5

Applications Information

Output Stage

To meet the requirements of low output impedance, wide

bandwidth, and large capacitive load driving capability, the

EL2021 has a fairly exotic output stage. Figure 1 shows a

simplified schematic of the circuit, only applicable in normal,

low impedance mode. External transistors are used to

handle the large load currents and peak power dissipations.

Since there is no need for good AC crossover distortion

performance in a pin driver, the output transistors are

operated class C. That is, for small output currents, neither

output transistor will conduct bias current, and when load

currents do flow, one of the devices is off. This is

accomplished by biasing the output transistors from Schottky

diodes D1 and D2. In operation, the diode forward voltage is

about 0.4V, whereas the “on” output transistor will have a

transistor's base-emitter junction, not nearly enough to

cause bias currents to flow in it. Schottky diodes have a

temperature drift similar to silicon transistors, so the class C

bias maintains over temperature. One caution is that the

diodes are in the IC package and are thermally separate

from the transistors, so there can exist temperature

differences between packages that can cause thermal

runaway. Runaway is avoided as long as the external

transistors are not hotter than the EL2021 package by more

than 80°C. The only way runaway has been induced as of

this writing is to use “freeze spray” on the IC package while

the output transistors are very hot.

Overshoot vs VSR

Overshoot vs VSR

Slew Rate vs VSR

Tri-State Leakage vs

Output Voltage

Supply Current vs

Supply Voltage

Supply Current vs VSR

Power Dissipation vs

Temperature

EL2021