MAX6391/MAX6392
Dual-Voltage µP Supervisory Circuits
with Sequenced Reset Outputs
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Applications Information
Selecting the Reset Timeout Capacitor
The RESET2 delay may be adjusted by the user with an
external capacitor connected from the CSRT pin to
ground. The MAX6391 includes a 600nA current source
that is switched to C
CSRT
to create a voltage ramp. The
voltage ramp is compared to the internal 1.25V refer-
ence to set the RESET2 delay period. The period is cal-
culated by:
∆t = C
✕
∆V/I
where ∆V = 1.25V, I = 600nA, and C is the external
capacitor.
Simplifying,
t
RP
= 2.08
✕
10
6
s / F
✕
C
CSRT
For C
CSRT
= 1500pF, t
RP
= 3.1ms
A fixed internal 140ms (min) reset delay time for
RESET2 may be chosen by connecting the CSRT pin to
V
CC
. The V
CC
to CSRT connection disables the voltage
ramp and enables a separate fixed delay counter
chain. The MAX6391 internally determines the CSRT
connection and provides the proper timing setup.
In all cases, RESET IN2 acts as the slave supply. V
CC
can assert the RESET2 output but RESET IN2 will have
no effect on the RESET1 output.
Monitoring Voltages Other Than V
CC
An external resistive-divider network is required at
RESET IN2 for most applications. The divider resistors,
R3 and R4, may be calculated by the following formula:
V
RST
= V
TH2
✕
(R3 + R4)/R4
where V
TH2
= 625mV (internal reference voltage) and
V
RST
is the desired reset threshold voltage. R4 may be
set to a conveniently high value (500kΩ for example, to
minimize current consumption) and the equation may
be solved for R3 by:
R3 = R4
✕
(V
RST
/V
TH2
- 1)
For single-supply operations requiring two reset out-
puts (RESET1 before RESET2), connect RESET IN2
directly to V
CC
and adjust RESET2 timeout delay with
C
CRST
as desired.
Pullup Resistors
The MAX6391 includes open-drain outputs for both
RESET1 and RESET2. Two internal resistors, R1 and
R2, of 47kΩ each are provided with internal connec-
tions to RESET1 and RESET2. These resistors may be
connected to the appropriate external voltage for inde-
pendent V
OH
drive with no additional component
requirements.
The MAX6392 includes a manual reset option, MR, that
replaces the R1 pullup resistor. The active-low manual
reset input forces both RESET1 and RESET2 low.
RESET2 is driven active before RESET1 in all cases
(10µs typ). The resets follow standard reset timing
specifications after the manual reset is released. The
manual reset is internally pulled up to V
CC
through a
47kΩ resistor.
Negative-Going V
CC
Transients
In addition to issuing a reset to the µP during power-up,
power-down, and brownout conditions, these devices
are relatively immune to short-duration, negative-going
V
CC
or RESET IN2 transients (glitches). The Typical
Operating Characteristics show the Maximum Transient
Duration vs. Reset Comparator Overdrive graph. The
graph shows the maximum pulse width that a negative-
going V
CC
transient may typically have without issuing
a reset signal. As the amplitude of the transient increas-
es, the maximum allowable pulse width decreases.
