MIC2777
Micrel, Inc.
Functional Diagram
V
DD
R
V
REF
IN
Delay
Line
One Shot
Q
/RST
S /Q
RST
V
REF
MIC2777
GND
Functional Description
IN, Under-Voltage Detector Input
The voltage present at the IN pin is compared to the internal
300mV reference voltage. A reset is triggered if and when V
IN
falls below V
REF
. Typically, a resistor divider is used to scale
the input voltage to be monitored such that V
IN
will fall below
V
REF
as the voltage being monitored falls below the desired
trip-point. Hysteresis is employed to prevent chattering due
to noise. The comparator on the IN input is relatively immune
to very brief negative-going transients.
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 Output
Typically, the MIC2777 is used to monitor the power supplies
of intelligent circuits such as microcontrollers and micropro-
cessors. By connecting the appropriate reset output of a
MIC2777 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.
The reset outputs are asserted any time V
DD
or V
IN
drops
below the corresponding threshold voltage. The reset outputs
remain asserted for t
RST
(min) after V
IN
and/or V
DD
subsequent
return above the threshold boundaries and/or /MR is released.
A reset pulse is also generated at power-on.
Manual Reset
The ability to initiate a reset via external logic or a manual
switch is provided in addition to the MIC2777’s automatic
supervisory functions. Typically, a momentary push-button
switch is connected such that IN is shorted to ground when
the switch contacts close. Assuming V
DD
and V
IN
are within
tolerance when the switch is released, the reset outputs will
be de-asserted no less than 140ms later. IN can also be driven
by an open-drain or open-collector logic signal.
November 2005
5
MIC2777
MICREL [ MICREL SEMICONDUCTOR ]
