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19-0366; Rev 1; 1/96
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits
_______________General Description
The MAX793/MAX794/MAX795 microprocessor (P) supervisory circuits monitor and control the activities of +3.0V/+3.3V Ps by providing backup-battery switchover, among other features such as low-line indication, P reset, write protection for CMOS RAM, and a watchdog (see the Selector Guide below). The backup-battery voltage can exceed VCC, permitting the use of 3.6V lithium batteries in systems using 3.0V to 3.3V for VCC. The MAX793/MAX795 offer a choice of reset threshold voltage range (denoted by suffix letter): 3.00V to 3.15V (T), 2.85V to 3.00V (S), and 2.55V to 2.70V (R). The MAX794's reset threshold is set externally with a resistor divider. The MAX793/MAX794 are available in 16-pin DIP and narrow SO packages, and the MAX795 comes in 8-pin DIP and SO packages. For similar devices designed for 5V systems, see the P Supervisory Circuits table at the back of this data sheet.
____________________________Features
MAX793/MAX794/MAX795 o Precision Supply-Voltage Monitor: Fixed Reset Trip Voltage (MAX793/MAX795) Adjustable Reset Trip Voltage (MAX794) o Guaranteed Reset Assertion to VCC = 1V o Backup-Battery Power Switching--Battery Voltage Can Exceed VCC o On-Board Gating of Chip-Enable Signals--7ns Max Propagation Delay MAX793/MAX794 Only o Battery Freshness Seal o Battery OK Output (MAX793) o Uncommitted Voltage Monitor for Power-Fail or Low-Battery Warning o Independent Watchdog Timer (1.6sec timeout) o Manual Reset Input
MAX793/MAX794/MAX795
_____________________Selector Guide
FEATURE Active-Low Reset Active-High Reset Programmable Reset Threshold Low-Line Early Warning Output Backup-Battery Switchover External Switch Driver Power-Fail Comparator Battery OK Output Watchdog Input Battery Freshness Seal Manual Reset Input Chip-Enable Gating Pins-Package MAX793 MAX794 MAX795
______________Ordering Information
PART* MAX793_CPE TEMP. RANGE 0C to +70C PIN-PACKAGE 16 Plastic DIP
MAX793_CSE 0C to +70C 16 Narrow SO MAX793_EPE -40C to +85C 16 Plastic DIP MAX793_ESE -40C to +85C 16 Narrow SO Ordering Information continued on last page. * The MAX793/MAX795 offer a choice of reset threshold voltage. Select the letter corresponding to the desired reset threshold voltage range (T = 3.00V to 3.15V, S = 2.85V to 3.00V, R = 2.55V to 2.70V) and insert it into the blank to complete the part number. The MAX794's reset threshold is adjustable.
__________Typical Operating Circuit
3.0V OR 3.3V 0.1F (OPTIONAL) Si9433DY SILICONIX 0.1F PMOS VCC BATT ON OUT BATT CE OUT
MAX793
16-DIP/SO 16-DIP/SO 8-DIP/SO
3.6V 0.1F
________________________Applications
Battery-Powered Computers and Controllers Embedded Controllers Intelligent Controllers Critical P Power Monitoring Portable Equipment
CMOS RAM VCC
WDO +5V SUPPLY FAILURE +5V MR PFO
CE IN WDI LOWLINE
ADDRESS DECODER
A0-A15 I/O NMI
VCC
P RESET
PFI
RESET BATT OK GND
Pin Configurations appear at end of data sheet.
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3.0V/3.3V Adjustable Microprocessor Supervisory Circuits MAX793/MAX794/MAX795
ABSOLUTE MAXIMUM RATINGS
Terminal Voltage (with respect to GND) VCC ........................................................................-0.3V to 6.0V VBATT .....................................................................-0.3V to 6.0V All Other Inputs ..................-0.3V to the higher of VCC or VBATT Continuous Input Current VCC .................................................................................200mA VBATT ................................................................................50mA GND ..................................................................................20mA Output Current VOUT................................................................................200mA All Other Outputs ..............................................................20mA Continuous Power Dissipation (TA = +70C) 8-Pin Plastic DIP (derate 9.09mW/C above +70C) .....727mW 8-Pin SO (derate 5.88mW/C above +70C)..................471mW 16-Pin Plastic DIP (derate 10.53mW/C above +70C) .842mW 16-Pin Narrow SO (derate 9.52mW/C above +70C) ...696mW Operating Temperature Ranges MAX793_C_ _/MAX794C_ _/MAX795_C_ _ ......... 0C to +70C MAX793_E_ _/MAX794E_ _/MAX795_E_ _ ........-40C to +85C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10sec) .............................+300C
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 = 3.17V to 5.5V for the MAX793T/MAX795T, VCC = 3.02V to 5.5V for the MAX793S/MAX795S, VCC = 2.72V to 5.5V for the MAX793R/MAX794/MAX795R, VBATT = 3.6V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Operating Voltage Range, VCC, VBATT (Note 1) VCC Supply Current (excluding IOUT, ICE OUT) VCC Supply Current in Battery-Backup Mode (excluding IOUT) BATT Supply Current (excluding IOUT) (Note 2) BATT Leakage Current, Freshness Seal Enabled Battery Leakage Current (Note 3) OUT Output Voltage in Normal Mode OUT Output Voltage in Battery-Backup Mode VOUT IOUT = 75mA IOUT = 30mA (Note 4) IOUT = 250A (Note 4) VBATT = 2.3V IOUT = 250A IOUT = 1mA VCC - 0.3 VCC - 0.12 VCC - 0.001 VBATT - 0.1 VCC - 0.125 VCC - 0.050 VCC - 0.5mV VBATT - 0.034 VBATT - 0.14 20 2.69 2.55 2.30 2.82 2.68 2.41 65 2.95 2.80 2.52 VCC = 0V, VOUT = 0V SYMBOL MAX79_C MAX79_E MAX793/MAX794, MR = VCC MAX795 VCC = 2.1V, VBATT = 2.3V VCC < 3.6V VCC < 5.5V VCC < 3.6V VCC < 5.5V MAX793/MAX794 MAX795 CONDITIONS MIN 1.0 1.1 46 62 35 49 32 24 TYP MAX 5.5 5.5 60 80 50 70 45 A 35 1 1 0.5 A A A A A UNITS V
