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 10Ω.
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 chip’s 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 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
