MIC803 Datasheet, PDF (8/10 Pages) Micrel Semiconductor – 3-Pin Microprocessor Supervisor Circuit with Open-Drain Reset Output

Micrel, Inc.
Application Information
Microprocessor Reset
The /RESET pin is asserted whenever V_CC falls below
the Reset Threshold Voltage, V_TH. The /RESET pin
remains asserted for the duration of the Reset Timeout
Period (t_RESET) after V_CC has risen above the Reset
Threshold Voltage. The reset function ensures the
microprocessor is properly reset and powers up in a
known condition after a power failure. /RESET will
remain valid with V_CC as low as 1.0V.
The /RESET output is a simple open-drain N-channel
MOSFET structure. A pull-up resistor must be used to
pull this output up to some voltage. For most
applications, this voltage will be the same power supply
that supplies V_CC to the MIC803. As shown in Figure 1, it
is possible, however, to tie this resistor to some other
voltage. This will allow the MIC803 to monitor one
voltage while level-shifting the /RESET output to some
other voltage. The pull-up voltage must be limited to
5.5V. The resistor must be small enough to supply
current to the inputs and leakage paths that are driven
by the /RESET output.
MIC803
Figure 2. /RESET at falling V_CC
Wire OR’ing The /RESET Output
Since the /RESET output is open-drain, several reset
sources can be wire-ORed, in parallel, to allow resets
from multiple sources.
V_CC Transients
The MIC803 is relatively immune to negative-going V_CC
glitches below the Reset Threshold. See the Typical
Characteristic Curve, Maximum Transient Duration vs
Overdrive on page 6 of the datasheet. As shown in
Figure 3, the Overdrive Voltage is the difference
between the Threshold Voltage and the minimum point
of the V_CC glitch. Typically, an overdrive of 100mV, with
duration of 15μs or less will not cause a reset. If
additional transient immunity is needed, a 0.1μF bypass
capacitor can be placed as close as possible to the
MIC803 on the V_CC pin.
Figure 1. MIC803 used in a Multiple Supply System
/RESET Valid at Low Voltage
As V_CC drops to 0V, the MIC803 will no longer be able to
pull the /RESET output low, and the pull-up resistor will
pull the output high. The value of the pull-up resistor and
the voltage it is connected to will affect the point at which
this happens.
Figure 3. V_CC Transient
December 2010
8
M9999-122310-A