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General Description

The MAX793/MAX794/MAX795 microprocessor (µP)

supervisory circuits monitor and control the activities of

+3.0V/+3.3V µPs by providing backup-battery switchover,

among other features such as low-line indication, µP

reset, write protection for CMOS RAM, and a watchdog

(see the

Selector Guide

below). The backup-battery volt-

age can exceed V

, permitting the use of 3.6V lithium

batteries in systems using 3.0V to 3.3V for V

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.

________________________Applications

Battery-Powered Computers and Controllers

Embedded Controllers

Intelligent Controllers

Critical µP Power Monitoring

Portable Equipment

____________________________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 V

= 1V

o Backup-Battery Power Switching—Battery

Voltage Can Exceed V

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 (1.6s timeout)

o Manual Reset Input

MAX793/MAX794/MAX795

3.0V/3.3V Adjustable Microprocessor

Supervisory Circuits

________________________________________________________________

Maxim Integrated Products

MAX793

RESET

LOWLINE

WDI

CE IN

CE OUT

3.0V OR 3.3V

+5V

BATT

A0-A15

+5V SUPPLY

FAILURE

BATT ON

PFI

WDO

OUT

CMOS

RAM

ADDRESS

DECODER

0.1µF

PMOS

0.1µF

PFO

GND

I/O

µP

NMI

RESET

0.1µF

3.6V

BATT OK

(OPTIONAL)

Si9433DY

SILICONIX

19-0366; Rev 6; 3/10

FEATURE

Active-Low Reset

Active-High Reset

Programmable Reset

Threshold

Low-Line Early Warning

Output

MAX793



MAX794



MAX795



Backup-Battery

Switchover

External Switch Driver

Power-Fail Comparator



Battery OK Output



_____________________Selector Guide

__________Typical Operating Circuit

Watchdog Input

Battery Freshness Seal



Manual Reset Input



Chip-Enable Gating

 

Pin-Package 16-DIP/SO 16-DIP/SO 8-DIP/SO

Pin Configurations appear at end of data sheet.

For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,

or visit Maxim’s website at www.maxim-ic.com.

Ordering Information

PART* TEMP RANGE

PIN-

PACKAGE

MAX793_CPE 0°C to +70°C 16 Plastic DIP

MAX793_CSE 0°C to +70°C 16 Narrow SO

Ordering Information continued at end of data sheet.

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.

Devices are available in both leaded and lead-free packaging.

Specify lead free by adding the + symbol at the end of the part

number when ordering.

Summary of content (20 pages)

