Datasheet

load for currents less than 250mA. The parallel diode
carries any current in excess of 250mA. Both the switch
and the diode have impedances less than 1Ω each. The
maximum continuous current is 250mA, but power-on
transients may reach a maximum of 1A.
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 opencircuit
outputs will, for example, assume the V
OUT
voltage in their
high states rather than the V
CC
voltage. At the maximum
source current of 250mA, V
OUT
will typically 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 decoupling
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 typically
±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 thresh-
Figure 8. V
CC
and VBATT to VOUT Switch Figure 9. SuperCap or MaxCap on VBATT
MAX691A
MAX693A
MAX800L
MAX800M
VBATT
V
CC
0.1µF
V
OUT
MAX691A
MAX693A
MAX800L
MAX800M
1
0.47F*
1N4148
+5V
2
3
V
CC
GND
VBATT
4
V
OUT
* MaxCap
MAX691A/MAX693A/
MAX800L/MAX800M
Microprocessor Supervisory Circuits
www.maximintegrated.com
Maxim Integrated │ 12