MIC2778-1BM5 MICREL [Micrel Semiconductor], MIC2778-1BM5 Datasheet - Page 5

• Manufacturer Part Number: MIC2778-1BM5
• Description: Voltage Monitor with Adjustable Hysteresis Preliminary Information
• 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
dominate. 470k 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
A value such as 1M
because it draws minimum battery current but has no signifi-
cant effect on accuracy.
When working with large resistors, a small amount of leakage
current can cause voltage offsets that degrade system accu-
racy. 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 1M = 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
to determine R2. A single lithium-ion cell should not be
discharged below 2.5V. Many applications limit the drain to
October 1999
MIC2778
BAT
to ground is 3M .
R3 344k
V
V
V
V
BAT(lo)
BAT(hi)
REF
BAT(hi)
1.240V
R1
R2
R3
Figure 1. Example Circuit
1.24
V
V
V
BATT
REF
REF
604k
1%
56k
1%
340k
1%
1M
R1 R2 R3
R1 R2 R3
R3
for R
R2 R3
VDD
LTH
HTH
MIC2778
R3
TOTAL
GND
RST
is a reasonable value
470k
BAT(lo)
can be used
TOTAL
BAT(hi)
.
5
3.1V. Using 3.1V for the V
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 duration
for a given amount of threshold overdrive that will not gener-
ate 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 4.
Figure 4. Interfacing to Bidirectional Reset Pin
R2 56k
R1 1M
R1 600k
V
BAT(lo)
R1
R2
R3
V
Figure 2. Input Transient Response
BATT
604k
1%
56k
1%
340k
1%
RESET COMP. OVERDRIVE, V
140
120
100
80
60
40
20
3.1V 1.24
0
1
R2 R3
VDD
LTH
HTH
MIC2778
Input Transient
GND
RST
10
BAT(lo)
Response
R2 344k
1M
100k
threshold allows calculation
100
REF
–V
Microprocessor
Supply In
Reset In
GND
1000
LTH
(mV)
MIC2778
BAT(lo)
Micrel