Microprocessor Supervisory Circuits
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
+5V
V
IN
+5V
R
1
V
CC
V
OUT
TO STATIC RAM
PFI
RESET
TO
µP
R
2
+
100k
0.1F
GND
TO
µP
R
3
C1*
PFO
V
CC
MAX692A
MAX802M
V
BATT
MAX690
MAX692A
MAX802L
MAX802M
MAX805L
GND
*OPTIONAL
+5V
Figure 5. Using a SuperCap™ as a Backup Power Source with
the MAX692A/MAX802M and a +5V ±10% Supply
PFO
0V
0V
V
L
V
H
V
BATT
is 0.6V or greater than V
CC
.
When V
BATT
connects to V
OUT
, backup mode is activat-
ed and the internal circuitry is powered from the battery
(Table 1). When V
CC
is just below V
BATT
, the current
drawn from V
BATT
is typically 30µA. When V
CC
drops to
more than 1V below V
BATT
, the internal switchover com-
parator shuts off and the supply current falls to less
than 1µA.
V
TRIP
V
IN
VTRIP
=
1.25/
R2
R2
+
R2
R2 || R3
VIH
=
1.25/
R1
+
R2 || R3
VL
−
1.25 5
−
1.25
1.25
+
=
R1
R3
R2
__________Applications Information
Using a SuperCap™ as a Backup
Power Source
SuperCaps are capacitors with extremely high capaci-
tance values, on the order of 0.1F. Figure 4 shows a
SuperCap used as a backup power source. Do not allow
the SuperCap’s voltage to exceed the maximum reset
threshold by more than 0.6V. In Figure 4’s circuit, the
SuperCap rapidly charges to within a diode drop of V
CC
.
However, after a long time, the diode leakage current will
pull the SuperCap voltage up to V
CC
. When using a
SuperCap with the MAX690A/MAX802L/MAX805L, V
CC
may not exceed 4.75V + 0.6V = 5.35V.
Use the SuperCap circuit of Figure 5 with a MAX692A
or MAX802M and a ±10% supply. This circuit
ensures that the SuperCap only charges to V
CC
-
0.5V. At the maximum V
CC
of 5.5V, the SuperCap
charges up to 5.0V, only 0.5V above the maximum
reset threshold—well within the requisite 0.6V.
™SuperCap is a trademark of Baknor Industries.
Figure 6. Adding Hysteresis to the Power-Fail Comparator
Allowable Backup Power-Source
Batteries
Lithium batteries work very well as backup batteries due to
very low self-discharge rates and high energy density.
Single lithium batteries with open-circuit voltages of 3.0V to
3.6V are ideal. Any battery with an open-circuit voltage
less than the minimum reset threshold plus 0.3V can be
Table 2. Allowable Backup-Battery Voltages
(see
Using a SuperCap as a Backup Power Source
section for use with a SuperCap)
MAXIMUM BACKUP-BATTERY
PART NO.
VOLTAGE (V)
MAX690A/
4.80
MAX802L/MAX805L
MAX692A/
4.55
MAX802M
8
_______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