ISUPPLY
ISUPPLY
V
VOUT VCC VBATT
V mV
VSW > VCC > 1.75V (Note 5) MAX793T/MAX795T MAX793S/MAX795S MAX793R/MAX795R/ MAX794 This value is identical to the reset threshold, VCC rising for VBATT > VRST VBATT > VCC (Note 6) VBATT < VRST
Battery Switch Threshold (VCC falling)
VSW
V
Battery Switch Threshold (VCC rising) (Note 7)
VCC VBATT
25
65
mV
2
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3.0V/3.3V Adjustable Microprocessor Supervisory Circuits
ELECTRICAL CHARACTERISTICS (continued)
(VCC = 3.17V to 5.5V for the MAX793T/MAX795T, VCC = 3.02V to 5.5V for the MAX793S/MAX795S, VCC = 2.72V to 5.5V for the MAX793R/MAX794/MAX795R, VBATT = 3.6V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER SYMBOL VCC Falling Reset Threshold (Note 8) VRST VCC Rising RESET IN Threshold (MAX794 only) RESET IN Leakage Current (MAX794 only) Reset Timeout Period LOWLINE-to-Reset Threshold, (V LOWLINE VRST), VCC Falling Low-Line Comparator Hysteresis LOWLINE Threshold, VCC Rising tRP VLR VCC < 3.6V MAX793 MAX794 MAX793 MAX794 MAX793T/MAX795T VLL MAX793S/MAX795S MAX793R/MAX795R MAX794 PFI Input Threshold PFI Input Current PFI Hysteresis, PFI Rising BATT OK Threshold (MAX793) INPUT AND OUTPUT LEVELS RESET Output Voltage High BATT OK, BATT ON, WDO, LOWLINE Output Voltage High PFO Output Voltage High BATT ON Output Voltage High RESET Output Leakage Current (Note 9) PFO Output Short to GND Current PFO, RESET, RESET, WDO, LOWLINE Output Voltage Low VOH VOH VOH VOH ILEAK ISC ISOURCE =300A, VCC = VRST min ISOURCE = 300A, VCC = VRST max ISOURCE = 65A, VCC = VRST max ISOURCE = 100A, VCC = 2.3V, VBATT = 3V VCC = VRST max VCC = 3.3V, V PFO = 0V ISINK = 1.2mA; RESET, LOWLINE tested with VCC = VRST min; RESET, BATTOK, WDO tested with VCC = VRST max 0.8VCC 0.8VCC 0.8VCC 0.8VBATT -1 180 -1 500 0.86VCC 0.86VCC V V V V A A VBOK 2.00 VTH VPFI falling VPFI rising 1.212 1.212 -25 1.240 1.250 2 10 2.25 VCC Falling VCC Rising CONDITIONS MAX793T/MAX795T MAX793S/MAX795S MAX793R/MAX795R MAX793T/MAX795T MAX793S/MAX795S MAX793R/MAX795R VRST IN MIN 3.00 2.85 2.55 3.00 2.85 2.55 1.212 1.212 -25 140 30 5 TYP 3.075 2.925 2.625 3.085 2.935 2.635 1.240 1.250 2 200 45 15 10 10 3.23 3.08 2.78 1.317 1.262 1.287 25 20 2.50 V nA mV V V MAX 3.15 3.00 2.70 3.17 3.02 2.72 1.262 1.282 25 280 60 25 mV mV V nA ms mV V UNITS
MAX793/MAX794/MAX795
VOL
0.08
0.2VCC
V
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3
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits MAX793/MAX794/MAX795
ELECTRICAL CHARACTERISTICS (continued)
(VCC = 3.17V to 5.5V for the MAX793T/MAX795T, VCC = 3.02V to 5.5V for the MAX793S/MAX795S, VCC = 2.72V to 5.5V for the MAX793R/MAX794/MAX795R, VBATT = 3.6V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER RESET Output Voltage Low BATT ON Output Voltage Low All Inputs Including PFO (Note 10) MANUAL RESET INPUT MR Pulse Width MR-to-Reset Delay MR Pull-Up Current CHIP-ENABLE GATING CE IN Leakage Current CE IN-to-CE OUT Resistance CE IN-to-CE OUT Propagation Delay VOH CE OUT Drive from CE IN VOL Reset to CE OUT High Delay CE OUT Output Voltage High (reset active) WDI Input Current Watchdog Timeout Period WDI Pulse Width tWD VOH IOH = 500A, VCC < 2.3V 0.8VBATT ILEAK Disable mode Enable mode, VCC = VRST max VCC = VRST max, Figure 9 VCC = VRST max, IOUT = -1mA, V CE IN = VCC VCC = VRST max, IOUT = 1.6mA, V CE IN = 0V 10 0.8VCC V 0.2VCC s V 10 46 2 7 nA ns tMR tMD MAX793/MAX794 only MAX793/MAX794 only MAX793/MAX794 only, MR = 0V 25 100 50 75 70 250 250 ns ns A SYMBOL VOL VOL VIH VIL CONDITIONS MAX79_C, VBATT = VCC = 1.0V, ISINK = 40A MAX79_E, VBATT = VCC = 1.2V, ISINK = 200A ISINK = 3.2mA, VCC = VRST max VRST max < VCC < 5.5V MIN TYP 0.13 0.17 MAX 0.3 0.3 0.2VCC 0.7VCC 0.3VCC UNITS V V V
WATCHDOG (MAX793/MAX794 only) 0V < VCC < 5.5V -1 1.00 1.00 0.01 1.60 1 2.25 A sec ns
Note 1: VCC supply current, logic input leakage, watchdog functionality (MAX793/MAX794), MR functionality (MAX793/MAX794), PFI functionality (MAX793/MAX794), state of RESET and RESET (MAX793/MAX794) tested at VBATT = 3.6V and VCC = 5.5V. The state of RESET is tested at VCC = VCC min. Note 2: Tested at VBATT = 3.6V, VCC = 3.5V and 0V. The battery current will rise to 10A over a narrow transition window around VCC = 1.9V. Note 3: Leakage current into the battery is tested under the worst-case conditions at VCC = 5.5V, VBATT = 1.8V and VCC = 1.5V, VBATT = 1.0V. Note 4: Guaranteed by design. Note 5: When VSW > VCC > VBATT, OUT remains connected to VCC until VCC drops below VBATT. The VCC-to-VBATT comparator has a small 15mV typical hysteresis to prevent oscillation. For VCC < 1.75V (typical), OUT switches to BATT regardless of VBATT. Note 6: When VBATT > VCC > VSW, OUT remains connected to VCC until VCC drops below the battery switch threshold (VSW). Note 7: OUT switches from BATT to VCC when VCC rises above the reset threshold, if VBATT > VRST. In this case, switchover back to VCC occurs at the exact voltage that causes reset to be asserted, however switchover occurs 200ms prior to reset. If VBATT < VRST, OUT switches from BATT to VCC when VCC exceeds VBATT. Note 8: The reset threshold tolerance is wider for VCC rising than for VCC falling to accommodate the 10mV typical hysteresis, which prevents internal oscillation. Note 9: The leakage current into or out of the RESET pin is tested with RESET not asserted (RESET output high impedance). Note 10: PFO is normally an output, but is used as an input when activating the battery freshness seal. 4 _______________________________________________________________________________________
