MIC2776 Datasheet, PDF (7/8 Pages) Micrel Semiconductor – Micro-Power Low Voltage Supervisor

MIC2776
V_CORE
1.0V ±5%
V_I/O
2.5V ±5%
R1
676k
1%
R2
324k
1%
MIC2776
VDD /RST
IN
/MR GND
Manual
Reset
MICROPROCESSOR
VCORE
VI/O
/RESET GND
Figure 1. MIC2776 Example Design
Interfacing to Processors With Bidirectional Reset Pins
Some microprocessors have reset signal pins that are bidi-
rectional, rather than input only. The Motorola 68HC11 family
is one example. Because the MIC2776N’s output is open-
drain, it can be connected directly to the processor’s reset pin
using only the pull-up resistor normally required. See Figure 2.
V_CC
MICROPROCESSOR
VCC
MIC2776N
VDD /RST
R1
IN
R2 _{/MR GND}
100k
/RESET GND
Micrel
Ensuring Proper Operation at Low Supply
At levels of V_DD below 1.2V, the MIC2776L’s /RST output
driver cannot turn on sufficiently to produce a valid logic-low
on the /RST output. In this situation, other 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 MIC2776L 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
leakage current and pull-down current. The resistor’s value is
not critical, however. See Figure 4.
The statements above also apply to the MIC2776H’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 5.
V_CC
R1
R2
Manual
Reset
MICROPROCESSOR
VCC
MIC2776L
VDD /RST
IN
/MR GND
/RESET GND
100k
Rpull-down
Figure 2. Interfacing to Bidirectional Reset Pin
Transient Response
The MIC2776 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, the narrower the transient, the deeper
the threshold overdrive that will be ignored by the MIC2776.
The graph represents the typical allowable transient duration
for a given amount of threshold overdrive that will not gener-
ate a reset.
Typical INPUT
Transient Response
40
35
30
25
20
15
10
5
0
0
100
200
300
RESET COMP. OVERDRIVE, V_REF–V_IN (mV)
Figure 3. Typical INPUT Transient Response
Figure 4. MIC2776L Valid /Reset Below 1.2V
V_CC
R1
R2
Manual
Reset
MIC2776H
VDD RST
IN
MICROPROCESSOR
VCC
100k
Rpull-up
RESET GND
/MR GND
Figure 5. MIC2776H Valid Reset Below 1.2V
September 29, 2000
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MIC2776