MIC833 View Datasheet(PDF) - Micrel
Part Name
Description
Manufacturer
MIC833 Datasheet PDF : 8 Pages
| |||
MIC833
Applications Information
Output
Since the MIC833 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. Note
that the output may be pulled up as high as 6V regardless of
IC supply voltage. See “Electrical Characteristics.”
Programming the Thresholds
The low-voltage threshold is calculated using:
VIN(lo)
=
VREF
R1+ R2 + R3
R2 + R3
The high-voltage threshold is calculated using:
VIN(hi)
=
VREF
R1+
R2 +
R3
R3
where, for both equations:
VREF = 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 = RTOTAL.
A value such as 1MΩ for RTOTAL is a reasonable value
because it draws minimum current but has no significant
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 VIN
to ground is 3MΩ.
VIN VDD
R1
604k
1%
R2
56k
1%
MIC833
5
4
VDD OUT
3
LTH
1
2
HTH GND
470k
VOUT
R3
340k
1%
Figure 1. Example Circuit
Once the desired trip points are determined, set the VIN(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:
VIN(hi)
=
3.6V
=
1.24
1MΩ
R3
R3 = 344kΩ
Once R3 is determined, the equation for VIN(lo) can be used
to determine R2. A single lithium-ion cell, for example, should
not be discharged below 2.5V. Many applications limit the
Micrel
drain to 3.1V. Using 3.1V for the VIN(lo) threshold allows
calculation of the two remaining resistor values.
VIN(lo)
=
3.1V
=
1.24
1MΩ
R2 + 344k
R2 = 56kΩ
1MΩ − (R2 − R3) = R1
R1= 600kΩ
The accuracy of the resistors can be chosen based upon the
accuracy required by the system.
The inputs may be subjected to voltages as high as 6V steady
state without adverse effects of any kind, regardless of the IC
supply voltage. This applies even if the supply voltage is zero.
This permits the situation in which the IC supply is turned off,
but voltage is still present on the inputs. See “Electrical
Characteritics.”
Input Transients
The MIC833 is inherently immune to very short negative-
going “glitches.” Very brief transients may exceed the VIN(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 MIC833.
The graph represents the typical allowable transient duration
for a given amount of threshold overdrive that will not toggle
the output.
Input Transient
Response
140
120
100
80
60
40
20
0
1
10
100
1000
RESET COMP. OVERDRIVE, VREF–VLTH (mV)
Figure 2. Input Transient Response
Initialization Behavior
When the MIC833 is powered up, the comparators and latch
become active before the reference voltage reaches its final
value. In most applications, this presents no problems. How-
ever, the user should be aware of this: when applying power
to the part, if the input voltage is between the two thresholds,
the output of the part will be high because input HTH will have
been higher than the 1.24V reference during initialization.
It is not very likely the part would be powered up in this state;
it is more likely the same power supply will power the part and
develop its inputs. However, if the above-described condition
should occur, the next HTH threshold crossing would not be
processed; that is, the latch would have been already set. The
next valid input condition would have to be a crossing of the
LTH threshold, which resets the latch, after which “normal”
operation is restored.
September 2001
5
MIC833
Share Link: 