MAX691A/MAX693A/MAX800L/MAX800M
Battery-Backup Input
The Battery-Backup Input (VBATT) is similar to the V
CC
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).
Output Supply Voltage
The Output Supply Voltage (V
OUT
) 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 V
OUT
volt-
age in their high states rather than the V
CC
voltage. At
the maximum source current of 250mA, V
OUT
will typi-
cally be 200mV below V
CC
. Decouple this terminal with
a 0.1µF capacitor.
__________Applications Information
The MAX691A/MAX693A/MAX800L/MAX800M are not
short-circuit protected. Shorting V
OUT
to ground, other
than power-up transients such as charging a decou-
pling capacitor, destroys the device.
All open-circuit outputs swing between V
OUT
and GND
rather than V
CC
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:
1) Normal operating mode with all circuitry powered
up. Typical supply current from V
CC
is 35µA while
only leakage currents flow from the battery.
2) Battery-backup mode where V
CC
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) Battery-backup mode where V
CC
is less than
VBATT by at least 0.7V. VBATT supply current is
1µA max.
Using SuperCap or MaxCap with the
MAX691A/MAX693A/MAX800L/MAX800M
VBATT has the same operating voltage range as V
CC
,
and the battery switchover threshold voltages are typi-
cally ±30mV centered at VBATT, allowing use of a
SuperCap and a simple charging circuit as a backup
source (Figure 9).
If V
CC
is above the reset threshold and VBATT is 0.5V
above V
CC
, current flows to V
OUT
and V
CC
from VBATT
until the voltage at VBATT is less than 0.5V above V
CC
.
For example, with a SuperCap connected to VBATT and
through a diode to V
CC
, if V
CC
quickly changes from 5.4V
to 4.9V, the capacitor discharges through V
OUT
and V
CC
until VBATT reaches 5.1V typ. Leakage current through
the SuperCap charging diode and the internal power
diode eventually discharges the SuperCap to V
CC
. Also, if
V
CC
and VBATT start from 0.1V above the reset threshold
and power is lost at V
CC
, the SuperCap on VBATT dis-
charges through V
CC
until VBATT reaches the reset
threshold; then the battery-backup mode is initiated and
the current through V
CC
goes to zero.
Microprocessor Supervisory Circuits
12 ______________________________________________________________________________________
