MAX691EPE+ [MAXIM]

Power Supply Management Circuit, Fixed, 1 Channel, CMOS, PDIP16, LEAD FREE, PLASTIC, DIP-16;
MAX691EPE+
型号: MAX691EPE+
厂家: MAXIM INTEGRATED PRODUCTS    MAXIM INTEGRATED PRODUCTS
描述:

Power Supply Management Circuit, Fixed, 1 Channel, CMOS, PDIP16, LEAD FREE, PLASTIC, DIP-16

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文件: 总16页 (文件大小:158K)
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19-0094; Rev 7a; 12/96  
Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s  
1693L/AX80M  
_______________Ge n e ra l De s c rip t io n  
____________________________Fe a t u re s  
200ms Power-OK/Reset Timeout Period  
The MAX691A/MAX693A/MAX800L/MAX800M micro-  
processor (µP) supervisory circuits are pin-compatible  
upgrades to the MAX691, MAX693, and MAX695. They  
improve performance with 30µA supply current, 200ms  
typ re s e t a c tive d e la y on p owe r-up , a nd 6ns c hip -  
enable propagation delay. Features include write pro-  
tection of CMOS RAM or EEPROM, separate watchdog  
outputs, backup-battery switchover, and a RESET out-  
put that is valid with VCC down to 1V. The MAX691A/  
MAX800L have a 4.65V typical reset-threshold voltage,  
and the MAX693A/MAX800M’s reset threshold is 4.4V  
typical. The MAX800L/MAX800M guarantee power-fail  
accuracies to ±2%.  
1µA Standby Current, 30µA Operating Current  
On-Board Gating of Chip-Enable Signals,  
10ns Max Delay  
MaxCapor SuperCapCompatible  
Guaranteed RESET Assertion to VCC = +1V  
Voltage Monitor for Power-Fail or Low-Battery  
Warning  
Power-Fail Accuracy Guaranteed to ±2%  
(MAX800L/M)  
Available in 16-Pin Narrow SO and Plastic  
DIP Packages  
________________________Ap p lic a t io n s  
______________Ord e rin g In fo rm a t io n  
Computers  
Controllers  
PART  
TEMP. RANGE  
0°C to +70°C  
PIN-PACKAGE  
16 Plastic DIP  
16 Narrow SO  
16 Wide SO  
Dice*  
MAX691ACPE  
MAX691ACSE  
MAX691ACWE  
MAX691AC/D  
MAX691AEPE  
MAX691AESE  
MAX691AEWE  
MAX691AEJE  
MAX691AMJE  
Intelligent Instruments  
Automotive Systems  
Critical µP Power Monitoring  
0°C to +70°C  
0°C to +70°C  
0°C to +70°C  
-40°C to +85°C  
-40°C to +85°C  
-40°C to +85°C  
-40°C to +85°C  
-55°C to +125°C  
16 Plastic DIP  
16 Narrow SO  
16 Wide SO  
16 CERDIP  
__________Typ ic a l Op e ra t in g Circ u it  
5V  
REGULATOR  
+8V  
16 CERDIP  
0.1µF  
Ordering Information continued on last page.  
* Dice are specified at T = +25 °C, DC parameters only.  
A
3
5
1N4148  
0.47F*  
V
BATT ON  
CC  
2
__________________P in Co n fig u ra t io n  
V
OUT  
1
VBATT  
12  
CE OUT  
TOP VIEW  
CMOS RAM  
MAX691A  
MAX693A  
MAX800L  
VBATT  
RESET  
RESET  
WDO  
CE IN  
CE OUT  
WDI  
1
2
3
4
5
6
7
8
16  
15  
14  
13  
12  
11  
10  
9
13  
11  
9
4
7
ADDRESS  
DECODE  
CE IN  
PFI  
V
OUT  
MAX800M  
V
CC  
GND  
A0-A15  
I/O  
GND  
MAX691A  
MAX693A  
MAX800L  
MAX800M  
WDI  
PFO  
µP  
OSC IN  
BATT ON  
LOW LINE  
OSC IN  
NO  
10  
15  
NMI  
CONNECTION  
8
OSC SEL  
PFO  
RESET  
RESET  
LOW LINE WDO  
OSC SEL  
PFI  
6
14  
AUDIBLE  
ALARM  
DIP/SO  
*MaxCap  
SYSTEM STATUS INDICATORS  
SuperCap is a registered trademark of Baknor Industries. MaxCap is a registered trademark of The Carborundum Corp.  
________________________________________________________________ Maxim Integrated Products  
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.  
For small orders, phone 408-737-7600 ext. 3468.  
Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s  
ABSOLUTE MAXIMUM RATINGS  
Terminal Voltage (with respect to GND)  
Continuous Power Dissipation (T = +70°C)  
A
V
.......................................................................-0.3V to +6V  
Plastic DIP (derate 10.53mW/°C above +70°C) ..........842mW  
CC  
VBATT...................................................................-0.3V to +6V  
All Other Inputs .....................................-0.3V to (V + 0.3V)  
Input Current  
Narrow SO (derate 8.70mW/°C above +70°C) ...........696mW  
Wide SO (derate 9.52mW/°C above +70°C)...............762mW  
CERDIP (derate 10.00mW/°C above +70°C)..............800mW  
Operating Temperature Ranges  
MAX69_AC_ _/MAX800_C_ _ .............................0°C to +70°C  
MAX69_AE_ _/MAX800_E_ _ ...........................-40°C to +85°C  
MAX69_AMJE ................................................-55°C to +125°C  
Storage Temperature Range .............................-65°C to +160°C  
Lead Temperature (soldering, 10sec) .............................+300°C  
OUT  
V
Peak...........................................................................1.0A  
Continuous.............................................................250mA  
CC  
V
CC  
VBATT Peak ..................................................................250mA  
VBATT Continuous ..........................................................25mA  
GND, BATT ON.............................................................100mA  
All Other Outputs ............................................................25mA  
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  
(MAX691A, MAX800L: V = +4.75V to +5.5V, MAX693A, MAX800M: V = +4.5V to +5.5V, VBATT = 2.8V, T = T  
to T  
,
CC  
CC  
A
MIN  
MAX  
unless otherwise noted.)  
