EL2003C, EL2033C
100MHz Video Line Driver
Capacitive Loads
Inductive Loads
The EL2003C and EL2033C are stable driving any type
of capacitive load. However, when driving a pure capac-
itance of less than a thousand picofarads the frequency
response has excessive peaking as shown in the charac-
teristic curves. The squarewave response will have large
overshoots and will ring for several hundred
nanoseconds.
The EL2003C and EL2033C can drive small motors,
solenoids, LDT's and other inductive loads. Foldback
current limiting is NOT used in the EL2003C or
EL2033C and current limiting into an inductive load
does NOT in and of itself cause spikes or kickbacks.
However, if the EL2003C or EL2033C is in current limit
and the input voltage is changing quickly (i.e., a square-
wave) the inductive load can kick the output beyond the
supply voltage. Motors are also able to generate kick-
backs when the EL2003C or EL2033C is in current
limit.
If the peaking and ringing cause system problems they
can be eliminated with an RC snubber circuit from the
output to ground. The values can be found empirically
by observing a squarewave or the frequency response.
First just put the resistor alone from output to ground
until the desired response is obtained. Of course the gain
will be reduced due to ROUT. Then put capacitance in
series with the resistor to restore the gain at low frequen-
cies. Start with a small capacitor and increase until the
response is optimum. Too large a capacitor will roll the
gain off prematurely and result in a longer settling time.
The figure below shows an example of an EL2003C
driving a 330pF load, which is similar to the input of a
flash converter.
To prevent damage to the EL2003C and EL2033C when
the output kicks beyond the supplies it is recommended
that catch diodes be placed from each supply to the
output.
Reverse Isolation
The EL2003C and EL2033C have excellent output to
input isolation over a wide frequency range. This char-
acteristic is very important when the buffer is used to
drive signals between different equipment over cables.
Often the cable is not perfect or the termination is
improper and reflections occur that act like a signal
source at the output of the buffer. Worst case the cable is
connected to a source instead of where it is supposed to
go. In both situations the buffer must keep these signals
from its input. The following curve shows the reverse
isolation of the EL2003C and EL2033C verses fre-
quency for various source resistors.
Driving a Pure Capacitance
Top Trace is without Snubber.
Bottom Trace is with Snubber Circuit.
9
ELANTEC [ ELANTEC SEMICONDUCTOR ]
