MAX690A(1993) 查看數據表（PDF） - Maxim Integrated
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MAX690A Datasheet PDF : 12 Pages
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Microprocessor Supervisory Circuits
+5V
VIN
+5V
VCC
VOUT
MAX692A
MAX802M
VBATT
RESET
+
100k 0.1F
GND
TO STATIC RAM
TO µP
R1
VCC
PFI
MAX690
MAX692A
MAX802L
R2
R3
C1*
MAX802M
MAX805L
PFO
GND
Figure 5. Using a SuperCap™ as a Backup Power Source with
the MAX692A/MAX802M and a +5V ±10% Supply
VBATT is 0.6V or greater than VCC.
When VBATT connects to VOUT, backup mode is activat-
ed and the internal circuitry is powered from the battery
(Table 1). When VCC is just below VBATT, the current
drawn from VBATT is typically 30µA. When VCC drops to
more than 1V below VBATT, the internal switchover com-
parator 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 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 VCC.
However, after a long time, the diode leakage current will
pull the SuperCap voltage up to VCC. When using a
SuperCap with the MAX690A/MAX802L/MAX805L, VCC
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 VCC -
0.5V. At the maximum VCC 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.
+5V
PFO
0V
0V
TO µP
VTRIP
=
1.25/
R
R2
2 +R
2
VIH
=
1.25/
R2 || R3
R1 + R2 || R3
VL − 1.25
R1
+
5
− 1.25
R3
1.25
= R2
*OPTIONAL
VL VTRIP VH
VIN
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)
PART NO.
MAXIMUM BACKUP-BATTERY
VOLTAGE (V)
MAX690A/
MAX802L/MAX805L
4.80
MAX692A/
MAX802M
4.55
8 _______________________________________________________________________________________
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