PARAMETER  
CONDITIONS  
MIN  
TYP  
MAX  
UNITS  
Operating Voltage Range,  
, VBATT (Note 1)  
0
5.5  
V
V
CC  
I
= 25mA  
V
CC  
- 0.02  
V
CC  
- 0.05  
OUT  
MAX69_AC  
V
- 0.2  
- 0.2  
V
- 0.3  
CC  
CC  
MAX69_AE,  
MAX800_C/E  
I
= 250mA  
V
CC  
V
CC  
- 0.35  
- 0.40  
- 0.3V  
OUT  
V
OUT  
Output  
V
CC  
= 4.5V  
V
MAX69_A/M  
V
CC  
MAX69_AC/AE,  
MAX800_C/E  
I
= 210mA  
V
CC  
- 0.17  
V
CC  
OUT  
MAX69_AC, MAX800_C  
0.8  
1.2  
V
CC  
-to-V  
On-Resistance  
V = 4.5V  
CC  
OUT  
MAX69_AE, MAX800_E  
MAX69_A/M  
0.8  
0.8  
1.4  
1.6  
VBATT = 4.5V, IOUT = 20mA  
VBATT = 2.8V, IOUT = 10mA  
VBATT = 2.0V, IOUT = 5mA  
VBATT = 4.5V  
VBATT - 0.3  
VBATT - 0.25  
VBATT - 0.15  
V
Mode  
in Battery-Backup  
OUT  
V
1693L/AX80M  
15  
25  
30  
VBATT-to-V  
On-Resistance  
OUT  
VBATT = 2.8V  
VBATT = 2.0V  
Supply Current in  
Normal Operating Mode  
V
> VBATT - 1V  
30  
100  
µA  
µA  
CC  
(Excludes I  
)
OUT  
Supply Current in  
Battery-Backup Mode  
T
= +25°C  
0.04  
1
5
A
V
< VBATT - 1.2V  
CC  
VBATT = 2.8V  
T
A
= T + T  
MIN MIN  
(Excludes I ) (Note 2)  
OUT  
T
= +25°C  
-0.1  
-1.0  
0.02  
0.02  
VBATT Standby Current  
(Note 3)  
A
VBATT + 0.2V V  
µA  
V
CC  
T
A
= T  
+ T  
MIN  
MIN  
Power-up  
VBATT + 0.3  
VBATT - 0.3  
Battery Switchover  
Threshold  
Power-down  
2
_______________________________________________________________________________________  
Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s  
1693L/AX80M  
ELECTRICAL CHARACTERISTICS (continued)  
(MAX691A, MAX800L: V = +4.75V to +5.5V, MAX693A, MAX800M: V = +4.5V to +5.5V, VBATT = 2.8V, T = T  
to T  
,
CC  
CC  
A
MIN  
MAX  
unless otherwise noted.)  
PARAMETER  
CONDITIONS  
MIN  
TYP  
MAX  
UNITS  
Battery Switchover  
Hysteresis  
60  
mV  
I
= 3.2mA  
= 25mA  
0.1  
0.7  
60  
0.4  
1.5  
BATT ON Output  
Low Voltage  
SINK  
V
I
SINK  
Sink current  
mA  
µA  
BATT ON Output  
Short-Circuit Current  
Source current  
1
15  
100  
RESET AND WATCHDOG TIMER  
MAX691A, MAX800L  
MAX693A, MAX800M  
MAX800L, T = +25°C, V falling  
4.50  
4.25  
4.55  
4.30  
4.65  
4.40  
4.75  
4.50  
4.70  
4.45  
Reset Threshold Voltage  
V
A
CC  
MAX800M, T = +25°C, V falling  
A
CC  
Reset Threshold Hysteresis  
15  
80  
mV  
µs  
V
to RESET Delay  
Power-down  
CC  
800  
ns  
LOW LINE-to-RESET Delay  
Reset Active Timeout Period,  
Internal Oscillator  
Power-up  
Power-up  
140  
200  
280  
ms  
Reset Active Timeout Period,  
External Clock (Note 4)  
Clock  
Cycles  
2048  
Long period  
Short period  
Long period  
Short period  
1.0  
70  
1.6  
100  
2.25  
140  
sec  
ms  
Watchdog Timeout Period,  
Internal Oscillator  
4096  
1024  
Watchdog Timeout Period,  
External Clock (Note 4)  
Clock  
Cycles  
Minimum Watchdog Input  
Pulse Width  
V
= 0.8V, V = 0.75 x V  
CC  
100  
3.5  
0.1  
ns  
V
IL  
IH  
I
= 50µA, V = 1V, VBATT = 0V, V falling  
0.004  
0.1  
0.3  
0.4  
SINK  
CC  
CC  
I
= 3.2mA, V = 4.25V  
CC  
RESET Output Voltage  
SINK  
I
= 1.6mA, V = 5V  
CC  
SOURCE  
RESET Output Short-Circuit  
Output source current  
7
20  
mA  
V
Current  
RESET Output Voltage Low  
(Note 5)  
I
= 3.2mA  
0.4  
SINK  
I
= 3.2mA, V = 4.25V  
0.4  
SINK  
CC  
V
LOW LINE Output Voltage  
I
= 1µA, V = 5V  
3.5  
1
SOURCE  
CC  
LOW LINE Output  
Short-Circuit Current  
Output source current  
= 3.2mA  
15  
3
100  
0.4  
µA  
V
I
SINK  
WDO Output Voltage  
I
= 500µA, V = 5V  
3.5  
SOURCE  
CC  
WDO Output  
Short-Circuit Current  
Output source current  
10  
mA  
V
V
0.75 x V  
WDI Threshold Voltage  
(Note 6)  
IH  
CC  
V
0.8  
50  
IL  
WDI = 0V  
WDI = V  
-50  
-10  
20  
WDI Input Current  
µA  
OUT  
_______________________________________________________________________________________  
3
Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s  
ELECTRICAL CHARACTERISTICS (continued)  
(MAX691A, MAX800L: V = +4.75V to +5.5V, MAX693A, MAX800M: V = +4.5V to +5.5V, VBATT = 2.8V, T = T  
to T  
,
CC  
CC  
A
MIN  
MAX  
unless otherwise noted.)  
