Datasheet
When V
BATT
connects to V
OUT
, backup mode is
activated 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 comparator shuts off and the supply current
falls to less than 1μA.
Applications Information
Using a SuperCap as a Backup Power Source
SuperCaps are capacitors with extremely high
capacitance 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.
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 volt-
ages 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 connected directly to the V
BATT
input of
the MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
with no additional circuitry (see the Typical Operating
Circuit). However, batteries with open-circuit voltages
that are greater cannot be used for backup, as current is
sourced into the substrate through the diode (D1 in Figure 3)
when V
CC
is close to the reset threshold.
Figure 5. Using a SuperCap as a Backup Power Source with
the MAX692A/MAX802M and a +5V ±10% Supply
Figure 6. Adding Hysteresis to the Power-Fail Comparator
Table 2. Allowable Backup-Battery
Voltages
(see Using a SuperCap as a Backup Power Source
section for use with a SuperCap)
PART NO.
MAXIMUM BACKUP-BATTERY
VOLTAGE (V)
MAX690A/
MAX802L/MAX805L
4.80
MAX692A/
MAX802M
4.55
V
BATT
V
CC
V
OUT
RESET
GND
TO STATIC RAM
TO µP
0.1F
MAX692A
MAX802M
+5V
+
100kΩ
PFO
V
CC
GND
TO µP
MAX690
MAX692A
MAX802L
MAX802M
MAX805L
+5V
R
2
R
1
R
3
PFI
*OPTIONAL
+5V
0V
PFO
V
L
V
H
V
TRIP
V
IN
0V
V
IN
C1*
V
TRIP
= 1.25/
R
2
R
1
+ =
R
2
+ R
2
(
(
V
IH
= 1.25/
R
2
|| R
3
R
1
+ R
2
|| R
3
(
(
V
L
- 1.25
R
3
5 - 1.25
R
2
1.25
MAX690A/MAX692A/
MAX802L/MAX802M/
MAX805L
Microprocessor Supervisory Circuits
www.maximintegrated.com
Maxim Integrated
│
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