MAX6342–MAX6345
of the reset output. The inverting input is internally con-
nected to a +1.25V reference.
To build an early-warning circuit for power failure, con-
nect the PFI pin to a voltage divider (see
Typical Oper-
ating Circuit
). Choose the voltage-divider ratio so that
the voltage at PFI falls below +1.25V just before the
+5V regulator drops out. Use PFO to interrupt the µP to
prepare for an orderly shutdown.
__________Applications Information
Negative-Going V
CC
Transients
The MAX6342–MAX6345 supervisors are immune to
short-duration, negative-going VCC transients (glitches)
that usually do not require the entire system to shut down.
Figure 4 shows typical transient duration vs. reset com-
parator overdrive, for which the MAX6342–MAX6345 do
not generate a reset pulse. The graph was generated
using a negative-going pulse applied to VCC, starting
0.5V above the actual reset threshold and ending below
it by the magnitude indicated (reset comparator over-
drive). The graph indicates the maximum pulse width a
negative-going VCC transient can have without causing
a reset pulse. As the magnitude of the transient increas-
es (goes further below the reset threshold), the maxi-
mum allowable pulse width decreases.
Typically, a VCC transient that goes 100mV below the
reset threshold and lasts 12µs or less will not cause a
reset pulse. A 1µF bypass capacitor mounted as close
as possible to the VCC pin provides additional transient
immunity.
Ensuring a Valid Reset Output
Down to V
CC
= 0
The MAX6342–MAX6345 are guaranteed to operate
properly down to VCC = +1V. In applications that require
valid reset levels down to VCC = 0, a pull-down resistor
to active-low outputs (MAX6342/MAX6345) and a pull-up
resistor to active-high outputs (MAX6344/MAX6345)
ensure that the reset line is valid when the reset output is
no longer sinking or sourcing current (Figures 5 and 6).
Note that this method does not work with the open-drain
output of the MAX6343. The resistor value used is not
critical, but it must be large enough not to load the reset
6-Pin µP Reset Circuit with
Power-Fail Comparator
6 _______________________________________________________________________________________