PARAMETER  
CONDITIONS  
MIN  
TYP  
MAX  
UNITS  
POWER-FAIL COMPARATOR  
MAX69_AC/AE/AM, V = 5V  
1.2  
1.25  
1.25  
1.3  
1.275  
±25  
0.4  
CC  
PFI Input Threshold  
V
nA  
V
MAX800_C/E, V = 5V  
1.225  
CC  
PFI Leakage Current  
PFO Output Voltage  
±0.01  
I
= 3.2mA  
SINK  
I
= 1µA, V = 5V  
3.5  
1
SOURCE  
CC  
PFO Output Short-Circuit  
Current  
Output source current  
= -20mV, V = 15mV  
15  
100  
µA  
µs  
V
IN  
25  
60  
OD  
PFI-to-PFO Delay  
V
IN  
= 20mV, V = 15mV  
OD  
CHIP-ENABLE GATING  
Disable mode  
Enable mode  
±0.005  
75  
±1  
µA  
CE IN Leakage Current  
CE IN-to-CE OUT Resistance  
(Note 7)  
150  
CE OUT Short-Circuit Current  
(Reset Active)  
0.1  
0.75  
6
2.0  
10  
mA  
ns  
Disable mode, CE OUT = 0V  
CE IN-to-CE OUT Propagation  
Delay (Note 8)  
50source impedance driver, C  
= 50pF  
LOAD  
V
= 5V, I  
= -100µA  
3.5  
2.7  
CC  
OUT  
CE OUT Output Voltage High  
(Reset Active)  
V
V
CC  
= 0V, VBATT = 2.8V, I  
= 1µA  
OUT  
Power-down  
12  
µs  
RESET-to-CE OUT Delay  
INTERNAL OSCILLATOR  
OSC IN Leakage Current  
OSC IN Input Pull-Up Current  
OSC SEL = 0V  
0.10  
10  
±5  
µA  
µA  
OSC SEL = V  
or floating, OSC IN = 0V  
100  
100  
OUT  
OSC SEL Input Pull-Up Current OSC SEL = 0V  
10  
µA  
OSC IN Frequency Range  
OSC SEL = 0V  
50  
kHz  
V
V
OUT  
- 0.3  
V
- 0.6  
IH  
OUT  
OSC IN External Oscillator  
Threshold Voltage  
V
V
IL  
3.65  
100  
2.00  
1693L/AX80M  
OSC IN Frequency with  
External Capacitor  
OSC SEL = 0V, COSC = 47pF  
kHz  
Note 1: Either V or VBATT can go to 0V, if the other is greater than 2.0V.  
CC  
Note 2: The supply current drawn by the MAX691A/MAX800L/MAX800M from the battery excluding I  
typically goes to 10µA  
OUT  
when (VBATT - 1V) < V < VBATT. In most applications, this is a brief period as V falls through this region.  
CC  
CC  
Note 3: +” = battery-discharging current, --” = battery-charging current.  
Note 4: Although presented as typical values, the number of clock cycles for the reset and watchdog timeout periods are fixed and  
do not vary with process or temperature.  
Note 5: RESET is an open-drain output and sinks current only.  
Note 6: WDI is internally connected to a voltage divider between V  
and GND. If unconnected, WDI is driven to 1.6V (typ),  
OUT  
disabling the watchdog function.  
Note 7: The chip-enable resistance is tested with V = +4.75V for the MAX691A/MAX800L and V = +4.5V for the  
CC  
CC  
MAX693A/MAX800M. CE IN = CE OUT = V / 2.  
CC  
Note 8: The chip-enable propagation delay is measured from the 50% point at CE IN to the 50% point at CE OUT.  
4
_______________________________________________________________________________________  
Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s  
1693L/AX80M  
__________________________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s  
(T = +25°C, unless otherwise noted.)  
A
V
SUPPLY CURRENT  
CC  
BATTERY SUPPLY CURRENT  
vs. TEMPERATURE  
(BATTERY-BACKUP MODE)  
CHIP-ENABLE ON-RESISTANCE  
vs. TEMPERATURE  
vs. TEMPERATURE  
(NORMAL OPERATING MODE)  
120  
100  
36  
34  
32  
30  
28  
2
1.5  
1
V
= 4.75V  
V
= 5V  
CC  
CC  
V = 5V  
CC  
VBATT = 2.8V  
NO LOAD  
VBATT = 2.8V  
= V /2  
VBATT = 2.8V  
PFI, CE IN = 0V  
V
CE IN CC  
80  
60  
0.5  
0
26  
40  
-60 -30  
0
30 60 90 120 150 180  
TEMPERATURE (°C)  
-60 -30  
0
30  
60  
90 120 150  
-60 -30  
0
30  
60  
90 120 150  
TEMPERATURE (°C)  
TEMPERATURE (°C)  
PFI THRESHOLD  
vs. TEMPERATURE  
VBATT to V ON-RESISTANCE  
OUT  
V
CC  
to V ON-RESISTANCE  
OUT  
vs. TEMPERATURE  
vs. TEMPERATURE  
1.50  
1.25  
1.00  
0.75  
0.50  
0.25  
0
20  
15  
10  
5
1.2  
1.1  
1.0  
0.9  
0.8  
0.7  
0.6  
V
= 5V,  
CC  
VBATT = 0V  
VBATT = 2.0V  
VBATT = 2.8V  
VBATT = 4.5V  
V
= +5V,  
CC  
VBATT = 0V  
NO LOAD ON PFO  
V
= 0V  
CC  
-60 -30  
0
30  
60  
90 120 150  
-60 -30  
0
30  
60  
90 120 150  
-60 -30  
0
30  
60  
90 120 150  
TEMPERATURE (°C)  
TEMPERATURE (°C)  
TEMPERATURE (°C)  
RESET THRESHOLD  
vs. TEMPERATURE  
RESET OUTPUT RESISTANCE  
vs. TEMPERATURE  
RESET DELAY  
vs. TEMPERATURE  
230  
220  
210  
200  
190  
180  
170  
4.75  
4.70  
4.65  
4.60  
4.55  
4.50  
4.45  
4.40  
4.35  
4.30  
600  
500  
400  
300  
VBATT = 2.8V  
V
= 0V TO 5V STEP  
CC  
VBATT = 2.8V  
V
= 5V, VBATT = 2.8V  
CC  
MAX691A  
MAX800L  
SOURCING CURRENT  
200  
100  
MAX693A  
MAX800M  
V
= 0V, VBATT = 2.8V  
CC  
SINKING CURRENT  
0
-60 -30  
0
30  
60  
90 120 150  
-60 -30  
0
30  
60  
90 120 150  
-60 -30  
0
30  
60  
90 120 150  
TEMPERATURE (°C)  
TEMPERATURE (°C)  
TEMPERATURE (°C)  
_______________________________________________________________________________________  
5
Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s  
____________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )  
(T = +25°C, unless otherwise noted.)  