PAGE 1
9-0366; Rev 6; 3/10 3.0V/3.3V Adjustable Microprocessor Supervisory Circuits The MAX793/MAX794/MAX795 microprocessor (µP) supervisory circuits monitor and control the activities of +3.0V/+3.3V µPs by providing backup-battery switchover, among other features such as low-line indication, µP 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.
PAGE 2
MAX793/MAX794/MAX795 3.0V/3.3V Adjustable Microprocessor Supervisory Circuits 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 .................................................................................
PAGE 3
.0V/3.3V Adjustable Microprocessor Supervisory Circuits (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 = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX MAX793T/MAX795T 3.00 3.075 3.15 MAX793S/MAX795S 2.85 2.925 3.00 MAX793R/MAX795R 2.55 2.625 2.70 MAX793T/MAX795T 3.00 3.085 3.17 MAX793S/MAX795S 2.85 2.
PAGE 4
MAX793/MAX794/MAX795 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 = +25°C.) PARAMETER SYMBOL CONDITIONS RESET Output-Voltage Low VOL MAX79_C, VBATT = VCC = 1.0V, ISINK = 40µA MAX79_E, VBATT = VCC = 1.
PAGE 5
3.0V/3.3V/Adjustable Microprocessor Supervisory Circuits 2.2 VCC = 3.0V 2.0 VCC = 3.3V 1.8 1.6 VCC = 5V 1.4 140 VBATT = 3.0V 120 VBATT = 3.6V 100 80 60 1.2 -40 -20 0 20 40 60 80 MAX793/4, VCC = 3.3V MAX795, VCC = 3.3V 30 20 VBATT = VCC = VOUT 0 -40 -20 0 20 40 60 80 100 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) TEMPERATURE (°C) BATTERY SUPPLY CURRENT vs. TEMPERATURE (BATTERY-BACKUP MODE) RESET TIMEOUT PERIOD vs. TEMPERATURE RESET COMPARATOR PROPAGATION DELAY vs.
PAGE 6
____________________________Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) MAX794 RESET IN THRESHOLD AND LOWLINE-TO-RESET IN THRESHOLD vs. TEMPERATURE 1.239 15 VLOWLINE - VRST 1.238 10 1.237 5 VCC FALLING 1.236 -20 0 20 40 60 80 1.5 1.0 0.
PAGE 7
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits PIN MAX793/ MAX794 MAX795 1 1 2 2 3 NAME FUNCTION OUT Supply Output for CMOS RAM. When VCC rises above the reset threshold or above VBATT, OUT is connected to VCC through an internal p-channel MOSFET switch. When VCC falls below VSW and VBATT, BATT connects to OUT. VCC Main Supply Input BATT OK (MAX793) Battery Status Output. High in normal operating mode when VBATT exceeds VBOK, otherwise low. VBATT is checked continuously.
PAGE 8
MAX793/MAX794/MAX795 3.0V/3.3V Adjustable Microprocessor Supervisory Circuits 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 70µA pullup 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.
PAGE 9
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits VLL 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.
PAGE 10
MAX793/MAX794/MAX795 3.0V/3.3V Adjustable Microprocessor Supervisory Circuits (OPTIONAL) Si9433DY SILICONIX 3.3V D 0.1µF VRST S 0.1µF PMOS VCC BATT ON OUT R1 RESET IN VCC CMOS RAM CE OUT tRP R2 VCC MAX794 3.6V 0.1µF CE IN BATT ADDRESS DECODER LOWLINE MR +5V SUPPLY +5V FAILURE NMI VCC RESET RESET PFI GND VRST = VRST IN (R1R2 + 1) Figure 3. MAX794 Standard Application Circuit Reset Threshold The MAX793T/MAX795T are intended for 3.
PAGE 11
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits 4.7kΩ MAX793/MAX794 VCC WDO RESET MAX793/MAX794/MAX795 VCC VRST tRP RESET TO µP MR WDO VCC tWD ∼10µs WDO WDI RESET WDO CONNECTED TO µP INTERRUPT RESET PULLED UP TO VCC Figure 5. Watchdog Timing Relationship a reset occurs or when a transition (low-to-high or highto-low) takes place at WDI. As long as reset is asserted, the timer remains cleared and does not count.
PAGE 12
MAX793/MAX794/MAX795 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. The CE propagation delay is production tested from the 50% point on CE IN to the 50% point on CE OUT using a 50Ω driver and 50pF of load capacitance (Figure 9).
PAGE 13
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits VCC BATT 3.6V MAX793 MAX794 MAX795 25Ω EQUIVALENT SOURCE IMPEDANCE 50Ω CABLE CE OUT CE IN 50Ω 50pF CL* 50Ω GND *CL INCLUDES LOAD CAPACITANCE AND SCOPE PROBE CAPACITANCE. Figure 9. CE Propagation Delay Test Circuit 10). If there is no easy access to the unregulated supply, the LOWLINE output can be used to generate an NMI to the µP (see LOWLINE Output section).
PAGE 14
MAX793/MAX794/MAX795 3.0V/3.3V Adjustable Microprocessor Supervisory Circuits VIN 3.0V OR 3.3V VCC R1 MAX793 MAX794 PFI 3.0V OR 3.3V VCC R1 PFO PFI R2 MAX793 MAX794 PFO R2 MR GND GND VIN VCC VCC PFO PFO VL VTRIP = R2 (VPFT + VPFH) VL = R2 (VPFT) 1 1 1 ( R1 + R2 ) – 1 ( R1 + R2 ) – VCC R1 VTRIP VCC R1 0V VIN VTRIP VTRIP = VPFT WHERE VPFT = 1.237V VPFH = 10mV R1 + R2 R2 VH = (VPFT + VPFH) NOTE: VTRIP, VL ARE NEGATIVE (a) ( ( VH VIN ) R1 + R2 R2 ) (b) Figure 12.
PAGE 15
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits VRST VCC VSW 3.6V 3.6V 3.3V VOUT VBATT = 3.6V Figure 13. Battery Switchover Timing Table 1.
PAGE 16
MAX793/MAX794/MAX795 3.0V/3.3V Adjustable Microprocessor Supervisory Circuits PMOS FET BODY DIODE TO CMOS RAM 3.0V OR 3.3V S D G VCC BATT ON OUT VCC BATT ON OUT 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 MAX793 MAX794 OUT VCC 3.0V OR 3.3V TO STATIC RAM RESET MAX793 MAX794 OUT VCC TO STATIC RAM 1N4148 1N4148 BATT VCC TO µP RESET BATT TO µP 0.47F 0.47F GND GND (a) (b) Figure 15.
PAGE 17
3.0V/3.3V Adjustable Microprocessor Supervisory Circuits MAX793/MAX794/MAX795 VIN VIN R1 VCC R1 MAX793 MAX794 PFI R2 VCC R2 R3 PFI MAX793 MAX794 PFO GND R3 C1* C1* PFO GND *OPTIONAL *OPTIONAL TO µP TO µP PFO PFO 0V VL 0V VTRIP = VPFT VH VTRIP (R1 R2+ R2) VL = R1 VPFT ( ( ) ) 1 R3 VCC R3 WHERE VPFT = 1.
PAGE 18
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 have without causing a reset pulse to be issued. As the amplitude of the transient increases (i.e.
PAGE 19
3.0V/3.
PAGE 20
MAX793/MAX794/MAX795 3.0V/3.3V Adjustable Microprocessor Supervisory Circuits Revision History REVISION NUMBER REVISION DATE DESCRIPTION 0 2/95 Initial release 5 2/07 Revised Electrical Characteristics. 6 3/10 Revised Absolute Maximum Ratings and Chip-Enable Input section. PAGES CHANGED — 4 1, 2 Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied.