5

Burn-In Circuit

Applications Information

The EL2008 is a monolithic buffer amplifier built on Elantec's

proprietary dielectric isolation process that produces NPN

and PNP transistors with essentially identical DC and AC

characteristics. The EL2008 takes full advantage of the

complementary process with a unique circuit topology.

Elantec has applied for two patents based on the EL2008’s

topology. The patents relate to the base drive and feedback

mechanism in the buffer. This feedback makes 3000V/µs

slew rates with 10

Ω load possible with modest supply

current.

Power Supplies

The EL2008 may be operated with single or split supplies

with total voltage difference between 10V (±5V) and 36V

(±18V). However, bandwidth, slew rate and output

impedance are affected by total supply voltages below 20V

(±10V) as shown by the characteristic curves. It is not

necessary to use equal split value supplies. For example -5V

and +12V would be excellent for signals from -2V to +9V.

Bypass capacitors from each supply pin to ground are highly

recommended to reduce supply ringing and the interference

it can cause. At a minimum a 10µF tantalum capacitor in

parallel with a 0.1µF capacitor with short leads should be

used for both supplies.

Input Characteristics

The input to the EL2008 looks like a resistance in parallel

with about 25pF in addition to a DC bias current. The DC

bias current is due to the mismatch in beta and collector

current between the NPN and PNP transistors connected to

the input pin. The bias current can be either positive or

negative. The change in input current with input voltage

curves are shown in the characteristic curves.

Internal clamp diodes from the input to the output are

provided. These diodes protect the transistor base emitter

junctions and limit the boost current during slew to avoid

saturation of internal transistors. The diodes begin

conduction at about ±2.5V input to output differential. When

that happens the input resistance drops dramatically. The

diodes are rated at 50mA. When conducting they have a

series resistance of about 20

Ω. If the output of the EL2008 is

accidentally shorted it is possible that some devices driving

the EL2008’s input could be damaged or destroyed driving

the EL2008’s load through the diodes while the EL2008 is

unaffected. In such cases a resistor in series with the input of

the EL2008 can limit the current.

Source Impedance

The EL2008 has good input to output isolation. Open loop,

capacitive and resistive sources up to 100k

Ω present no

oscillation problem driving resistive loads as long as care is

used in board layout to minimize output to input coupling and

the supplies are properly bypassed. When driving capacitive

loads in the 100pF to 1000pF region source resistances

above 25

Ω can cause peaking and oscillation. Such

problems can be eliminated by placing a capacitor from the

EL2008s input to ground. The value should be about 1/4 the

load capacitance. In a feedback loop there is a speed

penalty and a possibility of oscillation when the EL2008 is

driven with a source impedance of 200

Ω or more. Significant

phase shift can occur due to the EL2008’s 25pF input

capacitance. Inductive sources can cause oscillations. A

series resistor of a few hundred ohms to 1k

Ω will usually

solve the problem.

Current Limit

The EL2008 has internal current limiting to protect the output

transistors. The current limit is about 1.5A at room

temperature and decreases with junction temperature. At

150°C junction temperature it is above 1A.

Heat Sinking

A suitable heat sink will be required for most applications.

The thermal resistance junction to case for the TO-220

package is 4°C per watt. No voltage appears at the heat sink

tab so no precautions need to be taken to avoid shorting the

tab to a supply voltage or ground. As there is a small

parasitic capacitance between the tab and the buffer

circuitry, it is recommended that the tab be connected to AC

ground (either supply voltage or DC ground). The center

lead is internally connected to the tab so the connection can

be made at the tab or the center lead.

Parallel Operation

If more than 1A is required or if heat management is a

problem, several EL2008s may be paralleled together. The

result is as through each device was driving only part of the

load. For example, if two units are paralleled then a 5

Ω load

looks like 10

Ω to each EL2008. Of course, parallel operation

reduces both the input and output impedance and increases

EL2008