EL2008 Datasheet, PDF (5/8 Pages) Intersil Corporation – 55MHz 1 Amp Buffer Amplifier

EL2008
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 10Ω 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
(R_IN) is affected by the output load, beta and the internal
boost. R_IN can actually appear negative over portions of the
input range in some units. A few typical input current (I_IN)
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 20Ω. 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 100kΩ 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 25Ω 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 200Ω 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 1kΩ 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 5Ω load
looks like 10Ω to each EL2008. Of course, parallel operation
reduces both the input and output impedance and increases
5