EL2004 datasheet(8/16 Pages) ELANTEC

8 / 16 page

EL2004EL2004C

350 MHz FET Buffer

Circuit Operation

Contd

Offset Zero Adjust

2004 – 10

Capacitive Loading

The EL2004 is designed to drive capacitive loads

up to several thousand picofarads without oscil-

lation

However

peak current resulting from

charging currents on fast edges should be limited

below the absolute maximum peak current rating

of 250 mA In some cases it may be necessary to

employ one of the current limit schemes shown

below

Short Circuit Protection

Dynamic response of the EL2004 was preserved

by excluding current limit circuits which are not

needed in most applications However in situa-

tions where operating conditions are not con-

trolled short circuit protection can be added by

inserting resistors between the output device col-

lectors and supplies as illustrated

Using Resistor

Current Limiting

2004 – 11

Suitable resistor values can be calculated as fol-

lows

RSC e

ISC

where ISC s 100 mA for EL2004

Current Limiting

Using Current Sources

2004 – 12

The inclusion of limiting resistors in the collec-

tors of the output devices will reduce the output

voltage swing and speed Decoupling VCa and

VCb pins with capacitors to ground will retain

full output swing for transient pulses

An alternate active current limit technique that

retains full DC output swing is shown above

Here the current sources are saturated during

normal operation thus applying full supply volt-

age to the VC pins Under fault conditions the

voltage decreases as the current source reaches its

limit

RLIM e

VBE

ISC

06V

100 mA

Power Supplies

The EL2004 has been characterized for both g15

and g5V dual supply operation but other combi-

nations can also be useful For example in many

video applications it is only necessary for the out-

put to swing g2V or less but speed and distor-

tion are important In this situation the input

stage can be operated at the full g15V supply

while the output collectors are returned to g5V

The speed and distortion will be almost as good

as if the whole circuit was operating at g15V

but the dissipation is substantially reduced and

higher load currents can be safely accommodat-

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