Datasheet

Chip-Enable Output
In enabled mode, CE OUT’s impedance is equivalent to
75Ω in series with the source driving CE IN. In disabled
mode, the 75Ω transmission gate is off and CE OUT is
actively pulled to the higher of V
CC
or V
BATT
. The
source turns off when the transmission gate is enabled.
__________Applications Information
The MAX801/MAX808 are not short-circuit protected.
Shorting OUT to ground, other than power-up transients
such as charging a decoupling capacitor, may destroy
the device. If long leads connect to the IC’s inputs,
ensure that these lines are free from ringing and other
conditions that would forward bias the IC’s protection
diodes. Bypass OUT, V
CC
, and BATT with 0.1µF
capacitors to GND.
The MAX801/MAX808 operate in two distinct modes:
1) Normal Operating Mode, with all circuitry powered
up. Typical supply current from V
CC
is 68µA (48µA
for the MAX808), while only leakage currents flow
from the battery.
2) Battery-Backup Mode, where V
CC
is below V
BATT
and V
RST
. The supply current from the battery is typ-
ically less than 1µA.
Using SuperCaps™ or MaxCaps™
with the MAX801/MAX808
BATT has the same operating voltage range as V
CC
, and
the battery-switchover threshold voltage is typically
V
BATT
when V
CC
is decreasing or V
BATT
+ 0.05V when
V
CC
is increasing. This hysteresis allows use of a
SuperCap (e.g., around 0.47F) and a simple charging
circuit as a backup source (Figure 8). Since V
BATT
can
exceed V
CC
while V
CC
is above the reset threshold, no
special precautions are needed when using these µP
supervisors with a SuperCap.
Backup-Battery Replacement
The backup battery can be disconnected while V
CC
is
above the reset threshold, provided BATT is bypassed
with a 0.1µF capacitor to ground. No precautions are
necessary to avoid spurious reset pulses.
Negative-Going V
CC
Transients
While issuing resets to the µP during power-up, power-
down, and brownout conditions, these supervisors are
relatively immune to short-duration, negative-going V
CC
transients (glitches). It is usually undesirable to reset
the µP when V
CC
experiences only small glitches.
The Typical Operating Characteristics show a graph of
Maximum Transient Duration vs. Reset Threshold
Overdrive, for which reset pulses are not generated.
The graph was produced using negative-going V
CC
pulses, starting at 5V and ending below the reset
threshold by the magnitude indicated (reset compara-
tor overdrive). The graph shows the maximum pulse
width that a negative-going V
CC
transient may typically
have without causing a reset pulse to be issued. As the
amplitude of the transient increases (i.e., goes farther
below the reset threshold), the maximum allowable
pulse width decreases. Typically, a V
CC
transient that
goes 40mV below the reset threshold and lasts for 3µs
or less will not cause a reset pulse to be issued. A
0.1µF bypass capacitor mounted close to the V
CC
pin
provides additional transient immunity.
MAX801L/M/N, MAX808L/M/N
8-Pin µP Supervisory Circuits
with ±1.5% Reset Accuracy
______________________________________________________________________________________ 11
MAX808
CE IN
50pF C
LOAD
CE OUT
GND
V
RST
(max)
50Ω DRIVER
V
CC
Figure 7. MAX808 CE Gate Test Circuit
MAX801
MAX808
0.47F
1N4148
+5V
V
CC
GND
BATT OUT
Figure 8. Using the MAX801/MAX808 with a SuperCap