C above +70°C) ....................................600mV16-Pin Small Outline(derate 7mW/°C above +70°C) ....................................600mV16-Pin CERDIP(derate 10mW/°C above +85°C) ..................................600mV
Storage Temperature Range ............................-65°C to +160°C
Lead Temperature (Soldering, 10s) ...................................300°C
VCC = full operating range, VBATT = 2.8V, TA = +25°C, unless otherwise noted.)
PARAMETERCONDITIONSMINTYPMAXUNITS
BATTERY BACKUP SWITCHINGOperating Voltage Range
(MAX690, MAX691, MAX694, MAX695 VCC)4.755.5
Operating Voltage Range (MAX690, MAX691,
MAX694, MAX695 VBATT)2.04.25
Operating Voltage Range
(MAX692, MAX693 VCC)4.55.5
Operating Voltage Range
(MAX692, MAX693 VBATT)2.04.0
VOUT Output Voltage
IOUT = 1mAVCC -
VCC -
0.1V
IOUT = 50mAVCC -
VCC -
VOUT in Battery Backup ModeIOUT = 250µA, VCC < VBATT - 0.2VVBATT
- 0.1
VBATT
- 0.02V
Supply Current (Excluded IOUT)IOUT = 1mA25mA
IOUT = 50mA3.510
Supply Current in Battery Backup ModeVCC = 0V, VBATT = 2.8V0.61µA
Battery Standby Current
(+ = Discharge, - = Charge)
5.5V > VCC >
VBATT + 1V
TA = +25°C-0.1+0.02TA = full operating
range-1.0+0.02
Battery Switchover Threshold
(VCC - VBATT)
Power-up70mVPower-down50
MAX690–MAX695Microprocessor Supervisory Circuits
Absolute Maximum RatingsStresses 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 = full operating range, VBATT = 2.8V, TA = +25°C, unless otherwise noted.)
PARAMETERCONDITIONSMINTYPMAXUNITSBattery Switchover Hysteresis20mV
BATT ON Output VoltageISINK = 3.2mA0.4V
BATT ON Output Short-Circuit CurrentBATT ON = VOUT = 4.5V sink current25mA
BATT ON = 0V source current0.5125µA
RESET AND WATCHDOG TIMERReset Voltage ThresholdTA = full
operating range
MAX690, MAX691,
MAX694, MAX6954.54.654.75V
MAX692, MAX6934.254.44.5
Reset Threshold Hysteresis40mV
Reset Timeout Delay (MAX690/MAX691/
MAX692/MAX693)Figure 6, OSC SEL HIGH, VCC = 5V355070ms
Reset Timeout Delay (MAX694/MAX695)Figure 6, OSC SEL HIGH, VCC = 5V140200280ms
Watchdog Timeout Period, Internal OscillatorLong period, VCC = 5V1.01.62.25s
Short period, VCC = 5V70100140ms
Watchdog Timeout Period, External Clock
Long period38404097Clock
CyclesShort period7681025
Minimum WDI Input Pulse WidthVIL = 0.4, VIH = 0.8VCC200ns
RESET and LOW LINE Output VoltageISINK = 1.6mA, VCC = 4.25V0.4VISOURCE = 1µA, VCC = 5V3.5
RESET and WDO Output Voltage
ISINK = 1.6mA0.4ISOURCE = 1µA, VCC = 5V3.5
Output Short-Circuit CurrentRESET, RESET, WDO, LOW LINE1325µA
WDI Input Threshold Logic-LowVCC = 5V (Note 2)0.8VWDI Input Threshold Logic-HighVCC = 5V (Note 2)3.5
WDI Input Current
WDI = VOUT2050WDI = 0V-50-15
POWER-FAIL DETECTORPFI ThresholdVCC = 5V, TA = full1.21.31.4V
PFI Current±0.01±25nA
PFO Output Voltage
ISINK = 3.2mA0.4V
ISOURCE = 1µA3.5V
PFO Short Circuit Source CurrentPFI = VIH, PFO = 0V1325µA
MAX690–MAX695Microprocessor Supervisory Circuits
Electrical Characteristics (continued)
VCC = full operating range, VBATT = 2.8V, TA = +25°C, unless otherwise noted.)
