05-10-2012, 01:44 PM
Microprocessor Supervisory Circuits
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General Description
The MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
reduce the complexity and number of components
required for power-supply monitoring and battery-control
functions in microprocessor (μP) systems. They significantly
improve system reliability and accuracy compared
to separate ICs or discrete components.
These parts provide four functions:
1) A reset output during power-up, power-down, and
brownout conditions.
2) Battery-backup switching for CMOS RAM, CMOS
μP, or other low-power logic.
3) A reset pulse if the optional watchdog timer has not
been toggled within 1.6sec.
4) A 1.25V threshold detector for power-fail warning or
low-battery detection, or to monitor a power supply
other than +5V.
The parts differ in their reset-voltage threshold levels
and reset outputs. The MAX690A/MAX802L/MAX805L
generate a reset pulse when the supply voltage drops
below 4.65V, and the MAX692A/MAX802M generate a
reset below 4.40V. The MAX802L/MAX802M guarantee
power-fail accuracies to ±2%. The MAX805L is the
same as the MAX690A except that RESET is provided
instead of RESET.
Backup-Battery Switchover
In the event of a brownout or power failure, it may be
necessary to preserve the contents of RAM. With a
backup battery installed at VBATT, the devices automatically
switch RAM to backup power when VCC fails.
As long as VCC exceeds the reset threshold, VOUT connects
to VCC through a 5½ PMOS power switch. Once VCC falls
below the reset threshold, VCC or VBATT (whichever is higher)
switches to VOUT. Unlike the MAX690/MAX692, the
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L don’t
always connect VBATT to VOUT when VBATT is greater than
VCC. VBATT connects to VOUT (through an 80½ switch) only
when VCC is below the reset threshold and VBATT is greater
than VCC.