el5185 Intersil Corporation, el5185 Datasheet - Page 8

• Manufacturer Part Number: el5185
• Description: 4ns High-speed Comparator
• Manufacturer: Intersil Corporation

Input Slew Rate
Most high speed comparators oscillate when the voltage of
one of the inputs is close to or equal to the voltage on the
other input due to noise or undesirable feedback. For clean
output waveform, the input must meet certain minimum slew
rate requirements. In some applications, it may be helpful to
apply some positive feedback (hysteresis) between the
output and the positive input. The hysteresis effectively
causes one comparator's input voltage to move quickly past
the other, thus taking the input out of the region where
oscillation occurs. For the EL5185, the propagation delay
increases when the input slew rate increases for low
overdrive voltages. With high overdrive voltages, the
propagation delay does not change much with the input slew
rate.
Latch Pin Dynamics
The EL5185 contains a transparent latch for each channel.
The latch pin is designed to be driven with either a TTL or
CMOS output. When the latch is connected to a logic high
level or left floating, the comparator is transparent and
immediately responds to the changes at the input terminals.
When the latch is switched to a logic low level, the
comparator output remains latched to its value just before
the latch’s high-to-low transition. To guarantee data
retention, the input signal must remain the same state at
least 1ns (hold time) after the latch goes low and at least 2ns
(setup time) before the latch goes low. When the latch goes
high, the new data will appear at the output in approximately
4ns (latch propagation delay).
Hysteresis
Hysteresis can be added externally. The following two
methods can be used to add hysteresis.
Inverting comparator with hysteresis:
R
and R
Select the threshold voltage V
The current through R
greater than the input bias current of the comparator:
V
3
TH
adds a portion of the output to the threshold set by R
2
=
. The calculation of the resistor values are as follows:
V
REF
---------------------
R
1
R
R
R
2
1
+
V
1
REF
R
2
1
/R
V
IN
2
bias string must be many times
8
TH
+
-
R
3
and calculate R
1
and R
1
2
.
EL5185
Let the hysteresis be V
where:
Recalculate R
Non inverting comparator with hysteresis:
R
that the current through R
input bias current in order to minimize errors. The calculation
of the resistor values as follows:
Pick the value of R
order to minimize the propagation delay time.
Choose the hysteresis V
Check the current through R
greater than the input bias current as follows:
The above two methods will generate hysteresis of up to a
few hundred millivolts. Beyond that, the impedance of R
low enough to affect the bias string and adjustment of R
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
An approximate equation for the device power dissipation is
as follows. Assume the power dissipation in the load is very
small:
R
I
R
JMAX
P
R
3
3
V
=
DISS
3
2
adds a portion of the output to the positive input. Note
O
1
=
0.5V
-------------------------------------- -
=
= V
=
V
--------
V
V
(125°C).
=
O
H
SD
SD
SD
R
V
V
REF
-0.8
3
-V
-0.8V (swing of the output)
S
R
V
2
REF
1
IN
to maintain the same value of V
I
- V
S
R
R
------- -
V
+
2
1
TH
H
1
V
. R
SD
R
H
1
1
, and calculate R
H
should be small (less than 1k ) in
3
V
I
V
-----------
and calculate R
SD
R
REF
TH
should be much greater than the
1
3
and make sure that it is much
+
V
-----------------------------------
+
-
TH
R
3
-0.5V
R
3
3
3
:
SD
:
TH
:
3
1
is