The present invention relates to an output stage for a voltage regulator or other type of circuit which must conduct load current at both low and high input/output voltage differentials. More particularly, the present invention relates to a circuit for distributing load current among multiple power transistors, each optimized to conduct current over a different range of input/output voltage differentials.
Power devices, such as voltage regulators and power switches, which often must supply current to a load while operating at a high input/output voltage differential, require an output stage power transistor that can conduct substantial load current at such high voltage differentials without creating a risk of destroying the power transistor due to thermal instabilities and overheating in localized areas of the transistor. To prevent the power transistor from becoming damaged in this fashion, ballast resistors are used in the emitter of the power transistor to stabilize the current in the transistor.
Ballast resistance increases the voltage dropped across the emitter of the transistor in proportion to the current conducted by the transistor. In certain circumstances, this increase in voltage may increase the minimum operating voltage of a power device. An adequately ballasted power transistor, for example, should have sufficient ballast resistance to drop approximately 100-500 mV when conducting a low current at high input/output voltage differentials. Under such conditions, the increased voltage drop across the transistor caused by the ballast resistance does not significantly degrade the performance of the power device because the input/output voltage differential at which the device is operating will generally exceed the voltage drop across the power transistor. At higher currents and lower voltage differentials, however, the ballast resistance can place undesirable limits on the power device. Having a resistance in the emitter of the transistor increases the minimum value of the saturation voltage of the transistor. At high currents the voltage drop across the ballast resistance alone may be between 200 mV and 2V, thus causing a great increase in the minimum operating voltage for the power transistor and the power device in which the transistor is employed.
For this reason, a power transistor having a low ballast resistance is desirable in the output stage of power devices which conduct high currents while operating at low input/output voltage differentials. However, such a power transistor is unstable at high input/output voltage differentials, and thus if used alone as an output stage would severely limit the operating range of the power device.
In view of the foregoing, it would be desirable to be able to provide an output stage for a power device such as a voltage regulator or a power switch that has both a low minimum operating voltage and the ability to conduct substantial load current safely at high input/output voltage dif- ferentials.