3.0V/3.3V/Adjustable Microprocessor Supervisory Circuits
__________________________________________Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
MAX793/MAX794/MAX795
VCC-TO-OUT ON-RESISTANCE vs. TEMPERATURE
MAX793 TOC1
BATT-TO-OUT ON-RESISTANCE vs. TEMPERATURE
MAX793 TOC2
VCC SUPPLY CURRENT vs. TEMPERATURE (NORMAL OPERATING MODE)
MAX793/4, VCC = 5V MAX795, VCC = 5V 50 40 30 20 10 VBATT = VCC = VOUT 0 100 -40 -20 0 20 40 60 80 100 MAX793/4, VCC = 3.3V MAX795, VCC = 3.3V
MAX793 TOC3
3.0 VCC-TO-OUT ON-RESISTANCE () 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 -40 -20 0 20 40 60 80 VCC = 5V VCC = 3.0V VCC = 3.3V IOUT = 30mA
160 BATT-TO-OUT ON-RESISTANCE () 140 120 VBATT = 3.6V 100 80 60 VBATT = 5V 40 IOUT = 250A VCC = 0V 40 60 80 VBATT = 3.0V
70 VCC SUPPLY CURRENT (A) 60
100
-40
-20
0
20
TEMPERATURE (C)
TEMPERATURE (C)
TEMPERATURE (C)
BATTERY SUPPLY CURRENT vs. TEMPERATURE (BATTERY-BACKUP MODE)
MAX793 TOC4
RESET TIMEOUT PERIOD vs. TEMPERATURE
MAX793 TOC5
RESET COMPARATOR PROPAGATION DELAY vs. TEMPERATURE (VCC FALLING)
MAX793 TOC6
0.10 VCC = 0V VBATT = 3.6V
250 RESET TIMEOUT PERIOD (ms)
30 25 20 15 10 5 0
SUPPLY CURRENT (A)
200
0.06
150
0.04
100
0.02
50 VCC RISING FROM OV TO VRST MAX
0 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
0 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
PROPAGATION DELAY (s)
0.08
-40
-20
0
20
40
60
80
100
TEMPERATURE (C)
MAX793 LOWLINE-TO-RESET THRESHOLD vs. TEMPERATURE
MAX793 TOC7
MAX793/MAX794 LOWLINE COMPARATOR PROPAGATION DELAY vs. TEMPERATURE
40mV OVERDRIVE PROPAGATION DELAY (s) 8 VCC RISING
MAX793 TOC8
MAX793/MAX794 PFI THRESHOLD vs. TEMPERATURE
MAX793 TOC9
100 LOWLINE-TO-RESET THRESHOLD (mV) 90 80 70 60 50 40 30 20 10 0 -40 -20 0 20 40 60 80 VCC FALLING
10
1.250
6
PFI THRESHOLD (V)
1.245
1.240
4 VCC FALLING 2
1.235
0 100 -40 -20 0 20 40 60 80 100 TEMPERATURE (C) TEMPERATURE (C)
1.230 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
_______________________________________________________________________________________
5
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits MAX793/MAX794/MAX795
____________________________Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
MAX794 RESET IN THRESHOLD AND LOWLINE-TO-RESET IN THRESHOLD vs. TEMPERATURE
MAX793 TOC10
MAX793 BATT OK THRESHOLD vs. TEMPERATURE
MAX793 TOC11
CE IN-TO-CE OUT ON-RESISTANCE vs. TEMPERATURE
CE IN-TO-CE OUT ON-RESISTANCE ()
MAX793 TOC12
LOWLINE-TO-RESET IN THRESHOLD (mV)
1.242 1.241 VRESET IN 1.240 1.239 VLOWLINE - VRST 1.238 1.237 VCC FALLING 1.236 -40 -20 0 20 40 60 80
30 25 20 15 10 5 0
2.5
60 50 40 30 20 10 VCC = VRST MAX 0 -40 -20 0 20 40 60 80
RESET IN THRESHOLD (V)
BATT OK THRESHOLD (V)
2.0
1.5
1.0
0.5 VBATT FALLING 0 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
100
100
TEMPERATURE (C)
TEMPERATURE (C)
MAX793/MAX794 WATCHDOG TIMEOUT PERIOD vs. TEMPERATURE
MAX793 TOC13
MAX793/MAX794 BATTERY FRESHNESS SEAL LEAKAGE CURRENT vs. TEMPERATURE
VBATT = 5.5V VCC = 0V VOUT = 0V
MAX793 TOC14
RESET THRESHOLD vs. TEMPERATURE (NORMALIZED)
MAX793 TOC15
1.70 WATCHDOG TIMEOUT PERIOD (sec)
20
1.002 1.001 VRST (NORMALIZED) 1.000 0.999 0.998 0.997 VCC FALLING
1.65
LEAKAGE CURRENT (nA)
15
1.60
10
1.55
5
1.50 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
0 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
0.996 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
MAX793/MAX794 PFI TO PFO PROPAGATION DELAY vs. TEMPERATURE
MAX793 TOC16
10
PROPAGATION DELAY (s)
8
6
4
2
VPFI FALLING 20mV OVERDRIVE -40 -20 0 20 40 60 80 100
0 TEMPERATURE (C)
6
_______________________________________________________________________________________
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits
______________________________________________________________Pin Description
PIN MAX793/ MAX794 NAME MAX795 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 VSW and VBATT, BATT connects to OUT. Main Supply Input Battery Status Output. High in normal operating mode when VBATT exceeds VBOK, otherwise low. VBATT is checked continuously. Disabled and logic low while VCC is below VSW. Reset Input. Connect to an external resistor divider to select the reset threshold. The reset threshold can be programmed anywhere in the VSW to 5.5V range. Power-Fail Comparator Input. When PFI is less than VPFT or when VCC falls below VSW, PFO goes low; otherwise, PFO remains high (see Power-Fail Comparator section). Connect to VCC if unused. Logic Output/External Bypass Switch-Driver Output. High when OUT switches to BATT. Low when OUT switches to VCC. Connect the base/gate of PNP/PMOS transistor to BATT ON for IOUT requirements exceeding 75mA. Ground Power-Fail Comparator Output. When PFI is less than VPFT or when VCC falls below VSW, 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). Manual Reset Input. A logic low on MR asserts reset. Reset remains asserted as long as MR is low and for 200ms after MR returns high. The active-low input has an internal 70A pull-up current. In can be driven from a TTL- or CMOS-logic line or shorted to ground with a switch. Leave open if unused. Watchdog Output. WDO goes low if WDI remains either high or low for longer than the watchdog timeout period. WDO returns high on the next transition of WDI. WDO is a logic high for VSW < VCC < VRST, and low when VCC is below VSW. Watchdog Input. If WDI remains either high or low for longer than the watchdog timeout