Note 1: The input voltage limits on PFI and WDI may be exceeded provided the input current is limited to less than 10mA.
Note 2: WDI is guaranteed to be in the mid-level (inactive) state if WDI is floating and VCC is in the operating voltage range. WDI is
internally biased to 38% of VCC with an impedance of approximately 125kΩ.
PARAMETERCONDITIONSMINTYPMAXUNITS
CHIP ENABLE GATINGCE IN ThresholdsVIL0.8V
VIH3.0
CE IN Pullup Current3µA
CE OUT Output Voltage
ISINK = 3.2mA0.4ISOURCE = 3.0mAVOUT - 1.5
ISOURCE = 1µA, VCC = 0VVOUT - 0.05
CE Propagation DelayVCC = 5V50200ns
OSCILLATOROSC IN Input Current±2µA
OSC SEL Input Pullup Current5µA
OSC IN Frequency RangeOSC SEL = 0V0250kHz
OSC IN Frequency
with External Capacitor
OSC SEL = 0V
COSC = 47pF4kHz
PIN
NAMEFUNCTIONMAX690/
MAX692/
MAX694
MAX691/
MAX693/
MAX6953VCCThe +5V Input1VBATTBackup Battery Input. Connect to Ground if a backup battery is not used.2VOUTThe higher of VCC or VBATT is internally switched to VOUT. Connect VOUT to VCC if
VOUT and VBATT are not used. Connect a 0.1µF or larger bypass capacitor to VOUT.4GND0V Ground Reference for All Signals15RESET
RESET goes low whenever VCC falls below either the reset voltage threshold or the
VBATT input voltage. The reset threshold is typically 4.65V for the MAX690/691/694/695,
and 4.4V for the MAX692 and MAX693. RESET remains low for 50ms after VCC returns
to 5V, (except 200ms in MAX694/695). RESET also goes low for 50ms if the Watchdog
Timer is enabled but not serviced within its timeout period. The RESET pulse width can
be adjusted as shown in Table 1.11WDI
Watchdog Input (WDI). WDI is a three level input. If WDI remains either high or low for
longer than the watchdog timeout period, RESET pulses low and WDO goes low. The Watchdog Timer is disabled when WDI is left loating or is driven to mid-supply. The
timer resets with each transition at the Watchdog Timer input.
MAX690–MAX695Microprocessor Supervisory Circuits
Electrical Characteristics (continued)
Pin Description
PIN
NAMEFUNCTIONMAX690/
MAX692/
MAX694
MAX691/
MAX693/
MAX69510PFOOutput of the Power-Fail Comparator. It goes low when PFI is less than 1.3V. The
comparator is turned off and PFO goes low when VCC is below VBATT.13CE INCE Gating Circuit Input. Connect to GND or VOUT if not used.12CE OUTCE OUT goes low only when CE IN is low and VCC is above the reset threshold (4.65V
for MAX691 and MAX695, 4.4V for MAX693). See Figure 6.5BATT ON
BATT ON goes high when VOUT is internally switched to the VBATT input. It goes low
when VOUT is internally switched to VCC. The output typically sinks 25mA and can
directly drive the base of an external PNP transistor to increase the output current
above the 50mA rating of VOUT. 6LOW LINELOW LINE goes low when VCC falls below the reset threshold. It returns high as soon
as VCC rises above the reset threshold. See Figure 6, Reset Timing.16RESETActive-High Output. It is the inverse of RESET.8OSC SEL
When OSC SEL is unconnected or driven high, the internal oscillator sets the reset time
delay and watchdog timeout period. When OSC SEL is low, the external oscillator input,
OSC IN, is enabled. OSC SEL has a 3µA internal pullup. See Table 1.7OSC IN
When OSC SEL is low, OSC IN can be driven by an external clock to adjust both the
reset delay and the watchdog timeout period. The timing can also be adjusted by
connecting and external oscillator to this pin. See Figure 8. When OSC SEL is high or loating, OSC IN selects between fast and slow Watchdog timeout periods. 14WDO
The Watchdog Output (WDO). WDO goes low if WDI remains either high or low for
longer than the watchdog timeout period. WDO is set high by the next transition at WDI.
If WDI is unconnected or at mid-supply, WDO remains high. WDO also goes high when
LOW LINE goes low.
