EL5481C/EL5482C
Quad 8ns High-Speed Comparators
Inverting comparator with hysteresis:
Choose the hysteresis V and calculate R :
H 3
R
V
REF
1
R
= (V
– 0.8 ) ´ --------
SD
3
R
3
V
H
R
R
2
1
+
-
Check the current through R and make sure that it is
3
V
IN
much greater than the input bias current as follows:
0.5V – V
SD
REF
I = ---------------------------------------
R
3
R adds a portion of the output to the threshold set by R
3
1
and R . The calculation of the resistor values are as
2
follows:
The above two methods will generate hysteresis of up to
a few hundred millivolts. Beyond that, the impedance of
Select the threshold voltage V and calculate R and
TH
1
R is low enough to affect the bias string and adjustment
3
R . The current through R /R bias string must be many
2
1
2
of R may be required.
1
times greater than the input bias current of the
comparator:
Power Dissipation
R
1
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
V
= V
´ -------------------
1
TH
REF
R
+ R
2
Let the hysteresis be V , and calculate R :
H
3
T
(125°C).
JMAX
V
O
||
R )
2
R
= -------- ´ (R
3
1
V
H
An approximate equation for the device power dissipa-
tion is as follows. Assume the power dissipation in the
load is very small:
where:
V =V -0.8V (swing of the output)
O
SD
P
= (V ´ I + V ´ I ) ´ N
S S SD SD
DISS
Recalculate R to maintain the same value of V
:
TH
2
V
V
– 0.5V
TH SD
æ
ö
÷
ø
where:
V is the analog supply voltage from V + to V -
TH
R 1 = (V
– V ) ¸ ----------- + -------------------------------------
ç
TH
2
REF
R
R
3
è
1
S
S
S
Non inverting comparator with hysteresis:
I is the analog quiescent supply current per comparator
S
V
SD
is the digital supply voltage from V to ground
SD
R
3
I
SD
is the digital supply current per comparator
R
1
V
+
-
IN
N is the number of comparators in the package
V
REF
I
strongly depends on the input switching frequency.
SD
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:
R
adds a portion of the output to the positive input.
3
Note that the current through R should be much greater
than the input bias current in order to minimize errors.
The calculation of the resistor values as follows:
3
T
= T
+ Q ´ P
MAX JA DISS
JMAX
Pick the value of R . R should be small (less than 1kW)
1
1
in order to minimize the propagation delay time.
9
ELANTEC [ ELANTEC SEMICONDUCTOR ]