period, the internal watchdog timer runs out and WDO goes low. WDO returns high on the next transition of WDI. Connect WDO to MR to generate a reset due to a watchdog fault. Chip-Enable Input. The input to the chip-enable gating circuit. Connect to GND if unused Chip-Enable Output. CE OUT goes low only when CE IN is low and reset is not asserted. If CE IN is low when reset is asserted, CE OUT will remain low for 10s or until CE IN goes high, whichever occurs first. CE OUT is pulled up to OUT. Active-High Reset Output. Sources and sinks current. RESET is the inverse of RESET. Early Power-Fail Warning Output. Low when VCC falls to VLR. This output can be used to generate an NMI to provide early warning of imminent power-failure. Open-Drain, Active-Low Reset Output. Pulses low for 200ms when triggered, and stays low whenever VCC is below the reset threshold or when MR is a logic low. It remains low for 200ms after either VCC rises above the reset threshold, the watchdog triggers a reset (WDO connected to MR), or MR goes low to high. Backup-Battery Input. When VCC falls below VSW and VBATT, OUT switches from VCC to BATT. When VCC rises above the reset threshold or above VBATT, OUT reconnects to VCC. VBATT may exceed VCC. Connect VCC, OUT, and BATT together if no battery is used. 7 FUNCTION
MAX793/MAX794/MAX795
1 2
1 2
OUT VCC BATT OK (MAX793)
3
-- RESET IN (MAX794)
4
--
PFI
5 6 7
3 4 --
BATT ON GND PFO
8
--
MR
9
--
WDO
10 11 12 13 14
-- 5 6 -- --
WDI CE IN CE OUT RESET LOWLINE
15
7
RESET
16
8
BATT
_______________________________________________________________________________________
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits MAX793/MAX794/MAX795
_______________Detailed Description
General Timing Characteristics
The MAX793/MAX794/MAX795 are designed for 3.3V and 3V systems, and provide a number of supervisory functions (see the Selector Guide on the front page). Figures 1 and 2 show the typical timing relationships of the various outputs during power-up and power-down with typical VCC rise and fall times. out period (tRP), the state of MR is ignored if PFO is externally forced low, to facilitate enabling the battery freshness seal. MR has an internal 70A pull-up current, so it can be left open if it is not used. This input can be driven with TTL- or CMOS-logic 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 0.1F capacitor from MR to ground to provide additional noise immunity.
Manual Reset Input (MAX793/MAX794)
Many microprocessor-based products require manualreset capability, allowing the operator, a test technician, or external logic circuitry to initiate a reset. On the MAX793/MAX794, a logic low on MR asserts reset. Reset remains asserted while MR is low, and for tRP (200ms) after it returns high. During the first half of the reset timeVRST VSW VLL
Reset Outputs
A microprocessor's (P's) reset input starts the P in a known state. These MAX793/MAX794/MAX795 P supervisory circuits assert a reset to prevent code execution errors during power-up, power-down, and
VCC 5s VLOWLINE (MAX793/MAX794) tRP VRESET (PULLED UP TO VCC)
VRESET (MAX793/MAX794) VBATT
tRP
VCE OUT
tRP/2
VWDO (MAX793/MAX794) VBOK (MAX793)
25s
25s
PFO (MAX793/MAX794)
tRP/2 25s (PFO FOLLOWS PFI) 25s
BATT ON SHOWN FOR VCC = 0V to 3.3V, VBATT = 3.6V, CE IN = GND. TYPICAL PROPAGATION DELAYS REFLECT A 40mV OVERDRIVE. MAX794: VRESET IN = VCC (VRST IN / VRST)
Figure 1. Timing Diagram, VCC Rising
8 _______________________________________________________________________________________
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits
brownout conditions. RESET is guaranteed to be a logic low for 0V < V CC < V RST , provided V BATT is greater than 1V. Without a backup battery (VBATT = VCC = VOUT), RESET is guaranteed valid for VCC 1V. Once V CC exceeds the reset threshold, an internal timer keeps RESET low for the reset timeout period (tRP); after this interval, RESET becomes high impedance (Figure 2). RESET is an open-drain output, and requires a pull-up resistor to VCC (Figure 3). Use a 4.7k to 1M pull-up resistor that will provide sufficient current to assure the proper logic levels to the P.
VLL VCC VRST VSW
If a brownout condition occurs (VCC dips below the reset threshold), RESET goes low. Each time RESET is asserted, it stays low for the reset timeout period. Any time VCC goes below the reset threshold, the internal timer restarts. The watchdog output (WDO) can also be used to initiate a reset. See the Watchdog Output section. The RESET output is the inverse of the RESET output, and it can both source and sink current.
MAX793/MAX794/MAX795
VLOWLINE (MAX793/MAX794)
4s
VRESET (RESET PULLED UP TO VCC)
20s
VRESET (MAX793/MAX794)
20s
VCE OUT 10s
25s VBATT
VWDO (MAX793/MAX794) 25s VBOK (MAX793) 25s VPFO (MAX793/MAX794) 25s
VBATT ON SHOWN FOR VCC = 3.3V to 0V, VBATT = 3.6V, CE IN = GND, PFI = VCC. TYPICAL DELAY TIMES REFLECT A 40mV OVERDRIVE MAX794: VRESET IN = VCC (VRST IN / VRST)
25s
VBATT
Figure 2. Timing Diagram, VCC Falling
_______________________________________________________________________________________ 9
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits MAX793/MAX794/MAX795
3.3V 0.1F R1 (OPTIONAL) Si9433DY SILICONIX D S 0.1F CMOS RAM tRP VCC
MAX794
VRST VCC
VRST
PMOS VCC BATT ON OUT RESET IN CE OUT
R2 CE IN WDI WDO MR +5V SUPPLY +5V FAILURE PFO RESET PFI GND LOWLINE VCC 4.7k ADDRESS DECODER
RESET
3.6V
0.1F
BATT
A0-A15 I/O NMI PFO (EXTERNALLY HELD AT 0V) RESET PULLED UP TO VCC RESET PFO STATE LATCHED, FRESHNESS SEAL ENABLED.
