5

Applications Information

The EL2003 and EL2033 are monolithic buffer amplifiers

built with Elantec's proprietary dielectric isolation process

that produces NPN and PNP complimentary transistors. The

circuits are connection of symmetrical common collector

transistors that provide both sink and source current

capability independent of output voltage while maintaining

constant output and input impedances. The high slew rate

and wide bandwidth of the EL2003 and EL2033 make them

useful beyond video frequencies.

Power Supplies

The EL2003 and EL2033 may be operated with single or

split supplies as low as ±2.5V (5V total) to as high as ±18V

(36V total). However, the bandwidth, slew rate, and output

impedance degrade significantly for supply voltages less

than ±5V (10V total) as shown in the characteristic curves. It

is not necessary to use equal value split supplies, for

example -5V and +12V would be excellent for 0V to 1V video

signals.

Bypass capacitors from each supply pin to a ground plane

are recommended. The EL2003 and EL2033 will not

oscillate even with minimal bypassing, however, the supply

will ring excessively with inadequate capacitance. To

eliminate a supply ringing and the interference it can cause,

a 10µF tantalum capacitor with short pins is recommended

for both supplies. Inadequate supply bypassing can also

result in lower slew rates and longer settling times.

Input Range

The input to the EL2003 and EL2033 looks like a high

resistance in parallel with a few picofarads in addition to a

DC bias current. The input characteristics change very little

with output loading, even when the amplifier is in current

limit. However, there are clamp diodes from the input to the

output that protect the transistor base emitter junctions.

These diodes start to conduct at about ±9.5V input to output

differential voltage. Of course the input resistance drops

dramatically when the diodes start conducting; the diodes

are rated at ±50mA.

The input characteristics also change when the input voltage

exceeds either supply by 0.5V. This happens because the

input transistor's base-collector junctions forward bias. If the

input exceeds the supply by LESS than 0.5V and then

returns to the normal input range, the output will recover in

less than 10ns. However, if the input exceeds the supply by

MORE than 0.5V, the recovery time can be hundreds of

nanoseconds. For this reason it is recommended that

schottky diode clamps from input to supply be used if a fast

recovery from large input overloads is required.

Source Impedance

The EL2003 and EL2033 have excellent input-output

isolation and are very tolerant of variations in source

impedances. Capacitive sources cause no problems at all,

resistive sources up to 100k

Ω present no problems as long

as care is used in board layout to minimize output to input

coupling. Inductive sources can cause oscillations; a 1k

Ω

resistor in series with the buffer input pin will usually

eliminate problems without sacrificing too much speed. An

unterminated cable or other resonant source can also cause

oscillations. Again, an isolating resistor will eliminate the

problem.

Current Limit

The EL2003 and EL2033 have internal current limits that

protect the output transistors. The current limit goes down

with junction temperature rise as shown in the characteristic

curves. At a junction temperature of +175°C the current

limits are at about 100mA. If the EL2003 or EL2033 output is

shorted to ground when operating on ±15V supplies, the

power dissipation will be greater than 1.5W. A heat sink is

required in order for the EL2003 or EL2033 to survive an

indefinite short. Recovery time to come out of current limit is

about 250ns.

Heat Sinking

When operating the EL2003 and EL2033 in elevated

ambient temperatures and/or high supply voltages and low

impedance loads, the internal power dissipation can force

the junction temperature above the maximum rating (150°C

for the plastic DIP). Also, an indefinite short of the output to

ground will cause excessive power dissipation.

The thermal resistance junction to case is 50°C/W for the

plastic DIP. A suitable heat sink will increase the power

dissipation capability significantly beyond that of the package

alone. Several companies make standard heat sinks for both

packages. Aavid and Thermalloy heat sinks have been used

successfully.

Parallel Operation

If more than 100mA output is required or if heat

management is a problem, several EL2003 or EL2033s may

be paralleled together. The result is as though each device

was driving only part of the load. For example, if two units

are paralleled then a 50

Ω load looks like 100Ω to each

EL2003. Parallel operation results in lower input and output

impedances, increased bias current but no increase in offset

voltage. An example showing three EL2003s in parallel and

also the addition of a FET input buffer stage is shown below.

By using a dual FET the circuit complexity is minimal and the

performance is excellent. Take care to minimize the stray

capacitance at the input of the EL2003s for maximum slew

rate and bandwidth.

EL2003, EL2033