MIC2753 Datasheet, PDF (8/12 Pages) Micrel Semiconductor – Power Supply Supervisors Preliminary Information

MIC2753/2754
Functional Description
The MIC275x family of devices are typically used to monitor
power supplies of intelligent circuits such as microcontrollers
and microprocessors. By connecting the appropriate reset
output of an MIC275x to the reset input of a microcontroller or
microprocessor, the processor will be properly reset at power-
on, and during power-down and brownout conditions. In
addition, asserting /MR (manual reset input) activates the
reset function.
Reset Outputs
The /RST and/or RST output is asserted any time /MR is
asserted or if V_IN deviates beyond the preset threshold
voltages, V_THU and V_THO. Reset output(s) remain asserted
for 140ms minimum after V_IN returns within the threshold
boundaries and/or /MR is released. A minimum 140ms reset
pulse is also generated at power-on. Hysteresis is included in
the overvoltage and undervoltage comparators to prevent
chattering of the outputs due to noise.
The MIC275x family offers a choice of two reset output
options: the MIC2753 has an active-high RST output, and the
MIC2754 has an active-low /RST output.
Manual Reset Input
The ability to initiate a reset from external logic or a manual
switch is provided in addition to the MIC275x’s automatic
supervisory functions. Driving the /MR input low causes an
immediate and unconditional reset to occur. Assuming V_IN is
within the tolerance thresholds when /MR is released (inter-
nally pulled to logic high), the reset output will be deasserted
Micrel
no less than 140ms later. /MR may be driven by a CMOS or
TTL logic signal or a mechanical switch. Typically, a momen-
tary push-button switch is connected such that /MR is shorted
to ground when the contacts close. Switch debouncing is
performed internally; the switch may be connected directly
between /MR and GND. /MR has an internal pull-up to V_IN
and may be left open if unused.
Overvoltage Output
The /HV and reset outputs are immediately and uncondition-
ally asserted any time V_IN exceeds V_THO (overvoltage thresh-
old). /HV is an active-low, open-drain logic output. Overvolt-
age on a system power supply rail is generally considered to
be a catastrophic condition which may cause permanent
system damage. If and when an overvoltage condition occurs
using a MIC275x, the system is immediately driven into reset
mode and /HV (overvoltage flag), is driven active.
Typically, an overvoltage signal, such as /HV, is connected to
a “crowbar circuit” using a latching power device such as an
SCR. See “Typical Application”. The crowbar circuit will
clamp the power rail to some low voltage by shunting power
supply current to ground. This should cause some upstream
power supply protection device to be activated, such as fold-
back current limiting, a fuse, or circuit breaker. In this way, the
system is most likely to be protected against damage and
spurious operation until the condition can be remedied. /HV
is an open-drain output and may be wire-ORed with other
open-drain logic signals. Some systems may require a pull-
up resistor.
System Block Diagram
Reset
HV
Switch
Crowbar
Circuit
Voltage Monitor
and
Reset Generator
Microprocessor
Reset(s)
System
Circuitry
MIC2753/2754
8
June 1999