High voltage current source circuit and method

• US 4,659,979 A
• Filed: 11/27/1985
• Issued: 04/21/1987
• Est. Priority Date: 11/27/1985
• Status: Expired due to Fees

• Current Assignee: Burr-Brown Corporation (Texas Instruments, Inc.)
• Original Assignee: Burr-Brown Corporation (Texas Instruments, Inc.)
• Inventors: Ulbrich, Stephen F., Burnham, Stephen R.
• Primary Examiner(s): Wong, Peter S.
• Application Number:
  US06/802,709
• Time in Patent Office: 510 Days
• Field of Search: 323/268, 323/270, 323/312, 323/315-316, 307/296 R, 307/302, 307/303, 357/20, 357/34-35
• US Class Current: 323/312
• CPC Class Codes: G05F 3/222 with compensation for device parameters, e.g. Early effect, gain, manufacturing process, or external variations, e.g. temperature, loading, supply voltage
• CPC Class Codes: H01L 27/082 including bipolar components only
• CPC Class Codes: H01L 29/402 Field plates
• CPC Class Codes: H01L 29/41708 Emitter or collector electrodes for bipolar transistors
• CPC Class Codes: H01L 29/7813 with trench gate electrode, e.g. UMOS transistors trench gate electrodes per se H01L29/4236
• CPC Class Codes: H03F 2200/282 the level shifting stage between two amplifying stages being realised by a diode
• CPC Class Codes: H03F 2203/45144 At least one follower being added at the input of a dif amp
• CPC Class Codes: H03F 3/45071 with semiconductor devices only

Abstract

A high voltage constant current source circuit includes first transistor biased as a constant current source and a second transistor connected in series with the first transistor. The second transistor has a punch-through voltage that is substantially less than any breakdown voltage of the second transistor. The emitter of the second transistor is connected to the collector of the first transistor. The collector of the second transistor supplies the constant current, provides an increased high output impedance, and allows low voltage operation if the collector-to-emitter voltage of the second transistor is less than its punch-through voltage. If its punch-through voltage is exceeded, that punch-through voltage adds to the collector-to-emitter breakdown voltage of the first transistor, allowing high voltage operation.

Claims

1. A constant current source circuit comprising in combination:

(a) a first transistor having a collector, a base, and an emitter;

(b) a second transistor having a collector, a base, and an emitter, the second transistor having a punch-through voltage V_PT that is less in magnitude than the magnitude of BV_CEO breakdown voltage of the second transistor;

(c) means for coupling the emitter of the first transistor to a first supply voltage conductor and means for coupling the collector of the first transistor to the emitter of the second transistor;

(d) means for applying a first bias voltage to the base of the first transistor to turn it on and means for applying a second bias voltage to the base of the second transistor to turn it on; and

(e) constant current conducting means for coupling the collector of the second transistor to an external circuit to which a constant current is to be supplied.

2. The constant current circuit of claim 1 wherein the magnitude of the BV_CBO breakdown voltage of the second transistor is greater than the sum of the magnitude of the punch-through voltage V_PT of the second transistor and the BV_CEO breakdown voltage of the first transistor.

3. The constant current circuit of claim 1 wherein the magnitude of the BV_CBO voltage of the second transistor is less than the sum of the punch-through voltage V_PT of the second transistor and the magnitude of the BV_CEO breakdown voltage of the first transistor.

4. The constant current source circuit of claim 1 wherein the first transistor and the second transistor are of the same type.

5. The constant current source circuit of claim 4 wherein said first and second transistors are lateral PNP transistors.

6. The constant current source circuit of claim 5 wherein the base width of the second transistor is less than the base width of the first transistor.

7. The constant current source circuit of claim 6 wherein the first and second transistors are disposed in a single integrated circuit substrate.

8. The constant current source circuit of claim 7 including a plurality of constant current source circuits which are substantially similar to said constant current source circuit, and a means for coupling the first bias voltage to the bases of the first transistor of each of the constant current source circuits and means for applying the second bias voltage to the bases of the second transistor of each of the constant current source circuits.

9. The constant current source circuit of claim 7 wherein the second transistor includes an annular collector region and also includes an emitter region surrounded by and spaced from the annular collector region, and includes an oxide layer extending over the collector and emitter regions, and also including metal field plate means disposed on the oxide layer and making electrical contact to the collector of the second transistor, for extending inwardly over the oxide layer toward the emitter region to cover a substantial portion of the collector-base junction of the second transistor and to cover enough of the base region between the collector region and the emitter region to cause the collector depletion region of the second transistor to extend to the emitter region when the magnitude of the collector-to-emitter voltage of the second transistor exceeds a predetermined level.

10. The constant current source circuit of claim 9 wherein the predetermined level is approximately equal to the punch-through voltage of the second transistor.

11. The constant current source circuit of claim 9 wherein the metal field plate means extends outwardly over the oxide to cover most of the outermost portion of collector-base junction of the second transistor, thereby increasing the BV_CBO breakdown voltage of the second transistor.

12. A constant current source circuit comprising:

(a) a first transistor having a collector, a base connected to a first bias voltage conductor, and an emitter coupled to a first supply voltage conductor;

(b) a second transistor having an emitter coupled to the collector to the first transistor, a base coupled to a second bias voltage conductor, and a collector, the second transistor having a punch-through voltage that is lower in magnitude than the BV_CEO breakdown voltage of the first transistor; and

(c) the collector of the second transistor supplying a substantially constant current over a wide range of voltages that are less in amplitude than the sum of the punch-through voltage of the second transistor and the BV_CEO breakdown voltage of the first transistor.

13. A method of increasing the breakdown voltage of a circuit including a first transistor, the method comprising the steps of:

(a) connecting a second transistor to the first transistor, and causing the second transistor to have a punch-through voltage that is less in magnitude than its non-punch-through limited BV_CEO breakdown voltage; and

(b) biasing the base of the second transistor to produce a predetermined relationship between the voltages of the bases of the first and second transistors.

14. The method of claim 13 wherein the first transistor is connected in a current mirroring relationship with a current mirror bias circuit, and wherein the first and second transistors in combination form a high impedance constant current source, the emitter of the second transistor being coupled to the collector of the first transistor, the collector of the second transistor supplying a constant current to another circuit.

15. The method of claim 13 wherein the first and second transistors are lateral PNP transistors in an integrated circuit.

16. The method of claim 15 including providing an annular collector region in the second transistor and also providing an emitter region surrounded by and spaced from the annular collector region, providing an oxide layer extending over a portion of the collector region and a portion of the emitter region, and also including a metal field plate means disposed on the oxide layer and making electrical contact to the collector of the second transistor for extending inwardly over the oxide layer toward the emitter region to cover a substantial portion of the collector-base junction of the second transistor and to cover enough of the base region between the collector region and the emitter region to cause the collector depletion region of the second transistor to extend to the emitter region when the magnitude of the collector-to-emitter voltage of the second transistor exceeds a predetermined level.

17. The method of claim 16 wherein the magnitude of the BV_CBO breakdown voltage of the second transistor is greater than the sum of the magnitudes of the punch-through voltage of the second transistor and the BV_CEO breakdown voltage of the first transistor.

18. The method of claim 17 wherein the predetermined level is approximately equal to the punch-through voltage of the second transistor.

19. The method of claim 18 wherein the metal field plate means extends outwardly over the oxide to cover the outermost portion of the collector-base junction of the second transistor, to thereby increase the BV_CBO breakdown voltage of the second transistor, to thereby increase the BV_CBO breakdown voltage of the second transistor.