A
BATTERY CURRENT  
vs. INPUT SUPPLY VOLTAGE  
CHIP-ENABLE PROPAGATION DELAY  
vs. CE OUT LOAD CAPACITANCE  
WATCHDOG AND RESET TIMEOUT PERIOD  
vs. OSC IN TIMING CAPACITOR (COSC)  
20  
20  
100  
10  
1
V
= 5V  
VBATT = 2.8V  
CC  
V
= 5V  
CC  
LONG WATCHDOG  
TIMEOUT PERIOD  
CE IN = 0V TO 5V  
DRIVER SOURCE  
IMPEDANCE = 50  
I
= 0A  
OUT  
VBATT = 2.8V  
16  
12  
8
16  
12  
RESET ACTIVE  
TIMEOUT PERIOD  
8
4
0
SHORT WATCHDOG  
TIMEOUT PERIOD  
4
0
0.1  
0
1
2
3
4
5
0
50  
100  
150  
(pF)  
200  
250 300  
10  
100  
COSC (pF)  
1000  
V
CC  
(V)  
C
LOAD  
V
CC  
to V vs. OUTPUT CURRENT  
OUT  
VBATT to V vs. OUTPUT CURRENT  
OUT  
V
CC  
to LOW LINE  
(NORMAL OPERATING MODE)  
(BATTERY-BACKUP MODE)  
AND CE OUT DELAY  
1000  
100  
10  
1000  
100  
10  
V
= 4.5V  
V
= 0V  
CC  
CC  
5V  
RESET  
THRESHOLD  
VBATT = 0V  
VBATT = 4.5V  
V
CC  
80µs  
HI  
LOW LINE  
LO  
800ns  
SLOPE = 8Ω  
HI  
RESET  
LO  
SLOPE = 0.8Ω  
HI  
CE OUT  
LO  
12µs  
1
1
1
10  
100  
1000  
1
10  
(mA)  
100  
I
(mA)  
I
OUT  
OUT  
1693L/AX80M  
6
_______________________________________________________________________________________  
Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s  
1693L/AX80M  
______________________________________________________________P in De s c rip t io n  
PIN  
NAME  
FUNCTION  
Battery-Backup Input. Connect to external battery or capacitor and charging circuit. If backup battery is not  
used, connect to GND.  
1
VBATT  
Output Supply Voltage. When V is greater than VBATT and above the reset threshold, V  
connects to  
CC  
OUT  
2
V
OUT  
V
. When V falls below VBATT and is below the reset threshold, V connects to VBATT. Connect a 0.1µF  
OUT  
CC  
CC  
capacitor from V  
to GND. Connect V to V if no backup battery is used.  
OUT CC  
OUT  
3
4
V
CC  
Input Supply Voltage, 5V input.  
GND  
Ground. 0V reference for all signals.  
Battery On Output. When V  
goes low. Connect the base of a PNP through a current-limiting resistor to BATT ON for V  
ments greater than 250mA.  
switches to VBATT, BATT ON goes high. When V  
switches to V  
BATT ON  
OUT  
OUT  
CC,  
5
6
BATT ON  
current require-  
OUT  
LOW LINE output goes low when V falls below the reset threshold. It returns high as soon as V rises above  
the reset threshold.  
CC  
CC  
LOW LINE  
External Oscillator Input. When OSC SEL is unconnected or driven high, a 10µA pull-up connects from V  
to  
OUT  
OSC IN, the internal oscillator sets the reset and watchdog timeout periods, and OSC IN selects between fast  
and slow watchdog timeout periods. When OSC SEL is driven low, the reset and watchdog timeout periods may  
be set either by a capacitor from OSC IN to ground or by an external clock at OSC IN (Figure 3).  
7
8
OSC IN  
Oscillator Select. When OSC SEL is unconnected or driven high, the internal oscillator sets the reset delay and  
watchdog timeout period. When OSC SEL is low, the external oscillator input (OSC IN) is enabled (Table 1).  
OSC SEL has a 10µA internal pull-up.  
OSC SEL  
Power-Fail Input. This is the noninverting input to the power-fail comparator. When PFI is less than 1.25V, PFO  
9
PFI  
goes low. When PFI is not used, connect PFI to GND or V  
.
OUT  
Power-Fail Output. This is the output of the power-fail comparator. PFO goes low when PFI is less than 1.25V.  
This is an uncommitted comparator, and has no effect on any other internal circuitry.  
10  
PFO  
Watchdog Input. WDI is a three-level input. If WDI remains either high or low for longer than the watchdog time-  
out period, WDO goes low and reset is asserted for the reset timeout period. WDO remains low until the next tran-  
sition at WDI. Leaving WDI unconnected disables the watchdog function. WDI connects to an internal voltage  
11  
WDI  
divider between V  
and GND, which sets it to mid-supply when left unconnected.  
OUT  
Chip-Enable Output. CE OUT goes low only when CE IN is low and V is above the reset threshold. If CE IN is  
low when reset is asserted, CE OUT will stay low for 15µs or until CE IN goes high, whichever occurs first.  
CC  
12  
13  
CE OUT  
CE IN  
Chip-Enable Input. The input to chip-enable gating circuit. If CE IN is not used, connect CE IN to GND or V  
OUT.  
Watchdog Output. If WDI remains high or low longer than the watchdog timeout period, WDO goes low and reset  
is asserted for the reset timeout period. WDO returns high on the next transition at WDI. WDO remains high if  
WDI is unconnected.  
14  
WDO  
RESET Output goes low whenever V falls below the reset threshold. RESET will remain low typically for  
200ms after V crosses the reset threshold on power-up.  
CC  
CC  
15  
16  
RESET  
RESET  
RESET is an active-high output. It is open drain, and the inverse of RESET.  
g ua ra nte e d to b e va lid d own to VCC = 1V, a nd a n  
external 10kpull-down resistor on RESET insures  
that it will be valid with VCC down to GND (Figure 1).  
As VCC goes below 1V, the gate drive to the RESET  
outp ut s witc h re d uc e s a c c ord ing ly, inc re a s ing the  
RDS(ON) and the saturation voltage. The 10kpull-  
down resistor insures the parallel combination of switch  
plus resistor is around 10kand the output saturation  
voltage is below 0.4V while sinking 40µA. When using  
a 10kexternal pull-down resistor, the high state for  
RESET output with VCC = 4.75V will be 4.5V typical.  
For battery voltages 2V connected to VBATT, RESET  
and RESET remain valid for VCC from 0V to 5.5V.  
_______________De t a ile d De s c rip t io n  
–————–  
R E S E T a n d RES ET Ou t p u t s  
The MAX691A/MAX693A/MAX800L/MAX800M’s RESET  
a nd RESET outp uts e ns ure tha t the µP (with re s e t  
inputs a sse rte d e ithe r hig h or low) p owe rs up in a  
known state, and prevents code-execution errors dur-  
ing power-down or brownout conditions.  
The RESET output is active low, and typically sinks  
3.2mA at 0.1V saturation voltage in its active state.  
When deasserted, RESET sources 1.6mA at typically  
VOUT - 0.5V. RESET output is open drain, active high,  
and typically sinks 3.2mA with a saturation voltage of  
0.1V. When no backup battery is used, RESET output is  
_______________________________________________________________________________________  
7
Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s  
WDI  
15  
RESET  
TO µP RESET  
WDO  
1k  
MAX691A  
MAX693A  
t
2
RESET  
t
1
t
1
t
3
t = RESET TIMEOUT PERIOD  
1
t = NORMAL WATCHDOG TIMEOUT PERIOD  
2
t = WATCHDOG TIMEOUT PERIOD IMMEDIATELY AFTER RESET  
3
Figure 2. Watchdog Timeout Period and Reset Active Time  
Figure 1. Adding an external pull-down resistor ensures  
———–  
RESET is valid with V down to GND.  
