EL5485CS-T7 Просмотр технического описания (PDF) - Intersil
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EL5485CS-T7 Datasheet PDF : 9 Pages
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EL5485, EL5486
R3 adds a portion of the output to the positive input. Note
that the current through R3 should be much greater than the
input bias current in order to minimize errors. The calculation
of the resistor values as follows:
Pick the value of R1. R1 should be small (less than 1kΩ) in
order to minimize the propagation delay time.
Choose the hysteresis VH and calculate R3:
R3
=
(V
SD
–
0.8
)
×
-R----1--
VH
Check the current through R3 and make sure that it is much
greater than the input bias current as follows:
I = 0----.--5---V-----S----D-----–----V-----R----E----F-
R3
The above two methods will generate hysteresis of up to a
few hundred millivolts. Beyond that, the impedance of R3 is
low enough to affect the bias string and adjustment of R1
may be required.
Power Dissipation
When switching at high speeds, the comparator's drive
capability is limited by the rise in junction temperature
caused by the internal power dissipation. For reliable
operation, the junction temperature must be kept below
TJMAX (125°C).
An approximate equation for the device power dissipation is
as follows. Assume the power dissipation in the load is very
small:
PDISS = (VS × IS + VSD × ISD ) × N
where:
VS is the analog supply voltage from VS+ to VS-
IS is the analog quiescent supply current per comparator
VSD is the digital supply voltage from VSD to ground
ISD is the digital supply current per comparator
N is the number of comparators in the package
ISD strongly depends on the input switching frequency.
Please refer to the performance curve to choose the input
driving frequency. Having obtained the power dissipation, the
maximum junction temperature can be determined as
follows:
TJMAX = TMAX + ΘJA × PDISS
where:
TMAX is the maximum ambient temperature
θJA is the thermal resistance of the package
Threshold Detector
The inverting input is connected to a reference voltage and
the non-inverting input is connected to the input. As the input
passes the VREF threshold, the comparator's output
changes state. The non-inverting and inverting inputs may
be reversed.
VIN
+
VREF
-
VOUT
Crystal Oscillator
A simple crystal oscillator using one comparator of an
EL5485 and EL5486 is shown below. The resistors R1 and
R2 set the bias point at the comparator's non-inverting input.
Resistors R3, R4, and C1 set the inverting input node at an
appropriate DC average voltage based on the output. The
crystal's path provides resonant positive feedback and stable
oscillation occurs. Although the EL5485 and EL5486 will give
the correct logic output when an input is outside the common
mode range, additional delays may occur when it is so
operated. Therefore, the DC bias voltages at the inputs are
set about 500mV below the center of the common mode
range and the 200Ω resistor attenuates the feedback to the
non-inverting input. The circuit will operate with most AT-cut
crystal from 1MHz to 8MHz over a 2V to 7V supply range.
The output duty cycle for this circuit is roughly 50% at 5V
VCC, but it is affected by the tolerances of the resistors. The
duty cycle can be adjusted by changing VCC value.
5V
R1 5kΩ
R2 1.5kΩ
C1
0.01µF
200Ω
+
-
R4
2kΩ
1MHz to
8MHz
R3
2kΩ
VOUT
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