MAX690–MAX695Microprocessor Supervisory Circuits
Pin Description (continued)
Typical Applications
MAX691, MAX693, and MAX695typical connection for the MAX691/693/695 is shown
in Figure 1. CMOS RAM is powered from VOUT. VOUT is
internally connected to VCC when 5V power is present,
or to VBATT when VCC is less than the battery voltage.
VOUT can supply 50mA from VCC, but if more current
is required, an external PNP transistor can be added.
When VCC is higher than VBATT, the BATT ON output
goes low, providing 25mA of base drive for the external
transistor. When VCC is lower than VBATT, an internal 200Ω MOSFET connects the backup battery to VOUT.
The quiescent current in the battery backup mode is 1µA
maximum when VCC is between 0V and VBATT–700mV.
Reset OutputA voltage detector monitors VCC and generates a RESET
output to hold the microprocessor’s Reset line low when
VCC is below 4.65V (4.4V for MAX693). An internal
monostable holds RESET low for 50ms* after VCC rises
above 4.65V (4.4V for MAX693). This prevents repeated
toggling of RESET even if the 5V power drops out and
recovers with each power line cycle.
The crystal oscillator normally used to generate the clock
for microprocessors takes several milliseconds to start.
Since most microprocessors need several clock cycles
to reset, RESET must be held low until the micropro-
cessor clock oscillator has started. The MAX690 family
power-up RESET pulse lasts 50ms* to allow for this
oscillator start-up time. The manual reset switch and
the 0.1µF capacitor connected to the reset bus can be
omitted if manual reset is not needed. An inverted, active
high, RESET output is also supplied.
Power-Fail DetectorThe MAX691/93/95 issues a nonmaskable interrupt (NMI)
to the microprocessor when a power failure occurs. The
+5V power line is monitored via two external resistors
connected to the power-fail input (PFI). When the volt-
age at PFI falls below 1.3V, the power-fail output (PFO)
drives the processor’s NMI input low. If a power-fail
threshold of 4.8V is chosen, the microprocessor will
have the time when VCC fails from 4.8V to 4.65V to save
data into RAM. An earlier power-fail warning can be
generated if the unregulated DC input of the 5V regulator
is available for monitoring.
RAM Write ProtectionThe MAX691/MAX693/MAX695 CE OUT line drives the
Chip Select inputs of the CMOS RAM. CE OUT follows
CE IN as long as VCC is above the 4.65V (4.4V for
MAX693) reset threshold. If VCC falls below the reset
threshold, CE OUT goes high, independent of the logic
level at CE IN. This prevents the microprocessor from
writing erroneous data into RAM during power-up, power-
down, brownouts, and momentary power interruptions.
The LOW LINE output goes low when VCC falls below
4.65V (4.4V for MAX693).
*200ms for MAX695
+5V
VCC
INPUT
NO CONNECTION
0.1µF
0.1µF
0.1µF
VCC
VBATT
I/O
NMI
RESET
PFI
GND
BATT ON
LOW LINEWDO
SYSTEM STATUS INDICATORS
BATTERY14
MAX691
MAX693
MAX695OSC IN
VOUT
CE IN
CE OUT
PFO
WDI
RESET
RESETOSC SEL
AUDIBLE
ALARM
ADDRESS
DECODE
CMOS
RAM
MICROPROCESSOR
OTHER SYSTEM RESET SOURCES
A0-A15
MAX690–MAX695Microprocessor Supervisory Circuits
Watchdog TimerThe microprocessor drives the WATCHDOG INPUT
(WDI) with an I/O line. When OSC IN and OSC SEL are
unconnected, the microprocessor must toggle the WDI
pin once every 1.6s to verify proper software execution. If
a hardware or software failure occurs such that WDI not
toggled the MAX691/MAX693 will issue a 50ms* RESET
pulse after 1.6s. This typically restarts the microproces-
sor’s power-up routine. A new RESET pulse is issued
every 1.6s until the WDI is again strobed.
The WATCHDOG OUTPUT (WDO) goes low if the
watchdog timer is not serviced within its timeout period.
Once WDO goes low it remains low until a transition
occurs at WDI. The watchdog timer feature can be dis-
abled by leaving WDI unconnected. OSC IN and OSC
SEL also allow other watchdog timing options, as shown
in Table 1 and Figure 8.