CC  
Watchdog Output  
RESET and RESET are asserted when VCC falls below  
the reset threshold (4.65V for the MAX691A/MAX800L,  
4.4V for the MAX693A/MAX800M) and remain asserted  
for 200ms typ after VCC rises above the reset threshold  
on p owe r-up (Fig ure 5). The d e vic e s ’ b a tte ry-  
switchover comparator does not affect reset assertion.  
However, both reset outputs are asserted in battery-  
b a c kup mod e s inc e VCC mus t b e b e low the re s e t  
threshold to enter this mode.  
The Watchdog Output (WDO) remains high if there is a  
transition or pulse at WDI during the watchdog timeout  
period. The watchdog function is disabled and WDO is  
a logic high when VCC is below the reset threshold, bat-  
tery-backup mode is enabled, or WDI is an open circuit.  
In watchdog mode, if no transition occurs at WDI during  
the watchdog timeout period, RESET and RESET are  
asserted for the reset timeout period (200ms typical).  
WDO goes low and remains low until the next transition  
at WDI (Figure 2). If WDI is held high or low indefinitely,  
RESET and RESET will generate 200ms pulses every  
1.6sec. WDO has a 2 x TTL output characteristic.  
Wa t c h d o g Fu n c t io n  
The watchdog monitors µP activity via the Watchdog  
Input (WDI). If the µP becomes inactive, RESET and  
RESET are asserted. To use the watchdog function,  
c onne c t WDI to a b us line or µP I/O line . If WDI  
remains high or low for longer than the watchdog time-  
out period (1.6sec nominal), WDO, RESET, and RESET  
are asserted (see RESET and RESET Outputs section,  
and the Watchdog Output discussion on this page).  
Selecting an Alternative  
Watchdog and Reset Timeout Period  
The OSC SEL and OSC IN inputs control the watchdog  
and reset timeout periods. Floating OSC SEL and OSC  
IN or tying them both to VOUT selects the nominal 1.6sec  
watchdog timeout period and 200ms reset timeout peri-  
od. Connecting OSC IN to GND and floating or connect-  
ing OSC SEL to VOUT s e le c ts the 100ms norma l  
watchdog timeout delay and 1.6sec delay immediately  
after reset. The reset timeout delay remains 200ms  
(Figure 2). Select alternative timeout periods by con-  
necting OSC SEL to GND and connecting a capacitor  
between OSC IN and GND, or by externally driving OSC  
IN (Table 1 and Figure 3). OSC IN is internally connect-  
ed to a ±100nA (typ) current source that charges and  
discharges the timing capacitor to create the oscillator  
frequency, which sets the reset and watchdog timeout  
periods (see Connecting a Timing Capacitor at OSC IN  
in the Applications Information section).  
1693L/AX80M  
Watchdog Input  
A change of state (high to low, low to high, or a mini-  
mum 100ns pulse) at the WDI during the watchdog  
p e riod re s e ts the wa tc hd og time r. The wa tc hd og  
default timeout is 1.6sec.  
To disable the watchdog function, leave WDI floating.  
An internal resistor network (100kequivalent imped-  
a nc e a t WDI) b ia s e s WDI to a p p roxima te ly 1.6V.  
Internal comparators detect this level and disable the  
watchdog timer. When VCC is below the reset thresh-  
old, the watchdog function is disabled and WDI is dis-  
c onne c te d from its inte rna l re s is tor ne twork, thus  
becoming high impedance.  
8
_______________________________________________________________________________________  
Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s  
1693L/AX80M  
Table 1. Reset Pulse Width and Watchdog Timeout Selections  
Watchdog Timeout Period  
OSC SEL  
OSC IN  
Reset Timeout Period  
Normal  
Immediately After Reset  
4096 clks  
Low  
Low  
External Clock Input  
External Capacitor  
Low  
1024 clks  
(600/47pF x C)ms  
100ms  
2048 clks  
(1200/47pF x C)ms  
200ms  
(2.4/47pF x C)sec  
1.6sec  
Floating  
Floating  
Floating  
1.6sec  
1.6sec  
200ms  
In the high-impedance mode, the leakage currents into  
this terminal are ±1µA max over temperature. In the  
low-impedance mode, the impedance of CE IN appears  
as a 75resistor in series with the load at CE OUT.  
EXTERNAL  
CLOCK  
EXTERNAL  
OSCILLATOR  
MAX691A  
MAX693A  
MAX800L  
MAX800M  
8
7
8
7
OSC SEL  
OSC SEL  
The propagation delay through the CE transmission  
gate depends on both the source impedance of the  
OSC IN  
OSC IN  
drive to CE IN and the capacitive loading on the Chip-  
50kHz  
Enable Output (CE OUT) (see Chip-Enable Propagation  
Delay vs. CE OUT Load Capacitance in the Typical  
Operating Characteristics). The CE propagation delay  
INTERNAL OSCILLATOR  
1.6sec WATCHDOG  
INTERNAL OSCILLATOR  
100ms WATCHDOG  
is production tested from the 50% point of CE IN to the  
8
8
50% point of CE OUT using a 50driver and 50pF of  
OSC SEL  
OSC SEL  
N.C.  
N.C.  
load capacitance (Figure 6). For minimum propagation  
delay, minimize the capacitive load at CE OUT, and  
use a low output-impedance driver.  
7
7
N.C.  
OSC IN  
OSC IN  
Chip-Enable Output  
Figure 3. Oscillator Circuits  
In the enabled mode, the impedance of CE OUT is  
equivalent to 75in series with the source driving CE  
IN. In the disabled mode, the 75transmission gate is  
off and CE OUT is actively pulled to VOUT. This source  
turns off when the transmission gate is enabled.  
Ch ip -En a b le S ig n a l Ga t in g  
The MAX691A/MAX693A/MAX800L/MAX800M provide  
internal gating of chip-enable (CE) signals to prevent  
erroneous data from being written to CMOS RAM in the  
event of a power failure. During normal operation, the  
CE gate is enabled and passes all CE transitions. When  
reset is asserted, this path becomes disabled, prevent-  
ing erroneous data from corrupting the CMOS RAM. All  
these parts use a series transmission gate from CE IN to  
CE OUT (Figure 4).  
–  
LOW LINE Ou t p u t  
LOW LINE is the buffered output of the reset threshold  
comparator. LOW LINE typically sinks 3.2mA at 0.1V.  
For normal operation (VCC above the LOW LINE thresh-  
old), LOW LINE is pulled to VOUT  
.
P o w e r-Fa il Co m p a ra t o r  
The power-fail comparator is an uncommitted comparator  
tha t ha s no e ffe c t on the othe r func tions of the IC.  
