Transistor circuit with E/C voltage limiter

• US 4,782,280 A
• Filed: 06/16/1987
• Issued: 11/01/1988
• Est. Priority Date: 07/02/1986
• Status: Expired due to Fees

• Current Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Original Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Inventors: Lommers, Anthonius J. J. C.
• Primary Examiner(s): Salce, Patrick R.
• Assistant Examiner(s): Voeltz, Emanuel Todd
• Application Number:
  US07/062,918
• Time in Patent Office: 504 Days
• Field of Search: 323/270-278, 323/281, 323/289, 323/303, 363/89, 363/50
• US Class Current: 323/276
• CPC Class Codes: G05F 1/569 for protection

Abstract

In a series-regulation transistor (T₁), which is driven by a drive circuit (10), for generating a constant voltage across a load (R_L) the occurrence of comparatively large substrate currents in the case of saturation of the series-regulation transistor (T₁) is precluded by a limiting circuit comprising a resistor (R₁) arranged in the base line of the series-regulation transistor (T₁) and a second transistor (T₂) whose base-emitter junction is arranged across the resistor (R₁) and the base-collector junction of the series-regulation transistor (T₁). The collector of the second transistor is connected to a control input (12) of the drive circuit.

Claims

1. A circuit arrangement comprising:

a first transistor of a first conductivity type having an emitter coupled to a first power-supply terminal, a collector coupled to an output terminal, and a base,

a drive circuit for driving the first transistor, said drive circuit being coupled to a second power supply terminal and having an output coupled to the base of the first transistor, and

a limiting circuit for limiting the voltage between the emitter and the collector of the first transistor to a specific value by reducing the drive to the first transistor when said voltage decreases below said specific value, wherein the limiting circuit comprises a first resistor connected between the output of the drive circuit and the base of the first transistor, and a second transistor of the first conductivity type having an emitter coupled to the collector of the first transistor, a collector coupled to a control input of the drive circuit, and a base coupled to that end of the first resistor which is situated nearest the drive circuit output.

2. A circuit arrangement as claimed in claim 1, wherein the drive circuit comprises a third transistor of a second conductivity type having an emitter coupled to the second power-supply terminal by means of a second resistor, a collector coupled to the output of the drive circuit, and a base coupled to a circuit for supplying a control voltage to the third transistor, and wherein the control input of the drive circuit comprises the emitter of the third transistor.

3. A circuit arrangement as claimed in claim 1, wherein the drive circuit comprises a third transistor of the first conductivity type having an emitter coupled to the output of the drive circuit, a collector coupled to the second power-supply terminal, and a base coupled to a circuit for supplying a control current to the third transistor, and wherein the control input of the drive circuit comprises the base of the third transistor.

4. A circuit arrangement as claimed in claim 3, wherein the circuit for supplying a control current to the third transistor comprises a constant-current source for supplying a first current and a detection circuit for supplying a second current proportional to the difference in the voltage between the output terminal and the second power-supply terminal and a reference voltage, wherein the control current is determined by the difference between the first current and the second current.

5. A circuit arrangement as claimed in claim 4, wherein the constant-current source comprises a fourth transistor of the first conductivity type having an emitter connected to the first power-supply terminal, a collector connected to the second power-supply terminal via a second resistor, and a base coupled to its collector, and the circuit arrangement further comprises a fifth transistor of the first conductivity type having an emitter coupled to the emitter of the fourth transistor, a collector connected to the collector of the fourth transistor, and a base, and a sixth transistor of the second conductivity type having a collector connected to the base of the fifth transistor via a third resistor, an emitter connected to the second power-supply terminal via a fourth resistor, and a base connected to a switching input for applying a switching voltage, and a seventh transistor of the first condctivity type having an emitter connected to the collector of the fifth transistor, a collector connected to the emitter of the sixth transistor, and a base connected to that end of the third resistor which is connected to the collector of the sixth transistor.

6. A transistor circuit comprising:

an input terminal for connection to a source of voltage,

an output terminal for connection to a load,

a first transistor connected in series between said input and output terminals for controlling current to a load,

a drive circuit having an output coupled to a control electrode of the first transistor via a resistor thereby to control the drive of said first transistor, and

a voltage limiting circuit controlled by the emitter-collector voltage of the first transistor for limiting said emitter-collector voltage to a given value by limiting the drive to the first transistor when said emitter-collector voltage drops below said given value, said voltage limiting circuit including said resistor and a second transistor of the same conductivity type as the first transistor with the second transistor connected between an output electrode of the first transistor and a control input of the drive circuit, and means coupling a control electrode of the second transistor to the resistor in a manner such that the second transistor is responsive to the emitter-collector voltage of the first transistor to signal the drive circuit to limit the drive to the first transistor when the emitter-collector voltage of the first transistor drops below said given value.

7. A transistor circuit as claimed in claim 6 wherein the drive circuit comprises a third transistor of opposite conductivity type to that of the first transistor and connected in series with a second resistor between the output of the drive circuit and a second input terminal for the source of voltage, means coupling a control electrode of the third transistor to a source of control voltage, and wherein said control input of the drive circuit is coupled to said second resistor.

8. A transistor circuit as claimed in claim 6 wherein the drive circuit comprises a third transistor of the same conductivity type as the first transistor and connected between the output of the drive circuit and a second input terminal for the source of voltage, means coupling a control electrode of the third transistor to a soure of control current, and wherein said control input of the drive circuit is coupled to said control electrode of the third transistor.

9. A transistor circuit as claimed in claim 8 wherein the control current source comprises, a constant current source coupled to the control electrode of the third transistor and supplying a first current, a detection circuit coupled to the control electrode of the third transistor for supplying a second current proportional to the difference in the voltage at the output terminal and a reference voltage whereby the control current is proportional to the difference of said first and second currents.

10. A transistor circuit as claimed in claim 9 wherein said reference voltage is derived from a voltage divider coupled to said output terminal.

11. A transistor circuit as claimed in claim 6 wherein the value of the resistor is chosen so that the second transistor is normally in cut-off when said emitter-collector voltage of the first transistor is above said given value.

12. A transistor circuit as claimed in claim 6 wherein the transistor circuit is part of an integrated circuit having a substrate and wherein said given value of the emitter-collector voltage of the first transistor is determined by a circuit operating point at which large substrate currents begin to flow.

13. A transistor circuit as claimed in claim 6 wherein the second transistor is normally in cut-off when said emitter-collector voltage of the first transistor is above said given value, said second transistor commencing conduction when the difference between the emitter-collector voltage of the first transistor and the sum of the voltages across the resistor and the base-emitter of the first transistor exceeds the base-emitter threshold voltage of the second transistor.

14. A transistor circuit as claimed in claim 6 wherein the input terminal is intended for connection to a source of unregulated d.c. voltage so that the transistor circuit will operate as a voltage regulator, the voltage regulator further comprising means coupled to the output terminal for deriving a reference voltage determined by the output voltage, and means coupling said reference voltage to the drive circuit so as to control the drive of the first transistor in a sense to maintain the voltage at the output terminal relatively constant.