MIC2775 Datasheet, PDF (6/7 Pages) Micrel Semiconductor – Micro-Power Voltage Supervisor Advance Information

MIC2775
Application Information
Ensuring Proper Operation at Low Supply
At levels of V_DD below 1.2V, the MIC2775’s /RST output
cannot 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
resistor to the /RST output. A value of 100kΩ is recom-
mended 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
100k
/RESET GND
RST
/MR GND
Manual
Reset
Micrel
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 tran-
sient 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
MICROPROCESSOR
VCC
Manual
Reset
MIC2775-XX
VDD /RST
RST
/MR GND
/RESET GND
100k
Rpull-down
Figure 2. MIC2775 Valid Reset Below 1.2V
MIC2775
6
September 29, 2000