MIC2777_05 Datasheet, PDF (5/8 Pages) Micrel Semiconductor – Dual Micro-Power Low Voltage Supervisor

MIC2777
Functional Diagram
V_DD
V_REF
IN
V_REF
GND
Micrel, Inc.
RQ
Delay
Line
One Shot
S /Q
/RST
RST
MIC2777
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