MIC2753/2754

Micrel

MIC2753/2754

8

June 1999

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

voltages, V

for 140ms minimum after V

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

within the tolerance thresholds when /MR is released (inter-

nally pulled to logic high), the reset output will be deasserted

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

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

Switch

Voltage Monitor

and

Reset Generator

Microprocessor

System

Circuitry

Reset(s)

Crowbar

Circuit

HV