Common uses include low-battery indication (Figure 7),  
and early power-fail warning (see Typical Operating  
Circuit).  
The 10ns max CE propagation delay from CE IN to CE  
OUT enables the parts to be used with most µPs.  
Chip-Enable Input  
The Chip-Enable Input (CE IN) is high impedance (dis-  
abled mode) while RESET and RESET are asserted.  
———–  
Power-Fail Input  
During a power-down sequence where V falls below  
CC  
the reset threshold or a watchdog fault, CE IN assumes  
Power Fail Input (PFI) is the input to the power-fail com-  
parator. It has a guaranteed input leakage of ±25nA  
max over temperature. The typical comparator delay is  
a high-impedance state when the voltage at CE IN  
goes high or 15µs after reset is asserted, whichever  
occurs first (Figure 5).  
25µs from V to V (power failing), and 60µs from V  
IL  
OL  
IH  
to VOH (power being restored). If PFI is not used, con-  
nect it to ground.  
During a p owe r-up s e q ue nc e , CE IN re ma ins hig h  
impedance, regardless of CE IN activity, until reset is  
deasserted following the reset timeout period.  
_______________________________________________________________________________________  
9
Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s  
5
BATT ON  
4.65V*  
6
LOW LINE  
3
V
CC  
2
V
OUT  
CHIP-ENABLE  
OUTPUT  
CONTROL  
1
VBATT  
CE IN  
13  
12  
16  
CE OUT  
RESET  
MAX691A  
MAX693A  
MAX800L  
MAX800M  
RESET  
GENERATOR  
15  
RESET  
7
8
TIMEBASE FOR  
RESET AND  
OSC IN  
OSC SEL  
WATCHDOG  
WATCHDOG  
TRANSITION  
DETECTOR  
11  
9
WATCHDOG  
TIMER  
14  
10  
WDI  
PFI  
WDO  
PFO  
1.25V  
4
GND  
* 4.4V FOR THE MAX693A/MAX800M  
Figure 4. MAX691A/MAX693A/MAX800L/MAX800M Block Diagram  
5.0V  
V
CC  
RESET  
THRESHOLD  
4.0V  
5.0V  
CE IN  
1693L/AX80M  
0V  
5V  
CE OUT  
0V  
15µs  
100µs  
100µs  
5V  
RESET  
RESET  
0V  
5V  
0V  
LOGIC LEVELS SHOWN ARE FROM 0V TO 5V.  
Figure 5. Reset and Chip-Enable Timing  
10 ______________________________________________________________________________________  
Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s  
1693L/AX80M  
+5V  
+5V  
V
CC  
V
CC  
VBATT  
VBATT  
2.8V  
MAX691A  
MAX693A  
MAX800L  
MAX800M  
MAX691A  
MAX693A  
MAX800L  
MAX800M  
2.0V to 5.5V  
PFI  
LOW BATT  
PFO  
CE OUT  
CE IN  
50Ω  
OUTPUT  
GND  
GND  
C
LOAD  
IMPEDANCE  
Figure 6. CE Propagation Delay Test Circuit  
Figure 7. Low-Battery Indicator  
Power-Fail Output  
Table 2. Input and Output Status in Battery-Backup  
Mode  
The Power-Fail Output (PFO) goes low when PFI goes  
below 1.25V. It typically sinks 3.2mA with a saturation  
voltage of 0.1V. With PFI above 1.25V, PFO is actively  
PIN  
NAME  
STATUS  
1
VBATT  
Supply current is 1µA max.  
pulled to VOUT  
.
V
is connected to VBATT through an  
OUT  
2
V
OUT  
Ba t t e ry-Ba c k u p Mo d e  
internal PMOS switch.  
Two conditions are required to switch to battery-back-  
up mode: 1) VCC must be below the reset threshold,  
and 2) VCC must be below VBATT. Table 2 lists the sta-  
tus of the inputs and outputs in battery-backup mode.  
Battery switchover comparator monitors  
3
4
5
V
CC  
V
CC  
for active switchover.  
GND  
GND 0V, 0V reference for all signals.  
Logic high. The open-circuit output is  
Ba t t e ry On Ou t p u t  
The Battery On (BATT ON) output indicates the status  
of the inte rna l VCC/b a tte ry-s witc hove r c omp a ra tor,  
which controls the internal VCC and VBATT switches.  
For VCC greater than VBATT (ignoring the small hys-  
teresis effect), BATT ON typically sinks 3.2mA at 0.1V  
saturation voltage. In battery-backup mode, this termi-  
nal sources approximately 10µA from VOUT. Use BATT  
ON to indicate battery-switchover status or to supply  
base drive to an external pass transistor for higher-cur-  
rent applications (see Typical Operating Circuit).  
BATT ON  
equal to V  
.
OUT  
6
7
8
Logic low*  
OSC IN is ignored.  
LOWLINE  
OSC IN  
OSC SEL OSC SEL is ignored.  
The power-fail comparator remains  
9
PFI  
active in the battery-backup mode for  
VBATT - 1.2V typ.  
V
CC  
The power-fail comparator remains  
active in the battery-backup mode for  
10  
PFO  
V
CC  
VBATT - 1.2V typ. Below this volt-  
In p u t S u p p ly Vo lt a g e  
The Input Supply Voltage (VCC) should be a regulated  
5V. VCC connects to VOUT via a parallel diode and a  
large PMOS switch. The switch carries the entire cur-  
rent load for currents less than 250mA. The parallel  
diode carries any current in excess of 250mA. Both the  
switch and the diode have impedances less than 1Ω  
each. The maximum continuous current is 250mA, but  
power-on transients may reach a maximum of 1A.  
age, PFO is forced low.  
11  
12  
13  
14  
WDI  
CE OUT  
CE IN  
Watchdog is ignored.  
Logic high. The open-circuit voltage is  
equal to V  
.
OUT  
High impedance  
Logic high. The open-circuit voltage is  
WDO  
equal to V  
.
OUT  
15  
16  
Logic low*  
High impedance*  
RESET  
RESET  
* V must be below the reset threshold to enter battery-backup  
CC  
mode.  
______________________________________________________________________________________ 11  
Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s  
1) Normal operating mode with all circuitry powered  
Ba t t e ry-Ba c k u p In p u t  
The Battery-Backup Input (VBATT) is similar to the VCC  
input except the PMOS switch and parallel diode are  
much smaller. Accordingly, the on-resistances of the  
diode and the switch are each approximately 10.  
Continuous current should be limited to 25mA and  
peak currents (only during power-up) limited to 250mA.  
The reverse leakage of this input is less than 1µA over  
temperature and supply voltage (Figure 8).  
up. Typical supply current from VCC is 35µA while  
only leakage currents flow from the battery.  
2) Battery-backup mode where VCC is typically within  
0.7V below VBATT. All circuitry is powered up  
and the supply current from the battery is typically  
less than 60µA.  