MAX690, MAX692, and MAX694The 8 pin MAX690, MAX692, and MAX694 have most
of the features of the MAX691, MAX693, and MAX695.
Figure 2 shows the MAX690/MAX692/MAX694 in a typical
application. Operation is much the same as with the
MAX691/MAX693/MAX695 (Figure 1), but in this case, the
power- fail input (PFI) monitors the unregulated input to the
7805 regulator. The MAX690/MAX694 RESET output goes
low when VCC falls below 4.65V. The RESET output of the
MAX692 goes low when VCC drops below 4.4V.
The current consumption of the battery-backed-up power
bus must be less than 50mA. The MAX690/MAX692/
MAX694 does not have a BATT ON output to drive an
external transistor. The MAX690/MAX692/MAX694 also
does not include chip enable gating circuitry that is avail-
able on the MAX690/MAX692/MAX694. In many systems
though, CE gating is not needed since a low input to
the microprocessor RESET line prevents the processor
from writing to RAM during power-up and power-down
transients.
The MAX690/MAX692/MAX694 watchdog timer has a
fixed 1.6s timeout period. If WDI remains either low or
high for more than 1.6s, a RESET pulse is sent to the
microprocessor. The watchdog timer is disabled if WDI is
left unconnected.
*200ms for MAX695
Figure 2. MAX690/692/694 Typical Application
3-TERMINAL
REGULATOR
+8V+5V
0.1µF
0.1µF2
MAX690
MAX692
MAX694GND
MICROPROCESSOR
MICRO-
PROCESSOR
POWER
POWER
CMOS
RAM
VCCVOUT
VBATT
RESET
PFI
RESET
PFONMI
WDII/O LINE
MAX690–MAX695Microprocessor Supervisory Circuits
Detailed Description
Battery-Switchover and VOUTThe battery switchover circuit compares VCC to the VBATT
input, and connects VOUT to whichever is higher. Switchover
occurs when VCC is 50mV greater than VBATT as VCC falls,
and VCC is 70mV more than VBATT as VCC rises (see
Figure 4). The switchover comparator has 20mV of hyster-
esis to prevent repeated, rapid switching if VCC falls very
slowly or remains nearly equal to the battery voltage.
When VCC is higher than VBATT, VCC is internally
switched to VOUT via a low saturation PNP transis-
tor. VOUT has 50mA output current capability. Use an
external PNP pass transistor in parallel with internal tran-
sistor if the output current requirement at VOUT exceeds
50mA or if a lower VCC-VOUT voltage differential is
desired. The BATT ON output (MAX691/MAX693/MAX695
only) can directly drive the base of the external transistor.
It should be noted that the MAX690–MAX695 need only
supply the average current drawn by the CMOS RAM if
there is adequate filtering. Many RAM data sheets specify 75mA maximum supply current, but this peak current
spike lasts only 100ns. A 0.1µF bypass capacitor at VOUT
supplies the high instantaneous current, while VOUT need
only supply the average load current, which is much less.
A capacitance of 0.1µF or greater must be connected to
the VOUT terminal to ensure stability.
A 200Ω MOSFET connects VBATT input to VOUT during
battery backup. This MOSFET has very low input-to-
output differential (dropout voltage) at the low current
levels required for battery backup of CMOS RAM or other
low power CMOS circuitry. When VCC equals VBATT the
supply current is typically 12µA. When VCC is between
0V and (VBATT - 700mV) the typical supply current is only
600nA typical, 1µA maximum.
The MAX690/MAX691/MAX694/MAX695 operate with
battery voltages from 2.0V to 4.25V while MAX692/
MAX693 operate with battery voltages from 2.0V to
4.0V. High value capacitors can also be used for short-
term memory backup. External circuitry is required to
ensure that the capacitor voltage does not rise above
the reset threshold, and that the charging resistor does
not discharge the capacitor when in backup mode. The
MAX691A and the MAX791 provide solutions requiring
fewer external components.
