SOT23, Low-Power µP Supervisory Circuits
with Battery Backup
Backup-Battery Switchover
Applications Information
In a brownout or power failure, it may be necessary to
preserve the contents of the RAM. With a backup bat-
tery installed at BATT, the MAX6361/MAX6363/
MAX6364 automatically switch the RAM to backup
Operation Without a Backup
Power Source
The MAX6361/MAX6363/MAX6364 were designed for
battery-backed applications. If a backup battery is not
power when V
falls. The MAX6363 has a BATT ON
CC
used, connect V
to OUT and connect BATT to GND.
output that goes high when in battery-backup mode.
These devices require two conditions before switching
to battery-backup mode:
CC
Replacing the Backup Battery
If BATT is decoupled with a 0.1µF capacitor to ground,
the backup power source can be removed while V
1) V
2) V
must be below the reset threshold.
CC
CC
remains valid without danger of triggering a reset pulse.
The device does not enter battery-backup mode when
must be below V
. Table 2 lists the status
CC
BATT
of the inputs and outputs in battery-backup mode.
The device will not power up if the only voltage
source is on BATT. OUT will only power up from
V
stays above the reset threshold voltage.
CC
Negative-Going V
CC
Transients
V
CC
at startup.
These supervisors are relatively immune to short-dura-
tion, negative-going V transients. Resetting the µP
Manual Reset Input (MAX6361 only)
CC
when V
desirable.
experiences only small glitches is usually not
Many µP-based products require manual reset capabili-
ty, allowing the operator, a test technician, or external
logic circuitry to initiate a reset. For the MAX6361, a logic
low on MR asserts reset. Reset remains asserted while
CC
The Typical Operating Characteristics section shows a
graph of Maximum Transient Duration vs. Reset
Threshold Overdrive for which reset is not asserted.
MR is low, and for a minimum of 150ms (t ) after it
RP
returns high. MR has an internal 20kΩ pull-up resistor.
This input can be driven with TTL/CMOS logic levels or
with open-drain/collector outputs. Connect a normally
open momentary switch from MR to GND to create a
manual reset function; external debounce circuitry is not
required. If MR is driven from long cables or the device
is used in a noisy environment, connect a 0.1µF capaci-
tor from MR to GND to provide additional noise immunity.
The graph was produced using negative-going V
CC
pulses, starting at V
and ending below the reset
CC
threshold by the magnitude indicated (reset threshold
overdrive). The graph shows the maximum pulse width
Table 1. Reset Threshold Ranges
RESET THRESHOLD RANGES (V)
SUFFIX
MIN
4.50
4.25
3.00
2.85
2.55
2.25
TYP
4.63
4.38
3.08
2.93
2.63
2.32
MAX
4.75
4.50
3.15
3.00
2.70
2.38
Reset In (MAX6364 only)
RESET IN is compared to an internal 1.235V reference.
If the voltage at RESET IN is less than 1.235V, reset is
asserted. The RESET IN comparator may be used as
an undervoltage detector to signal a failing power sup-
ply. It can also be used as a secondary power-supply
reset monitor.
46
44
31
29
26
23
Reset Output
A µP’s reset input starts the µP in a known state. The
MAX6361/MAX6363/MAX6364 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 high depend-
ing on the device chosen (see Ordering Information).
Table 2. Input and Output Status in
Battery-Backup Mode
PIN
STATUS
Disconnected from OUT
Connected to BATT
V
CC
OUT
RESET or RESET asserts when V
is below the reset
CC
Connected to OUT. Current drawn from
threshold and for at least 150ms (t ) after V
rises
CC
RP
BATT
the battery is less than 1µA (at V
=
BATT
= 0.
above the reset threshold. RESET or RESET also
asserts when MR is low (MAX6361) and when RESET
IN is less than 1.235V (MAX6364).
2.8V, excluding I
) when V
OUT
CC
Asserted
RESET/RESET
BATT ON
High state
Inputs ignored
MR, RESET IN
6
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