3) Ba tte ry-b a c kup mod e whe re VCC is le s s tha n  
VBATT by at least 0.7V. VBATT supply current is  
1µA max.  
Ou t p u t S u p p ly Vo lt a g e  
The Output Supply Voltage (VOUT) pin is internally con-  
nected to the substrate of the IC and supplies current  
to the external system and internal circuitry. All open-  
circuit outputs will, for example, assume the VOUT volt-  
age in their high states rather than the VCC voltage. At  
the maximum source current of 250mA, VOUT will typi-  
cally be 200mV below VCC. Decouple this terminal with  
a 0.1µF capacitor.  
Us in g S u p e rCa p ™ o r Ma x Ca p ™ w it h t h e  
MAX6 9 1 A/MAX6 9 3 A/MAX8 0 0 L/MAX8 0 0 M  
VBATT has the same operating voltage range as VCC  
,
and the battery switchover threshold voltages are typi-  
c a lly ±30mV c e nte re d a t VBATT, a llowing use of a  
SuperCap and a simple charging circuit as a backup  
source (Figure 9).  
If VCC is above the reset threshold and VBATT is 0.5V  
above VCC, current flows to VOUT and VCC from VBATT  
until the voltage at VBATT is less than 0.5V above VCC.  
__________Ap p lic a t io n s In fo rm a t io n  
The MAX691A/MAX693A/MAX800L/MAX800M are not  
short-circuit protected. Shorting VOUT to ground, other  
than power-up transients such as charging a decou-  
pling capacitor, destroys the device.  
For example, with a SuperCap connected to VBATT and  
through a diode to VCC, if VCC quickly changes from 5.4V  
to 4.9V, the capacitor discharges through VOUT and VCC  
until VBATT reaches 5.1V typ. Leakage current through  
the SuperCap charging diode and the internal power  
diode eventually discharges the SuperCap to VCC. Also, if  
VCC and VBATT start from 0.1V above the reset threshold  
and power is lost at VCC, the SuperCap on VBATT dis-  
charges through VCC until VBATT reaches the reset  
threshold; then the battery-backup mode is initiated and  
the current through VCC goes to zero.  
All open-circuit outputs swing between VOUT and GND  
rather than VCC and GND.  
If long leads connect to the chip inputs, insure that  
these leads are free from ringing and other conditions  
that would forward bias the chips protection diodes.  
There are three distinct modes of operation:  
+5V  
3
V
CC  
1N4148  
0.47F*  
1693L/AX80M  
VBATT  
1
2
VBATT  
V
OUT  
MAX691A  
MAX693A  
MAX800L  
MAX800M  
V
OUT  
MAX691A  
MAX693A  
MAX800L  
MAX800M  
0.1µF  
V
CC  
GND  
4
* MaxCap  
Figure 8. V and VBATT to V  
Switch  
Figure 9. SuperCap or MaxCap on VBATT  
CC  
OUT  
12 ______________________________________________________________________________________  
Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s  
1693L/AX80M  
V
IN  
Rp*  
+5V  
CE  
CE  
R1  
RAM 1  
RAM 2  
V
CC  
V
OUT  
C1*  
R3  
PFI  
CE IN  
CE OUT  
CE  
CE  
MAX691A  
MAX693A  
MAX800L  
MAX800M  
MAX691A  
MAX693A  
MAX800L  
MAX800M  
R2  
CE  
CE  
PFO  
RAM 3  
RAM 4  
GND  
GND  
TO µP  
*OPTIONAL  
CE  
CE  
5V  
PFO  
0V  
0V  
V
L
V
V
TRIP H  
V
IN  
R1 + R2  
R2  
V
= 1.25  
TRIP  
*MAXIMUM Rp VALUE DEPENDS ON  
THE NUMBER OF RAMS.  
MINIMUM Rp VALUE IS 1kΩ.  
ACTIVE-HIGH  
CE LINES  
FROM LOGIC  
R2 I I R3  
R1 + R2 I I R3  
V - 1.25 5 - 1.25 1.25  
L
V = 1.25/  
H
+
=
R1  
R3  
R2  
Figure 10. Alternate CE Gating  
Figure 11. Adding Hysteresis to the Power-Fail Comparator  
Us in g S e p a ra t e P o w e r S u p p lie s  
fo r VBATT a n d V  
CC  
+5V  
If using separate power supplies for VCC and VBATT,  
VBATT must be less than 0.3V above VCC when VCC is  
above the reset threshold. As described in the previ-  
ous section, if VBATT exceeds this limit and power is  
lost at VCC, current flows continuously from VBATT to  
VCC via the VBATT-to-VOUT diode and the VOUT-to-VCC  
switch until the circuit is broken (Figure 8).  
R1  
V
CC  
PFO  
PFI  
MAX691A  
MAX693A  
MAX800L  
MAX800M  
Alt e rn a t e Ch ip -En a b le Ga t in g  
Using memory devices with both CE and CE inputs  
allows the CE loop to be bypassed. To do this, con-  
R2  
GND  
nect CE IN to ground, pull up CE OUT to VOUT, and  
c onne c t CE OUT to the CE inp ut of e a c h me mory  
V-  
5V  
PFO  
0V  
device (Figure 10). The CE input of each part then  
connects directly to the chip-select logic, which does  
not have to be gated.  
V
TRIP  
V-  
0V  
Ad d in g Hys t e re s is t o t h e  
P o w e r-Fa il Co m p a ra t o r  
5 - 1.25 1.25 - V  
TRIP  
=
R1  
R2  
Hysteresis adds a noise margin to the power-fail com-  
parator and prevents repeated triggering of PFO when  
NOTE: V IS NEGATIVE  
TRIP  
V
is near the power-fail comparator trip point. Figure  
IN  
11 shows how to add hysteresis to the power-fail com-  
Figure 12. Monitoring a Negative Voltage  
______________________________________________________________________________________ 13  
Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s  
Ba c k u p -Ba t t e ry Re p la c e m e n t  
The backup battery may be disconnected while VCC is  
above the reset threshold. No precautions are neces-  
sary to avoid spurious reset pulses.  
100  
80  
V
= 5V  
CC  
T = +25°C  
A
0.1µF CAPACITOR  
FROM V TO GND  
Ne g a t ive -Go in g V  
Tra n s ie n t s  
CC  
OUT  
While issuing resets to the µP during power-up, power-  
down, and brownout conditions, these supervisors are  
relatively immune to short-duration, negative-going VCC  
transients (glitches). It is usually undesirable to reset  
the µP when VCC experiences only small glitches.  