A small charging current of typically 10nA (0.1µA max)
flows out of the VBATT terminal. This current varies with the
amount of current that is drawn from VOUT but its polarity
is such that the backup battery is always slightly charged,
and is never discharged while VCC is in its operating volt-
age range. This extends the shelf life of the backup battery
by compensating for its self-discharge current. Also note
that this current poses no problem when lithium batteries
are used for backup since the maximum charging current
(0.1µA) is safe for even the smallest lithium cells.
If the battery-switchover section is not used, connect
VBATT to GND and connect VOUT to VCC. Table 2 shows
the state of the inputs and output in the low power battery
backup mode.
RESET GENERATOR
TIMEBASE FOR RESET
AND
WATCHDOG
WATCHDOG TRANSITION
DETECTOR
WATCHDOG
TIMER1.3V
4.65V
VBATT
BATT ON15
VCC
CHIP-ENABLE INPUT
OSC IN
*4.4V (MAX693)
OSC SEL
WATCHDOG INPUT
POWER FAIL
INPUT
VOUT
CHIP ENABLE OUTPUT
LOW LINE
RESET
GROUND
RESET
WATCHDOG OUTPUT
POWER FAIL OUTPUT
MAX690–MAX695Microprocessor Supervisory Circuits
Reset OutputRESET is an active-low output which goes low when-
ever VCC falls below 4.5V (MAX690/MAX691/MAX694/
MAX695) or 4.25V (MAX692/MAX693). It will remain low
until VCC rises above 4.75V (MAX690/691/694/695) or 4.5V
(MAX692/MAX693) for milliseconds*. See Figures 5 and 6.
The guaranteed minimum and maximum thresholds of
MAX690/MAX691/MAX694/MAX695 are 4.5V and 4.75V,
while the guaranteed thresholds of the MAX692/MAX693
are 4.25V and 4.5V. The MAX690/MAX691/MAX694/
MAX695 is compatible with 5V supplies with a +10%, -5%
tolerance while the MAX692/MAX693 is compatible with
5V ±10% supplies. The reset threshold comparator has
approximately 50mV of hysteresis, with a nominal thresh-
old of 4.65V in the MAX690/MAX691/MAX694/MAX695,
and 4.4V in the MAX692/MAX693.
The response time of the reset voltage comparator is
about 100µs. VCC should be bypassed to ensure that
glitches do not activate the RESET output.
RESET also goes low if the watchdog timer is enabled
and WDI remains either high or low longer than the watch-
dog timeout period. RESET has an internal 3µA pullup,
and can either connect to and open collector Reset bus
or directly drive a CMOS gate without and external pullup
resistor.
CE Gating and RAM Write ProtectionThe MAX691/MAX693/MAX695 use two pins to control
the Chip Enable or Write inputs of CMOS RAMs. When
VCC is +5V, CE OUT is a buffered replica of CE IN, with
a 50ns propagation delay. If VCC input falls below 4.65V
(4.5V min, 4.75V max) an internal gate forces CE OUT
high, independent of CE IN. The MAX693 CE OUT goes
high whenever VCC is below 4.4V (4.25V min, 4.5V max).
The CE output of both devices is also forced high when
VCC is less than VBATT. (See Figure 5.)
CE OUT typically drives the CE, CS, or Write input of
battery backed up CMOS RAM. This ensures the integ-
rity of the data in memory by preventing write operations
when VCC is at and invalid level. Similar protection of
EEPROMs can be achieved by using the CE OUT to
drive the Store or Write inputs of an EEPROM, EAROM,
or NOVRAM.
If the 50ns typical propagation delay of CE OUT is too
long, connect CE IN to GND and use the resulting CE
OUT to control a high speed external logic gate. A second
alternative is to AND the LOW LINE output with the CE or
WR signal. An external logic gate and the RESET output
of the MAX690/MAX692/MAX694 can also be used for
CMOS RAM write protection.
Figure 4. Battery-Switchover Block Diagram+
BASE DRIVE
LOW IQ MODE
SELECT
VOUT
0.1F
TO CMOS
RAM AND
REALTIME
CLOCK
VCC IN
BATTERY
INPUT
P CHANNEL
MOSFET
BATT ON
(MAX691, MAX693, MAX695 ONLY)
INTERNAL
SHUTDOWN
SIGNAL WHEN
VBATT > VCC + 0.7V
VCC
VCC
+5V
MAX690–MAX695Microprocessor Supervisory Circuits
