EL2003C datasheet(9/17 Pages) ELANTEC

9 / 17 page

EL2003C, EL2033C

100MHz Video Line Driver

Capacitive 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.

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.

Inductive Loads

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.

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.

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