MAX6365–MAX6368
SOT23, Low-Power µP Supervisory Circuits
with Battery Backup and Chip-Enable Gating
10 ______________________________________________________________________________________
Watchdog Input (MAX6366 Only)
The watchdog monitors µP activity through the watch-
dog input (WDI). If the µP becomes inactive, reset
asserts. To use the watchdog function, connect WDI to
a bus line or µP I/O line. A change of state (high to low,
low to high, or a minimum 100ns pulse) resets the
watchdog timer. If WDI remains high or low for longer
than the watchdog timeout period (t
WD
), the internal
watchdog timer runs out and a reset pulse is triggered
for the reset timeout period (t
RP
). The internal watchdog
timer clears whenever reset asserts or whenever WDI
sees a rising or falling edge. If WDI remains in either a
high or low state, a reset pulse asserts periodically after
every t
WD
(Figure 2).
BATT ON Indicator (MAX6367 Only)
BATT ON is a push-pull output that drives high when in
battery-backup mode. BATT ON typically sinks 3.2mA
at 0.1V saturation voltage. In battery-backup mode, this
terminal sources approximately 10µA from OUT. Use
BATT ON to indicate battery-switchover status or to
supply base drive to an external pass transistor for
higher current applications (Figure 3).
RESET IN Comparator (MAX6368 Only)
RESET IN is compared to an internal 1.235V reference.
If the voltage at RESET IN is less than 1.235V, reset
asserts. Use the RESET IN comparator as an undervolt-
age detector to signal a failing power supply or as a
secondary power-supply reset monitor.
To program the reset threshold (V
RTH
) of the secondary
power supply, use the following (see
Typical Operating
Circuit
):
V
RTH
= V
REF
(R1 / R2 + 1)
where V
REF
= 1.235V. To simplify the resistor selection,
choose a value for R2 and calculate R1:
R1 = R2 [(V
RTH
/ V
REF
) - 1]
Since the input current at RESET IN is 25nA (max),
large values (up to 1MΩ) can be used for R2 with no
significant loss in accuracy. For example, in the
Typical
Operating Circuit
, the MAX6368 monitors two supply
voltages. To monitor the secondary 5V logic or analog
supply with a 4.60V nominal programmed reset thresh-
old, choose R2 = 100kΩ, and calculate R1 = 273kΩ.
Reset Output
A µP’s reset input starts the µP in a known state. The
MAX6365–MAX6368 µP supervisory circuits assert a
reset to prevent code-execution errors during power-
up, power-down, and brownout conditions. RESET is
guaranteed to be a logic low or logic high, depending
on the device chosen (see the
Ordering Information
).
RESET or RESET asserts when V
CC
is below the reset
threshold and for at least 150ms (t
RP
) after V
CC
rises
above the reset threshold. RESET or RESET also
asserts when MR is low (MAX6365) and when RESET IN
is less than 1.235V (MAX6368). The MAX6366 watch-
dog function will cause RESET (or RESET) to assert in
pulses following a watchdog timeout (Figure 2).
Applications Information
Operation Without
a Backup Power Source
The MAX6365–MAX6368 provide battery-backup func-
tions. If a backup power source is not used, connect
BATT to GND and OUT to V
CC
.
Figure 2. MAX6366 Watchdog Timeout Period and Reset Active Time