60  
40  
Figure 13 shows maximum transient duration vs. reset-  
comparator overdrive, for which reset pulses are not  
generated. The graph was produced using negative-  
going VCC pulses, starting at 5V and ending below the  
reset threshold by the magnitude indicated (reset com-  
parator overdrive). The graph shows the maximum  
pulse width a negative-going VCC transient may typical-  
ly have without causing a reset pulse to be issued. As  
the amplitude of the transient increases (i.e., goes far-  
ther below the reset threshold), the maximum allowable  
pulse width decreases. Typically, a VCC transient that  
goes 100mV below the reset threshold and lasts for  
40µs or less will not cause a reset pulse to be issued.  
20  
0
10  
100  
RESET COMPARATOR OVERDRIVE,  
(Reset Threshold Voltage - V ) (mV)  
1000  
10000  
CC  
Figure 13. Maximum Transient Duration without Causing a  
Reset Pulse vs. Reset Comparator Overdrive  
parator. Select the ratio of R1 and R2 such that PFI sees  
1.25V when V falls to the desired trip point (VTRIP).  
Resistor R3 adIdNs hysteresis. It will typically be an order  
of ma g nitud e g re a te r tha n R1 or R2. The c urre nt  
through R1 and R2 should be at least 1µA to ensure that  
the 25nA (max) PFI input current does not shift the trip  
point. R3 should be larger than 10kto prevent it from  
loading down the PFO pin. Capacitor C1 adds noise  
rejection.  
A 100nF bypass capacitor mounted close to the VCC  
pin provides additional transient immunity.  
Co n n e c t in g a Tim in g Ca p a c it o r a t OS C IN  
When OSC SEL is connected to ground, OSC IN dis-  
connects from its internal 10µA (typ) pull-up and is  
inte rna lly c onne c te d to a ± 100nA c urre nt s ourc e .  
When a capacitor is connected from OSC IN to ground  
(to select alternative reset and watchdog timeout peri-  
ods), the current source charges and discharges the  
timing capacitor to create the oscillator that controls the  
reset and watchdog timeout period. To prevent timing  
errors or oscillator start-up problems, minimize external  
current leakage sources at this pin, and locate the  
capacitor as close to OSC IN as possible. The sum of  
PC-board leakage plus OSC capacitor leakage must be  
small compared to ±100nA.  
Mo n it o rin g a Ne g a t ive Vo lt a g e  
The power-fail comparator can be used to monitor a  
negative supply voltage using Figure 12s circuit. When  
the negative supply is valid, PFO is low. When the neg-  
ative supply voltage drops, PFO goes high. This cir-  
c uit’s a c c ura c y is a ffe c te d b y the PFI thre s hold  
tolerance, the VCC voltage, and resistors R1 and R2.  
1693L/AX80M  
14 ______________________________________________________________________________________  
Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s  
1693L/AX80M  
Ma x im u m V  
Fa ll Tim e  
CC  
The VCC fall time is limited by the propagation delay of  
the b a tte ry s witc hove r c omp a ra tor a nd s hould not  
exceed 0.03V/µs. A standard rule of thumb for filter  
capacitance on most regulators is on the order of 100µF  
per amp of current. When the power supply is shut off or  
the main battery is disconnected, the associated initial  
VCC fall rate is just the inverse or 1A/100µF = 0.01V/µs.  
The VCC fall rate decreases with time as VCC falls expo-  
nentially, which more than satisfies the maximum fall-time  
requirement.  
START  
SET  
WDI  
LOW  
Wa t c h d o g S o ft w a re Co n s id e ra t io n s  
A way to help the watchdog timer keep a closer watch  
on software execution involves setting and resetting the  
wa tc hd og inp ut a t d iffe re nt p oints in the p rog ra m,  
rather than pulsing” the watchdog input high-low-high  
or low-high-low. This technique avoids a stuck” loop  
where the watchdog timer continues to be reset within  
the loop, keeping the watchdog from timing out. Figure  
14 shows an example flow diagram where the I/O dri-  
ving the watchdog input is set high at the beginning of  
the program, set low at the beginning of every subrou-  
tine or loop, then set high again when the program  
returns to the beginning. If the program should “hang”  
in any subroutine, the I/O is continually set low and the  
watchdog timer is allowed to time out, causing a reset  
or interrupt to be issued.  
SUBROUTINE  
OR PROGRAM LOOP  
SET WDI  
HIGH  
RETURN  
END  
Figure 14. Watchdog Flow Diagram  
______________________________________________________________________________________ 15  
Mic ro p ro c e s s o r S u p e rvis o ry Circ u it s  
_Ord e rin g In fo rm a t io n (c o n t in u e d )  
___________________Ch ip To p o g ra p h y  
V
VBATT RESET RESET  
OUT  
PART  
TEMP. RANGE  
0°C to +70°C  
0°C to +70°C  
0°C to +70°C  
0°C to +70°C  
-40°C to +85°C  
-40°C to +85°C  
-40°C to +85°C  
-40°C to +85°C  
-55°C to +125°C  
0°C to +70°C  
0°C to +70°C  
-40°C to +85°C  
-40°C to +85°C  
0°C to +70°C  
0°C to +70°C  
-40°C to +85°C  
-40°C to +85°C  
PIN-PACKAGE  
16 Plastic DIP  
16 Narrow SO  
16 Wide SO  
MAX693ACPE  
MAX693ACSE  
MAX693ACWE  
MAX693AC/D  
MAX693AEPE  
MAX693AESE  
MAX693AEWE  
MAX693AEJE  
MAX693AMJE  
MAX800LCPE  
MAX800LCSE  
MAX800LEPE  
MAX800LESE  
MAX800MCPE  
MAX800MCSE  
MAX800MEPE  
MAX800MESE  
V
Dice*  
CC  
WDO  
16 Plastic SO  
16 Narrow SO  
16 Wide SO  
CE IN  
GND  
CE OUT  
0. 11"  
(2. 794mm)  
16 CERDIP  
16 CERDIP  
16 Plastic DIP  
16 Narrow SO  
16 Plastic DIP  
16 Narrow SO  
16 Plastic DIP  
16 Narrow SO  
16 Plastic DIP  
16 Narrow SO  
BATT ON  
LOW LINE  
WDI  
PFI PFO  
OSC SEL  
OSC IN  
0. 07"  
(1. 778mm)  
* Dice are specified at T = +25°C, DC parameters only.  
A
TRANSISTOR COUNT: 729  
SUBSTRATE CONNECTED TO V  
OUT  
________________________________________________________P a c k a g e In fo rm a t io n  
1693L/AX80M  
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.  
16 ____________________Ma x im In t e g ra t e d P ro d u c t s , 1 2 0 S a n Ga b rie l Drive , S u n n yva le , CA 9 4 0 8 6 4 0 8 -7 3 7 -7 6 0 0  
© 1996 Maxim Integrated Products  
Printed USA  
is a registered trademark of Maxim Integrated Products.  

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