MIC2778-2BM5 MICREL [Micrel Semiconductor], MIC2778-2BM5 Datasheet - Page 5

• Manufacturer Part Number: MIC2778-2BM5
• Description: Voltage Monitor with Adjustable Hysteresis
• Manufacturer: MICREL [Micrel Semiconductor]

Applications Information
Output
Since the MIC2778 output is an open-drain MOSFET, most
applications will require a pull-up resistor. The value of the
resistor should not be too large or leakage effects may domi-
nate. 470kΩ is the maximum recommended value.
Programming the Thresholds
The low-voltage threshold is calculated using:
The high-voltage threshold is calculated using:
where, for both equations:
In order to provide the additional criteria needed to solve
for the resistor values, the resistors can be selected such
that they have a given total value, that is, R1 + R2 + R3 =
R
value because it draws minimum battery current but has no
significant effect on accuracy.
When working with large resistors, a small amount of leak-
age current can cause voltage offsets that degrade system
accuracy. The maximum recommended total resistance from
V
Once the desired trip points are determined, set the V
threshold first.
For example, use a total of 1MΩ = R1 + R2 + R3. For a typical
single-cell lithium ion battery, 3.6V is a good “high threshold”
because at 3.6V the battery is moderately charged. Solving
for R3:
Once R3 is determined, the equation for V
used to determine R2. A single lithium-ion cell should not be
discharged below 2.5V. Many applications limit the drain to
3.1V. Using 3.1V for the V
January 2005
MIC2778
BAT
TOTAL
to ground is 3MΩ.
V
V
V
V
R3
BAT(lo)
BAT(hi)
REF
BAT(hi)
. A value such as 1MΩ for R
=
344k
=
1.240V
=
=
=
R1
R2
R3
1.24
V
V
Figure 1. Example Circuit
V
Ω
REF
REF
BATT
604k
1%
56k
1%
340k
1%









1M
R1 R2
R1 R2
R3
R2
Ω
+
+
VDD
LTH
HTH
BAT(lo)



MIC2778
R3
+
R3
+
+
GND
R S T
R3
R3
threshold allows calculation






TOTAL
470k
is a reasonable
BAT(lo)
can be
BAT(hi)
5
of the two remaining resistor values.
The accuracy of the resistors can be chosen based upon the
accuracy required by the system.
Input Transients
The MIC2778 is inherently immune to very short negative-
going “glitches.” Very brief transients may exceed the V
threshold without tripping the output.
As shown in Figure 2, the narrower the transient, the deeper
the threshold overdrive that will be ignored by the MIC2778.
The graph represents the typical allowable transient dura-
tion for a given amount of threshold overdrive that will not
generate a reset.
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 MIC2778’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 3.
Figure 3. Interfacing to Bidirectional Reset Pin
V
R2
R1 1M
R1
BAT(lo)
R1
R2
R3
=
=
=
V
Figure 2. Input Transient Response
BATT
600k
56k
604k
1%
56k
1%
340k
1%
Ω
=
Ω
140
120
100
Ω
3.1V
80
60
40
20
RESET COMP. OVERDRIVE, V
−
0
R2
1
VDD
LTH
HTH
MIC2778
−
=
R3
1.24
Input T rans ient
GND
R S T
10
R es pons e



R2
1M
100k
+
100
344k
Ω
REF
–V



Microprocessor
LTH
Supply In
Reset In
GND
1000
(mV)
M9999-011405
BAT(lo)
Micrel