MIC2774
Micrel
Functional Diagram
V
DD
R
V
REF
IN
V
DD
I
PU
/MR
GND
* Pinout andpolarity vary by device type.
See ordering information table.
ing info
f
tion
Q
/RST*
One Shot
Delay
Line
S
/Q
RST*
V
REF
MIC2774
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 pin 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 MIC2774 is used to monitor the power supplies
of intelligent circuits such as microcontrollers and micropro-
cessors. By connecting the appropriate reset output of a
MIC2774 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 output is asserted any time /MR is asserted or if V
IN
or V
DD
drops below the corresponding threshold voltage. The
reset output remains asserted for t
RST
(min) after V
IN
and/or
V
DD
subsequently return above the threshold boundaries
and/or /MR is released. A reset pulse is also generated at
power-on. Hysteresis is included in the comparators to prevent
chattering of the output 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 MIC2774’s automatic
supervisory functions. Driving the /MR input to a logic low
causes an immediate and unconditional reset to occur.
Assuming V
IN
and V
DD
are within tolerance when /MR is
released (returns high), the reset output 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. Switch de-bouncing is performed
internally; the switch may be connected directly between
/MR and GND. /MR is internally pulled-up to V
DD
and may
be left open if unused.
October 2005
5
M9999-102605
MICREL [ MICREL SEMICONDUCTOR ]