Figure 4. Battery Freshness Seal Enable Timing
VRST = VRST IN R1 + 1 R2
(
)
Using the standard application circuit (Figure 3), the reset threshold may be programmed anywhere in the range of VSW (the battery switch threshold) to 5.5V. Reset is asserted when VCC falls below VSW.
Figure 3. MAX794 Standard Application Circuit
Battery Freshness Seal
The MAX793/MAX794's battery freshness seal disconnects the backup battery from internal circuitry 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, connect a battery to BATT, ground PFO, bring VCC above the reset threshold and hold it there until reset is deasserted following the reset timeout period, then bring VCC back down again (Figure 4). Once the battery freshness seal is enabled (disconnecting the backup battery from the internal circuitry and anything connected to OUT), it remains enabled until VCC is brought above VRST. Note that connecting PFO to MR will not interfere with battery freshness seal operation.
Reset Threshold
The MAX793T/MAX795T are intended for 3.3V systems with a 5% power-supply tolerance and a 10% systems tolerance. Except when MR is asserted, reset will not assert as long as the power supply remains above 3.15V (3.3V - 5%). Reset is guaranteed to assert before the power supply falls below 3.0V (3.3V - 10%). The MAX793S/MAX795S are designed for 3.3V 10% power supplies. Except when MR is asserted, they are guaranteed not to assert reset as long as the supply remains above 3.0V (3.0V is just above 3.3V - 10%). Reset is guaranteed to assert before the power supply falls below 2.85V (3.3V - 14%). The MAX793R/MAX795R are optimized to monitor 3.0V 10% power supplies. Reset will not occur until VCC falls below 2.7V (3.0V - 10%), but is guaranteed to occur before the supply falls below 2.55V (3.0V - 15%). Program the MAX794's reset threshold with an external voltage divider to RESET IN. The reset-threshold tolerance will be a combination of the RESET IN tolerance and the tolerance of the resistors used to make the external voltage divider. Calculate the reset threshold as follows: VRST = VRST IN (R1 / R2 + 1)
BATT OK Output (MAX793)
BATT OK indicates the status of the backup battery. When reset is not asserted, the MAX793 checks the battery voltage continuously. If VBATT is below VBOK (2.0V min), BATT OK goes low; otherwise, it remains pulled up to VCC. BATT OK also goes low when VCC goes below VSW.
Watchdog Input (MAX793/MAX794)
In the MAX793/MAX794, the watchdog circuit monitors the P's activity. If the P does not toggle the watchdog input (WDI) within 1.6sec, WDO goes low. The internal 1.6sec timer is cleared and WDO returns high
10
______________________________________________________________________________________
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits MAX793/MAX794/MAX795
VRST VCC tRP VCC 4.7k
MAX793/MAX794
RESET WDO MR RESET TO P
WDO
tWD WDI
VCC
WDO
10s
WDO CONNECTED TO P INTERRUPT RESET PULLED UP TO VCC
RESET
tRP
tWP
tRP
Figure 5. Watchdog Timing Relationship
WDI
either when a reset occurs or when a transition (low-tohigh or high-to-low) takes place at WDI. As long as reset is asserted, the timer remains cleared and does not count. As soon as reset is released or WDI changes state, the timer starts counting (Figure 5). WDI can detect pulses as short as 100ns. Unlike the 5V MAX690 family, the watchdog function cannot be disabled.
Figure 6. Generating a Reset on Each Watchdog Fault
Chip-Enable Signal Gating
Internal gating of chip-enable (CE) signals prevents erroneous data from corrupting CMOS RAM in the event of an undervoltage condition. The MAX793/MAX794/MAX795 use a series transmission gate from CE IN to CE OUT (Figure 7). 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 these P supervisors to be used with most Ps. If CE IN is low when reset asserts, CE OUT remains low for typically 10s to permit completion of the current write cycle.
Watchdog Output (MAX793/MAX794)
In the MAX793/MAX794, WDO remains high (WDO is pulled up to VCC) if there is a transition or pulse at WDI during the watchdog timeout period. WDO goes low if no transition occurs at WDI during the watchdog timeout period. The watchdog function is disabled and WDO is a logic high when reset is asserted if VCC is above VSW. WDO is a logic low when VCC is below VSW. If a system reset is desired on every watchdog fault, simply diode-OR connect WDO to MR (Figure 6). When a watchdog fault occurs in this mode, WDO goes low, pulling MR low, which causes a reset pulse to be issued. Ten microseconds after reset is asserted, the watchdog timer clears and WDO returns high. This delay results in a 10s pulse at WDO, allowing external circuitry to "capture" a watchdog fault indication. A continuous high or low on WDI will cause 200ms reset pulses to be issued every 1.6sec.
Chip-Enable Input 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 at the moment CE IN goes high, or 10s after reset asserts, whichever occurs first (Figure 8). This permits the current write cycle to complete during power-down.
______________________________________________________________________________________
11
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits MAX793/MAX794/MAX795
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 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 50 driver and 50pF of load capacitance (Figure 9). For minimum propagation delay, minimize the capacitive load at CE OUT, and use a low-output-impedance driver.
CE OUT
MAX793 MAX794 MAX795
CHIP-ENABLE OUTPUT CONTROL RESET GENERATOR CE IN
OUT
P
P
N
Chip-Enable Output When the CE transmission gate is enabled, the impedance of CE OUT is equivalent to a 46 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 8). This pull-up turns off when the transmission gate is enabled.
Early Power-Fail Warning (MAX793/MAX794)
Figure 7. Chip-Enable Transmission Gate
The CE transmission gate remains disabled and CE IN remains high impedance (regardless of CE IN activity) for the first half of the reset timeout period (tRP / 2), any time a reset is generated. While disabled, CE IN is high impedance. When the CE transmission gate is enabled, the impedance of CE IN appears as a 46 resistor in series with the load at CE OUT.
VRST VCC VSW VRST
Critical systems often require an early warning indicating that power is failing. This warning provides time for the P to store vital data and take care of any additional "housekeeping" functions, before the power supply gets too far out of tolerance for the P to operate reliably. The MAX793/MAX794 offer two methods of achieving this early warning. If access to the unregulated supply is feasible, the power-fail comparator input (PFI) can be connected to the unregulated supply
VRST VRST VSW
CE OUT VBATT tRP/2 10s VBATT VCC
tRP RESET (PULLED TO VCC)
CE IN VBATT = 3.6V RESET PULLED UP TO VCC
Figure 8. Chip-Enable Timing
12 ______________________________________________________________________________________
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits
VCC VCC BATT 3.6V
pulled to VCC. Use LOWLINE to provide an NMI to the P when power begins to fall. 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 also contend with a more rapid VCC fall time, such as when the main battery is disconnected or a highside switch is opened during normal operation, use capacitance on the VCC line to provide time to execute the shutdown routine (Figure 11).
