MIC2775_09 Datasheet, PDF (6/7 Pages) Micrel Semiconductor – Micro-Power Voltage Supervisor

MIC2775
Application Information
Ensuring Proper Operation at Low Supply
At levels of V_DD below 1.2V, the MIC2775’s /RST output can-
not turn on sufficiently to produce a valid logic-low on /RST.
In this situation, circuits driven by /RST could be allowed to
float, causing undesired operation. (In most cases, however,
it is expected that the circuits driven by the MIC2775 will be
similarly inoperative at V_DD ≤ 1.2V.)
If a given application requires that /RST be valid below V_DD
= 1.2V, this can be accomplished by adding a pull-down re-
sistor to the /RST output. A value of 100kΩ is recommended
as this is usually an acceptable compromise of quiescent
current and pull-down current. The resistor’s value is not
critical, however. See Figure 1.
V_CC
MICROPROCESSOR
VCC
MIC2775-XX
VDD /RST
RST
/RESET GND
100k
/MR GND
Manual
Reset
Micrel, Inc.
Transient response
The MIC2775 is inherently immune to very short negative-
going “glitches.” Very brief transients may exceed the voltage
threshold without tripping the output.
As shown in Figure 3, in general the narrower the transient,
the deeper the threshold overdrive that will be ignored by
the MIC2775. The graph represents the typical allowable
transient duration for a given amount of threshold overdrive
that will not generate a reset.
Typical V_DD
100 ^{Transient Response}
80
60
40
20
00
500 1000 1500 2000
RESET COMP. OVERDRIVE, V_REF–V_DD(mV)
Figure 3. Typical V_DD Transient Response
Figure 1.  MIC2775 Valid /Reset Below 1.2V
The statements above also apply to the MIC2775’s RST
output. That is, to ensure valid RST signal levels at V_DD <
1.2V, a pull-up resistor (as opposed to a pull-down) should
be added to the RST output. A value of 100kΩ is typical for
this application as well. See Figure 2.
V_CC
Manual
Reset
MIC2775-XX
VDD /RST
MICROPROCESSOR
VCC
100k
/RESET GND
RST
/MR GND
Figure 2.  MIC2775 Valid Reset Below 1.2V
MIC2775
6
June 2009