MAX791
Applications Information
The MAX791 is 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, ensure that
these lines 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 60µA, 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
less than 1µA max.
Using SuperCaps or MaxCaps
with the MAX791
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 12).
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 5.3V typ. Leakage
current through the SuperCap charging diode and
MAX791 internal power diode eventually discharges the
SuperCap to V
CC
. Also, if V
CC
and VBATT start from
0.5V above the reset threshold and power is lost at
V
CC
, the SuperCap on VBATT discharges through V
CC
until VBATT reaches the reset threshold; the MAX791
then switches to battery-backup mode and the current
through V
CC
goes to zero (Figure 10).
Using Separate Power Supplies
for VBATT and V
CC
If using separate power supplies for V
CC
and VBATT,
VBATT must be less than 0.3V above V
CC
when V
CC
is
above the reset threshold. As described in the previous
section, if VBATT exceeds this limit and power is lost at
V
CC
, current flows continuously from VBATT to V
CC
via
the VBATT-to-V
OUT
diode and the V
OUT
-to-V
CC
switch
until the circuit is broken (Figure 10).
Alternative Chip-Enable Gating
Using memory devices with CE and CE inputs allows
the MAX791 CE loop to be bypassed. To do this, con-
nect CE IN to ground, pull up CE OUT to V
OUT
, and
connect CE OUT to the CE input of each memory
device (Figure 13). The CE input of each part then con-
nects directly to the chip-select logic, which does not
have to be gated by the MAX791.
Adding Hysteresis to the
Power-Fail Comparator
Hysteresis adds a noise margin to the power-fail com-
parator and prevents repeated triggering of PFO when
VIN is near the power-fail comparator trip point. Figure
14 shows how to add hysteresis to the power-fail com-
parator. Select the ratio of R1 and R2 so that PFI sees
1.25V when VIN falls to the desired trip point (V
TRIP
).
Resistor R3 adds hysteresis. It will typically be an order
of magnitude greater than R1 or R2. The current
through R1 and R2 should be at least 1µA to ensure
that the 25nA (max) PFI input current does not shift the
trip point. R3 should be larger than 10kΩ to prevent it
from loading down the PFO pin. Capacitor C1 adds
additional noise rejection.
Microprocessor Supervisory Circuit
14 ______________________________________________________________________________________
