EL5285C - Preliminary
Dual and Window 4ns High-Speed Comparators
Select the threshold voltage VTH and calculate R1 and
R2. The current through R1/R2 bias string must be many
times greater than the input bias current of the
comparator:
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.
R
1
Power Dissipation
V
= V
´ -------------------
TH
REF
R
+ R
2
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
TJMAX (125°C).
Let the hysteresis be VH, and calculate R3:
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:
VO=VSD-0.8V (swing of the output)
Recalculate R2 to maintain the same value of VTH
:
P
= (V ´ I + V ´ I
)
SD
DISS
S
S
SD
V
V
– 0.5V
TH SD
æ
ö
÷
ø
TH
R 1 = (V
– V ) ¸ ----------- + ------------------------------------
ç
TH
2
REF
R
R
3
è
1
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
Non inverting comparator with hysteresis:
R
3
R
1
V
+
-
IN
V
REF
ISD 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:
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:
T
= T
+ Q ´ P
MAX JA DISS
JMAX
Pick the value of R1. R1 should be small (less than 1kW)
in order to minimize the propagation delay time.
where:
Choose the hysteresis VH and calculate R3:
TMAX is the maximum ambient temperature
R
1
R
= (V
– 0.8) ´ -------
qJA is the thermal resistance of the package
3
SD
V
H
Threshold Detector
Check the current through R3 and make sure that it is
much greater than the input bias current as follows:
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
0.5V – V
SD
REF
---------------------------------------
I =
R
3
9
ELANTEC [ ELANTEC SEMICONDUCTOR ]