50pF CL*
MAX793/MAX794/MAX795
25 EQUIVALENT SOURCE IMPEDANCE 50 CABLE
MAX793 MAX794 MAX795
CE IN CE OUT
50 50 GND
*CL INCLUDES LOAD CAPACITANCE AND SCOPE PROBE CAPACITANCE.
Figure 9. CE Propagation Delay Test Circuit
through a voltage divider, with the power-fail comparator output (PFO) providing the NMI to the P (Figure 10). If there is no easy access to the unregulated supply, the LOWLINE output can be used to generate an NMI to the P (see LOWLINE Output section).
First, calculate the worst-case time required for the system to perform its shutdown routine. Then, with the worstcase shutdown time, the worst-case load current, and the minimum low-line to reset threshold (VLR min), calculate the amount of capacitance required to allow the shutdown routine to complete before reset is asserted: CHOLD > ILOAD x tSHDN / VLR where I LOAD is the current being drained from the capacitor, VLR is the low-line to reset threshold difference (VLL - VRST), and tSHDN is the time required for the system to complete an orderly shutdown routine.
LOWLINE Output (MAX793/MAX794) The low-line comparator monitors VCC with a threshold voltage typically 45mV above the reset threshold (10mV of hysteresis) for the MAX793, and 15mV above RESET IN (4mV of hysteresis) for the MAX794. For normal operation (VCC above the reset threshold), LOWLINE is
Power-Fail Comparator (MAX793/MAX794) The MAX793/MAX794's 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 12). However, the comparator does not need to be dedicated to this function because it is completely separate from the rest of the circuitry.
UNREGULATED SUPPLY REGULATOR
3.0V OR 3.3V 3.0V OR 3.3V REGULATOR VCC CHOLD VCC LOWLINE TO P NMI
R1
MAX793 MAX794
PFI PFO TO P NMI CHOLD > ILOAD x tSHDN VLR
MAX793 MAX794
R2 GND
GND
Figure 10. Using the Power-Fail Comparator to Generate Power-Fail Warning
Figure 11. Using LOWLINE to Provide Power-Fail Warning to the P
13
______________________________________________________________________________________
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits MAX793/MAX794/MAX795
3.0V OR 3.3V R1 PFI R2 VCC R1 PFO PFI R2 MR GND VIN VCC PFO VL VTRIP = R2 (VPFT + VPFH) VL = R2 (VPFT) VTRIP VCC R1 0V VIN R1 + R2 R2 VCC PFO VTRIP VTRIP = VPFT WHERE VPFT = 1.237V VPFH = 10mV NOTE: VTRIP, VL ARE NEGATIVE VH VIN GND VIN VCC 3.0V OR 3.3V
MAX793 MAX794
MAX793 MAX794
PFO
( R1 + R2 ) - )
VCC - R1
1
1
(
) )
(
1 1 + R1 R2
VH = (VPFT + VPFH)
(
R1 + R2 R2
(a)
(b)
Figure 12. Using the Power-Fail Comparator to Monitor an Additional Power Supply: (a) VIN Is Negative, (b) VIN Is Positive
The power-fail comparator turns off and PFO goes low when VCC falls below VSW on power-down. During the first half of the reset timeout period (tRP), PFO is forced high, irrespective of VPFI. At the beginning of the second half of tRP, the power-fail comparator is enabled and PFO follows PFI. If the comparator is unused, connect PFI to VCC and leave PFO unconnected. PFO may be connected to MR so that a low voltage on PFI will generate a reset (Figure 12b). 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 200ms after PFO pulls MR high when the monitored supply is above the programmed threshold.
Backup-Battery Switchover
In the event of a brownout or power failure, it may be necessary to preserve the contents of RAM. With a backup battery installed at BATT, the devices automatically switch RAM to backup power when VCC falls. In order to allow the backup battery (e.g., a 3.6V lithium cell) to have a higher voltage than VCC, this family of P supervisors (designed for 3.3V and 3V systems) does not always connect BATT to OUT when V BATT is greater than VCC. BATT connects to OUT (through a 140 switch) either when VCC falls below VSW and
VBATT is greater than VCC, or when VCC falls below 1.75V (typ) regardless of the BATT voltage. Switchover at VSW ensures that battery-backup mode is entered before VOUT gets too close to the 2.0V minimum required to reliably retain data in most CMOS RAM, (switchover at higher V CC voltages would decrease backup-battery life). When VCC recovers, switchover is deferred either until VCC crosses VBATT if V BATT is below V RST, or when V CC rises above the reset threshold (VRST) if VBATT is above VRST. This power-up switchover technique prevents V CC from charging the backup battery through OUT when using an external transistor driven by BATT ON. OUT connects to VCC through a 4 (max) PMOS power switch when VCC crosses the reset threshold (Figure 13).
BATT ON (MAX793/MAX794)
BATT ON is high when OUT is connected to BATT. Although BATT ON can be used as a logic output to indicate the battery switchover status, it is most often used as a gate or base drive for an external pass transistor for high-current applications (see Driving an External Switch with BATT ON in the Applications Information section). When V CC exceeds V RST on power-up, BATT ON sinks 3.2mA at 0.4V. In batterybackup mode, this terminal sources 100A from BATT.
14
______________________________________________________________________________________
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits
VRST VCC 3.3V
VSW
to VCC, the collector to OUT, and the base to BATT ON (Figure 14a). No current-limiting resistor is required, but a resistor connecting the base of the PNP to BATT ON can be used to limit the current drawn from V CC, prolonging battery life in portable equipment. If you are using a PMOS transistor, however, it must be connected backwards from the traditional method. Connect the gate to BATT ON, the drain to VCC, and the source to OUT (Figure 14b). This method orients the body diode from V CC to OUT and prevents the backup battery from discharging through the FET when its gate is high. Two PMOS transistors in the Siliconix LITTLE FOOTTM series are specified with VGS down to -2.7V. The Si9433DY has a maximum 100m drainsource on-resistance with 2.7V of gate drive and a 2A drain-source current. The Si9434DY specifies a 60m drain-source on-resistance with 2.7V of gate drive and a 5.1A drain-source current.
