7

Power Supplies

In typical operation, V+ and V- can be as much as ±15V and

heavy output currents, however, it is wise to have 5V of

saturation of devices within the EL2021 and attendant

minimum operating voltages are +10, -7V. It is very important

to bypass the supply terminals with low-inductance

capacitors to ground, since the full base drive currents of the

output transistors are derived from these supplies. Because

the pulse currents can reach 60mA, the capacitors should be

at least a microfarad; 4.7µF tantalum are ideal and require

no small bypasses in parallel.

B+ and B- can be any voltage within V+ and V- and some

currents can flow when the EL2021 attempts to bring an

output transistor's base beyond the collector voltage. The

bypassing care of the V± lines apply to the B± lines, as well

as the fact that ampere currents can occur. Large (100µF–

500µF) capacitors should be used to bypass perhaps every

tenth EL2021.

as to not pick up magnetic interference from the output. The

inductance of interconnects to these lines can allow coupling

to cause waveshape anomalies or even oscillations. If long

lines are unavoidable, local 1k resistors or 50pF–100pF

capacitors to ground can also serve the purpose.

Data Pin

The slew rate of the input to the Data pin should be kept less

than 1000V/µs. Some feedthrough can occur for large Slew

Rates which will distort the output waveshape. A 1k–2k

resistor in series with the data pin will reduce feedthrough.

Current Sense

The output current is sensed by comparing the voltage

dropped across the external shunt resistors to an internal

0.45V reference. The center of the trip level is adjusted for

the particular output transistor betas listed in the data

specifications. Transistors with less beta at high currents will

cause the sense comparators to trip at slightly higher output

currents. The 1

Ω shunt resistors should be non-inductive.

The family of wirewound resistors called “non-inductive” are

too inductive for these shunts.

The response of the Sense Out can be thought of as slow

attack and fast decay. A continuous overcurrent condition

must last for at least 2µs before Sense Out will go high, but

will clear to low only about 200ns after the overcurrent is

withdrawn. This allows transient currents due to slewing

capacitive load to not generate a flag. On the other hand, the

output transistors will not be damaged with only a 2µs

system reaction time to a short-circuit.

Construction Practices

The major cautions in connecting to the EL2021 involve

magnetic rather than capacitive parasitic concerns. The

circuits can output as much as 100A/µs. Even with normal

Slew Rates and moderately large capacitive loads, the dI/dT

can cause magnetic fields in harmless looking wires to fill

adjacent lines with noise, and sometimes ringing or even

sustained feedback. Thus, rules for wiring the EL2021 are:

1. Keep leads short and large. Short wires are less

inductive, as are wires with large surface area. The large

surface area also reduces skin resistance at high

frequencies, important at high currents (at 100MHz,

current penetrates only a few microns in metals).

2. Use a ground plane. Due to inductance and skin effect,

“ground” voltages will be different only inches apart on a

copper ground plane. Individual wires do not create

ground at high frequencies. The common “star” ground is

a very bad idea for high-current and high-frequency

circuits.

3. Dress all wires against the ground plane. The magnetic

fields that the wires would have generated will be

intercepted by the ground plane and absorbed, thus

reducing the wire's effective inductance. The capacitance

added by this method is not important to EL2021

operation.

4. The external transistors should have short interconnects

to the EL2021, the collector shunt resistors, and the

bypass capacitors. As previously stated, the shunt

resistors must not be wire wound because of their

inductance.

5. The bypass capacitors should have low series resistance

and inductance, but should not have a high Q. This may

seem contradictory, but a 4.7µF tantalum capacitor

seems to work the best. An electrolytic capacitor should

be added to help bolster the supply levels in the 0.1µs–

1µs after a transition. No small capacitors are needed in

parallel with the tantalums. The bypasses' ground returns

FIGURE 2. SIMPLIFIED OUTPUT STAGE (HIGH-IMPEDANCE

MODE)

EL2021