MIC2778 Datasheet, PDF (5/8 Pages) Micrel Semiconductor – Voltage Monitor with Adjustable Hysteresis Preliminary Information

MIC2778
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. 470kΩ is the maximum recommended value.
Programming the Thresholds
The low-voltage threshold is calculated using:
V_BAT(lo) = V_REF ^{ R1R+}_2R+^2R+_3R3^{}
The high-voltage threshold is calculated using:
V_BAT(hi)
=
V_REF

R1+
R2 +
R3
R3 
where, for both equations:
V_REF = 1.240V
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_TOTAL.
A value such as 1MΩ for R_TOTAL is a reasonable value
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_BAT to ground is 3MΩ.
V_BATT
R1
604k
1%
R2
56k
1%
R3
340k
1%
MIC2778
VDD
LTH
HTH
RST
GND
470k
Figure 1. Example Circuit
Once the desired trip points are determined, set the V_BAT(hi)
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:
V_BAT(hi)
=
1.24

1MΩ
R3

R3 = 344kΩ
Once R3 is determined, the equation for V_BAT(lo) can be used
to determine R2. A single lithium-ion cell should not be
discharged below 2.5V. Many applications limit the drain to
Micrel
3.1V. Using 3.1V for the V_BAT(lo) threshold allows calculation
of the two remaining resistor values.
V_BAT(lo)
=
3.1V
=
1.24 ^{}
1MΩ
R2 + 344k

R2 = 56kΩ
R1= 1MΩ − R2 − R3
R1= 600kΩ
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_BAT(lo)
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.
Input Transient
Response
140
120
100
80
60
40
20
0
1
10
100
1000
RESET COMP. OVERDRIVE, V_REF–V_LTH (mV)
Figure 2. Input Transient Response
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.
V_BATT
Microprocessor
R1
604k
1%
R2
56k
1%
MIC2778
VDD RST
LTH
HTH GND
100k
Supply In
Reset In
GND
R3
340k
1%
Figure 4. Interfacing to Bidirectional Reset Pin
October 1999
5
MIC2778