MAX793/MAX794/MAX795
3.6V 3.3V VOUT VBATT = 3.6V
3.6V
Figure 13. Battery Switchover Timing
Table 1. Input and Output Status in Battery-Backup Mode
PIN NAME OUT VCC BATT ON BATT OK PFI PFO MR WDO WDI RESET RESET BATT LOWLINE CE IN CE OUT STATUS Connected to BATT through an internal 140 switch Disconnected from OUT Pulled up to BATT Logic low Disabled Logic low Disabled, but still pulled up to VCC Logic low Disabled Logic low Pulled up to VCC Connected to OUT Logic low High impedance Pulled to BATT
Using a SuperCapTM as a Backup Power Source
SuperCapsTM are capacitors with extremely high capacitance values (e.g., order of 0.47F) for their size. Figure 15 shows two ways to use a SuperCap as a backup power source. The SuperCap can be connected through a diode to the 3V input (Figure 15a); or, if a 5V supply is also available, the SuperCap can be charged up to the 5V supply (Figure 15b), allowing a longer backup period. Since VBATT can exceed VCC while VCC is above the reset threshold, there are no special precautions when using these P supervisors with a SuperCap.
Operation without a Backup Power Source
These P supervisors were designed for batterybacked applications. If a backup battery is not used, connect BATT, OUT, and VCC together, or use a different P supervisor. See the P Supervisory Circuits table at the end of this data sheet.
__________Applications Information
These P supervisory circuits are not short-circuit protected. Shorting VOUT to ground, excluding power-up transients such as charging a decoupling capacitor, destroys the device. Decouple both VCC and BATT pins to ground by placing 0.1F ceramic capacitors as close to the device as possible.
Replacing the Backup Battery
The backup power source can be removed while VCC remains valid, without danger of triggering a reset pulse, provided that BATT is decoupled with a 0.1F capacitor to ground. As long as VCC stays above the reset threshold, battery-backup mode cannot be entered.
Driving an External Switch with BATT ON
BATT ON can be directly connected to the base of a PNP transistor or the gate of a PMOS transistor. The PNP connection is straightforward: connect the emitter
TM LITTLE FOOT is a trademark of Siliconix Inc. SuperCap is a trademark of Baknor Industries. 15
______________________________________________________________________________________
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits MAX793/MAX794/MAX795
PMOS FET BODY DIODE 3.0V OR 3.3V TO CMOS RAM D G VCC BATT ON OUT VCC BATT ON OUT S
MAX793 MAX794 MAX795
MAX793 MAX794 MAX795
GND
GND
(a)
(b)
Figure 14. Driving an External Transistor with BATT ON
3.0V OR 3.3V
+5V
VCC 1N4148 BATT 0.47F
MAX793 MAX794
OUT VCC
TO STATIC RAM
3.0V OR 3.3V 1N4148
VCC
MAX793 MAX794
OUT VCC
TO STATIC RAM
RESET
TO P 0.47F
BATT
RESET
TO P
GND
GND
(a)
(b)
Figure 15. Using a SuperCapTM as a Backup Source
____________________________Erratum
Initial versions of the MAX793 and MAX794 have a logic design error that can cause the loss of output voltage (OUT) when VCC is absent even though a backup battery is connected to the BATT input. Applications that do not use the MR input (including all MAX795 applications) are unaffected by this phenomenon. Also, applications that do not use PFO are unaffected if PFI is tied to VCC. The loss of output voltage is caused by the IC incorrectly entering the battery "freshness seal" mode. Normally, freshness seal mode is activated by grounding PFO during a power-up reset timeout period. Then, the removal of VCC powers the IC down without connecting the backup battery to OUT.
16
The IC decides whether or not to enter freshness seal mode during all reset timeout periods. During a powerup reset timeout period (which occurs when V CC is raised above the MAX793's reset threshold or the voltage on the MAX794's RESET IN pin is raised above the RESET IN threshold), the IC momentarily disconnects the PFO pin from the comparator output and lightly pulls PFO up to VCC. The voltage level on the PFO pin is then tested and, if it is low, freshness seal mode is chosen. (PFO is reconnected to the comparator output before the end of the reset timeout period.) However, when a reset is initiated by MR, the PFO pin incorrectly remains connected to the comparator output during the entire timeout period and is not pulled up. If the comparator is driving PFO low during an MR reset
______________________________________________________________________________________
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits MAX793/MAX794/MAX795
VIN R1 PFI R2 R3 VCC R1 PFI R2 C1* C1* PFO GND TO P *OPTIONAL TO P PFO GND *OPTIONAL R3 VIN VCC
MAX793 MAX794
MAX793 MAX794
PFO 0V 0V VL VTRIP VH VIN
PFO 0V 0V VTRIP VH VIN
VTRIP = VPFT
+ (R1 R2R2) 1 1 + + R1 R2 1 1 1 + + - R1 R2 R3
VTRIP = VPFT
VH = (VPFT + VPFH) (R1) VL = R1 VPFT
(
(
)
1 R3 VCC R3
)
WHERE VPFT = 1.237V VPFH = 10mV
(R1 +R2R2 )
1 1 1 ( R1 + R2 + R3 ) - VD R3
VH = R1 (VPFT + VPFH)
(a)
(b)
WHERE VPFT = 1.237V VPFH = 10mV VD = DIODE FORWARD VOLTAGE DROP VL = VTRIP
Figure 16. Adding Hysteresis to the Power-Fail Comparator: (a) Symmetrical Hysteresis, (b) Hysteresis Only on Rising VIN
timeout period (because PFI is below the PFI threshold), the IC will test the voltage level on PFO, find that it is low, and incorrectly decide to enter freshness seal mode. If VCC is later removed, the backup battery will not be connected to OUT and any devices powered by OUT will lose power. Applications that do not use the PFO comparator need not be affected by this problem. Simply connect PFI to VCC and PFO will be driven high during all reset timeout periods. Freshness seal mode can be entered only when PFO is low. The IC is under revision to correct this problem. The revised IC will disable PFO during all reset timeout periods including MR-initiated ones. This revision will not affect applications that either do not use MR or do not use PFO, but could affect applications that require the use of the PFO output during MR-initiated reset timeout periods. The revised ICs are expected to be available in late 1996. For technical assistance, please contact Maxim Applications at 1-800-998-8800 or at http:// www. maxim-ic.com.
