el5181 Intersil Corporation, el5181 Datasheet - Page 8

• Manufacturer Part Number: el5181
• Description: 8ns High-speed Comparator
• Manufacturer: Intersil Corporation

Applications Information
Power Supplies and Circuit Layout
The EL5181 comparator operates with single and dual sup-
ply with 5V to 12V between V
the comparator is supplied by a single supply from 2.7V to
5V. The rail to rail output swing enables direct connection of
the comparator to both CMOS and TTL logic circuits. As with
many high speed devices, the supplies must be well
bypassed. Elantec recommends a 4.7µF tantalum in parallel
with a 0.1µF ceramic. These should be placed as close as
possible to the supply pins. Keep all leads short to reduce
stray capacitance and lead inductance. This will also mini-
mize unwanted parasitic feedback around the comparator.
The device should be soldered directly to the PC board
instead of using a socket. Use a PC board with a good,
unbroken low inductance ground plane. Good ground plane
c o n s t r u c t i o n t e c h n i q u e s e n h a n c e s t a b i l i t y o f t h e
comparators.
Input Voltage Considerations
The EL5181 input range is specified from 0.1V below V
2.25V below V
output still responds correctly to a small differential input sig-
nal. The differential input stage is a pair of PNP transistors,
therefore, the input bias current flows out of the device.
When either input signal falls below the negative input volt-
age limit, the parasitic PN junction formed by the substrate
and the base of the PNP will turn on, resulting in a significant
increase of input bias current. If one of the inputs goes above
the positive input voltage limit, the output will still maintain
the correct logic level as long as the other input stays within
the input range. However, the propagation delay will
increase. When both inputs are outside the input voltage
range, the output becomes unpredictable. Large differential
voltages greater than the supply voltage should be avoided
to prevent damages to the input stage.
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 out-
put 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 EL5181, the propagation delay increases
when the input slew rate increases for low overdrive volt-
ages. With high overdrive voltages, the propagation delay
does not change much with the input slew rate.
Latch Pin Dynamics
The EL5181 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 imme-
diately responds to the changes at the input terminals. When
the latch is switched to a logic low level, the comparator out-
put remains latched to its value just before the latch’s high-
S
+. The criterion for the input limit is that the
S
8
+ and V
S
-. The output side of
S
- to
EL5181
to-low transition. To guarantee data retention, the input sig-
nal 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 6ns (latch propagation
delay).
Hysteresis
Hysteresis can be added externally. The following two meth-
ods can be used to add hysteresis.
Inverting comparator with hysteresis:
R
R
Select the threshold voltage V
The current through R
greater than the input bias current of the comparator:
Let the hysteresis be V
where:
Recalculate R
Non inverting comparator with hysteresis:
R
the current through R
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
3
2
3
V
. The calculation of the resistor values are as follows:
adds a portion of the output to the threshold set by R
adds a portion of the output to the positive input. Note that
O
=V
R
SD
2
1
-0.8V (swing of the output)
=
2
V
IN
to maintain the same value of V
V
REF
V
1
R
R
R
R
. R
TH
2
1
3
V
3
3
REF
- V
1
R
should be much greater than the input
H
1
/R
=
1
=
=
, and calculate R
H
should be small (less than 1k ) in
TH
V
V
--------
V
2
V
V
and calculate R
REF
V
IN
O
H
REF
bias string must be many times
SD
TH
-0.8
R
V
-----------
R
+
-
1
---------------------
R
TH
R
and calculate R
1
+
-
1
3
R
R
R
+
3
1
2
+
------- -
V
R
R
V
-----------------------------------
H
1
2
TH
3
3
:
:
-0.5V
R
3
TH
SD
:
1
and R
1
and
2
.