MIC2775
Micrel, Inc.
Functional Diagram
V
DD
R
V
REF
V
DD
I
PU
/MR
GND
Delay
Line
One Shot
Q
/RST
S /Q
MIC2775
RST
Functional Description
V
DD
Input
The V
DD
pin is both the power supply terminal and a monitored
input voltage. The voltage at this pin is continually compared
against the internal reference. The trip-point at which a reset
occurs is factory programmed. A reset is triggered if and
when V
DD
falls below the trip-point. Hysteresis is employed
to prevent chattering due to noise. The comparator on the
V
DD
input is relatively immune to very brief negative-going
transients.
RST, /RST Reset Outputs
Typically, the MIC2775 is used to monitor the power supply
of intelligent circuits such as microcontrollers and micropro-
cessors. By connecting the appropriate reset output of a
MIC2775 to the reset input of a µC or µP, the processor will
be properly reset at power-on and during power-down and
brown-out conditions. In addition, asserting /MR, the manual
reset input, will activate the reset function.
The reset outputs are asserted any time /MR is asserted or
if V
DD
drops below the threshold voltage. The reset outputs
remain asserted for t
RST
(min) after V
DD
subsequently returns
above the threshold boundary and/or /MR is released. A reset
pulse is also generated at power-on. Hysteresis is included
in the comparator to prevent chattering of the outputs due
to noise.
/MR, Manual Reset Input
The ability to initiate a reset via external logic or a manual
switch is provided in addition to the MIC2775’s automatic su-
pervisory functions. Driving the /MR input to a logic low causes
an immediate and unconditional reset to occur. Assuming
V
DD
is within tolerance when /MR is released (returns high),
the reset outputs will be de-asserted no less than t
RST
later.
/MR may be driven by a logic signal or mechanical switch.
Typically, a momentary push-button switch is connected
such that /MR is shorted to ground when the switch contacts
close. The switch may be connected directly between /MR
and GND. /MR has an internal 100nA pull-up current to V
DD
and may be left open if unused.
November 2005
5
MIC2775
MICREL [ MICREL SEMICONDUCTOR ]