VCC
VCC RESET RESET
VCC
RESET GENERATOR
N
P
MAX793 MAX794 MAX795
GND GND
Figure 17. Interfacing to Ps with Bidirectional Reset I/O
______________________________________________________________________________________ 17
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits MAX793/MAX794/MAX795
Adding Hysteresis to the Power-Fail Comparator (MAX793/MAX794)
The power-fail comparator has a typical input hysteresis of 10mV. This is sufficient for most applications where a power-supply line is being monitored through an external voltage divider (see the section Monitoring an Additional Power Supply). If additional noise margin is desired, connect a resistor between PFO and PFI as shown in Figure 16a. Select the ratio of R1 and R2 such that PFI sees V PFT when VIN falls to its trip point (VTRIP). R3 adds the additional hysteresis and should typically be more than 10 times the value of R1 or R2. The hysteresis window extends both above (VH) and below (VL) the original trip point (VTRIP). Connecting an ordinary signal diode in series with R3, as shown in Figure 16b, causes the lower trip point (VL) to coincide with the trip point without hysteresis (VTRIP), so the entire hysteresis window occurs above VTRIP. This method provides additional noise margin without compromising the accuracy of the power-fail threshold when the monitored voltage is falling. It is useful for accurately detecting when a voltage falls past a threshold. The current through R1 and R2 should be at least 1A to ensure that the 25nA (max over temperature) PFI input current does not shift the trip point. R3 should be larger than 82k so it does not load down the PFO pin. Capacitor C1 is optional, and adds noise rejection. PFI. PFO can be used to generate an interrupt to the P or to cause reset to assert (Figure 12).
Interfacing to Ps with Bidirectional Reset Pins
Since the RESET output is open drain, the MAX793/ MAX794/MAX795 interface easily with Ps that have bidirectional reset pins, such as the Motorola 68HC11. Connecting the RESET output of the P supervisor directly to the RESET input of the microcontroller with a single pull-up resistor allows either device to assert reset (Figure 17).
Negative-Going VCC Transients These supervisors are relatively immune to short-duration negative-going VCC transients (glitches) while issuing resets to the P during power-up, power-down, and brownout conditions. Therefore, resetting the P when VCC experiences only small glitches is usually not recommended. Figure 18 shows maximum transient duration vs. resetcomparator overdrive, for which reset pulses are not generated. The graph was produced using negativegoing VCC pulses, starting at 3.3V and ending below the reset threshold by the magnitude indicated (reset comparator overdrive). The graph shows the maximum pulse width a negative-going VCC transient can typically
Monitoring an Additional Power Supply
These P supervisors can monitor either positive or negative supplies using a resistor voltage divider to
START
MAXIMUM PULSE DURATION (s)
90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80
MAX793-FIG 18
100
SET WDI HIGH
PROGRAM CODE
Subroutine or Program Loop SET WDI LOW
90 100
RESET COMPARATOR OVERDRIVE, VRST - VCC (mV)
RETURN
Figure 18. Maximum Transient Duration without Causing a Reset Pulse vs. Reset Comparator Overdrive
18
Figure 19. Watchdog Flow Diagram
______________________________________________________________________________________
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits
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 10s or less will not cause a reset pulse to be issued. A 0.1F bypass capacitor mounted close to the VCC pin provides additional transient immunity.
_________________Pin Configurations
TOP VIEW
OUT 1 VCC 2 (RESET IN) BATT OK 3 PFI 4 BATT ON 5 GND 6 PFO 7 MR 8 16 BATT 15 RESET 14 LOWLINE
MAX793/MAX794/MAX795
Watchdog Software Considerations
There is a way to help the watchdog timer monitor software execution more closely, which involves setting and resetting 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 19 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, causing a reset or interrupt to be issued.
MAX793 MAX794
13 RESET 12 CE OUT 11 CE IN 10 WDI 9 WDO
DIP / Narrow SO
OUT 1 VCC 2 BATT ON 3 GND 4
8
BATT
7 RESET
MAX795
6 CE OUT 5 CE IN
DIP/SO
( ) ARE FOR MAX794
_Ordering Information (continued)
PART* MAX794CPE MAX794CSE MAX794EPE MAX794ESE MAX795_CPA MAX795_CSA MAX795_EPA MAX795_ESA TEMP. RANGE 0C to +70C 0C to +70C -40C to +85C -40C to +85C 0C to +70C 0C to +70C -40C to +85C -40C to +85C PIN-PACKAGE 16 Plastic DIP 16 Narrow SO 16 Plastic DIP 16 Narrow SO 8 Plastic DIP 8 SO 8 Plastic DIP 8 SO
___________________Chip Information
TRANSISTOR COUNT: 1271
* The MAX793/MAX795 offer a choice of reset threshold voltage. Select the letter corresponding to the desired reset threshold voltage range (T = 3.00V to 3.15V, S = 2.85V to 3.00V, R = 2.55V to 2.70V) and insert it into the blank to complete the part number. The MAX794's reset threshold is adjustable.
______________________________________________________________________________________
19
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits MAX793/MAX794/MAX795
________________________________________________________Package Information
E D A3 A A2 E1
DIM A A1 A2 A3 B B1 C D1 E E1 e eA eB L INCHES MAX MIN 0.200 - - 0.015 0.175 0.125 0.080 0.055 0.022 0.016 0.065 0.045 0.012 0.008 0.080 0.005 0.325 0.300 0.310 0.240 - 0.100 - 0.300 0.400 - 0.150 0.115 INCHES MIN MAX 0.348 0.390 0.735 0.765 0.745 0.765 0.885 0.915 1.015 1.045 1.14 1.265 MILLIMETERS MIN MAX - 5.08 0.38 - 3.18 4.45 1.40 2.03 0.41 0.56 1.14 1.65 0.20 0.30 0.13 2.03 7.62 8.26 6.10 7.87 2.54 - 7.62 - - 10.16 2.92 3.81 MILLIMETERS MIN MAX 8.84 9.91 18.67 19.43 18.92 19.43 22.48 23.24 25.78 26.54 28.96 32.13
21-0043A
L A1 e B D1
0 - 15 C B1 eA eB
Plastic DIP PLASTIC DUAL-IN-LINE PACKAGE (0.300 in.)
PKG. DIM PINS P P P P P N D D D D D D 8 14 16 18 20 24
DIM
D A e B
0.101mm 0.004in.
0-8
A1
C
L
A A1 B C E e H L
INCHES MAX MIN 0.069 0.053 0.010 0.004 0.019 0.014 0.010 0.007 0.157 0.150 0.050 0.244 0.228 0.050 0.016
MILLIMETERS MIN MAX 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 3.80 4.00 1.27 5.80 6.20 0.40 1.27
E
H
Narrow SO SMALL-OUTLINE PACKAGE (0.150 in.)
DIM PINS D D D 8 14 16
INCHES MILLIMETERS MIN MAX MIN MAX 0.189 0.197 4.80 5.00 0.337 0.344 8.55 8.75 0.386 0.394 9.80 10.00
21-0041A
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.
20 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600 (c) 1996 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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