EL5285C - Preliminary
EL5285C - Preliminary
Dual and Window 4ns High-Speed Comparators
Select the threshold voltage V
TH
and calculate R
1
and
R
2
. The current through R
1
/R
2
bias string must be many
times greater than the input bias current of the
comparator:
R
1
V
T H
= V
REF
×
-------------------
R +R
1
2
The above two methods will generate hysteresis of up to
a few hundred millivolts. Beyond that, the impedance of
R
3
is low enough to affect the bias string and adjustment
of R
1
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
T
JMAX
(125°C).
An approximate equation for the device power dissipa-
tion is as follows. Assume the power dissipation in the
load is very small:
P
DISS
=
(
V
S
×
I
S
+ V
SD
×
I
SD
)
Let the hysteresis be V
H
, and calculate R
3
:
V
O
R
3
= -------
× (
R
1
||
R
2
)
-
V
H
where:
V
O
=V
SD
-0.8V (swing of the output)
Recalculate R
2
to maintain the same value of V
TH
:
V
TH
V
T H
– 0.5V
SD
R
2
1 =
(
V
REF
– V
T H
) ÷
----------- + ------------------------------------
-
R
3
R
1
where:
V
S
is the analog supply voltage from V
S
+ to V
S
-
I
S
is the analog quiescent supply current per comparator
V
SD
is the digital supply voltage from V
SD
to ground
I
SD
is the digital supply current per comparator
I
SD
strongly depends on the input switching frequency.
Please refer to the performance curve to choose the input
driving frequency. Having obtained the power dissipa-
tion, the maximum junction temperature can be
determined as follows:
T
JMAX
= T
MAX
+
Θ
JA
×
P
DISS
Non inverting comparator with hysteresis:
R
3
V
IN
R
1
V
REF
+
-
R
3
adds a portion of the output to the positive input.
Note that the current through R
3
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 R
1
. R
1
should be small (less than 1kΩ)
in order to minimize the propagation delay time.
Choose the hysteresis V
H
and calculate R
3
:
R
1
R
3
=
(
V
SD
– 0.8
) ×
-------
-
V
H
where:
T
MAX
is the maximum ambient temperature
θ
JA
is the thermal resistance of the package
Threshold Detector
Check the current through R
3
and make sure that it is
much greater than the input bias current as follows:
0.5V
SD
– V
REF
-
I = ---------------------------------------
R
3
The inverting input is connected to a reference voltage
and the non-inverting input is connected to the input. As
the input passes the V
REF
threshold, the comparator's
9
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