Output driver with current compensation circuit for variation of common mode voltage

• US 6,329,843 B1
• Filed: 10/10/2000
• Issued: 12/11/2001
• Est. Priority Date: 10/14/1999
• Status: Expired due to Fees

• Current Assignee: Matsushita Electric Industrial Company Limited (Panasonic Holdings Corporation)
• Original Assignee: Matsushita Electric Industrial Company Limited (Panasonic Holdings Corporation)
• Inventors: Hirata, Takashi, Takahashi, Satoshi, Terada, Yutaka, Komatsu, Yoshihide, Akamatsu, Hironori
• Primary Examiner(s): Tokar, Michael
• Assistant Examiner(s): LE, DON P
• Application Number:
  US09/684,979
• Time in Patent Office: 427 Days
• Field of Search: 326/82, 326/26, 326/27, 326/30, 326/83, 326/86, 326/89, 326/90
• US Class Current: 326/82
• CPC Class Codes: H04L 25/0276 Arrangements for coupling common mode signals
• CPC Class Codes: H04L 25/028 Arrangements specific to the transmitter end

Abstract

A current driver, a common mode voltage monitoring circuit and a current compensator are provided to drive a twisted pair cable, which is made up of two signal lines coupled to a terminal bias voltage through respective terminal resistors. The common mode voltage monitoring circuit monitors a difference between a common mode voltage of the twisted pair cable and a supply voltage level for the current driver. And the current compensator is coupled to the twisted pair cable to compensate for an output current of the current driver in accordance with a result of monitoring performed by the common mode voltage monitoring circuit. If the current driver has decreased its current drivability due to a drop of the supply voltage level of the current driver or a variation in the common mode voltage of the twisted pair cable, then the current compensator compensates for the decrease. Thus, the current driver can continuously operate in a broad voltage range to supply a constant current.

Claims

1. A current driver circuit for driving a twisted pair cable, the cable being made up of two signal lines that are coupled to a terminal bias voltage through respective terminal resistors, the driver circuit comprising:

a current driver coupled to the twisted pair cable;

a common mode voltage monitoring circuit for monitoring a difference between a common mode voltage of the twisted pair cable and at least one supply voltage level for the current driver; and

a current compensator, coupled to the twisted pair cable, for compensating for an output current of the current driver in accordance with a result of monitoring performed by the common mode voltage monitoring circuit.

2. The current driver circuit of claim 1, wherein the current compensator compensates stepwise for the output current of the current driver.

3. The current driver circuit of claim 1, wherein the current compensator changes a total size of current source transistors to be activated nonlinearly relative to the difference between the common mode voltage and the supply voltage level.

4. The current driver circuit of claim 1, wherein the current driver includes a PMOS current source transistor and an NMOS current source transistor, and

wherein the common mode voltage monitoring circuit estimates a difference between a first supply voltage level coupled to the source of the PMOS current source transistor and the common mode voltage and a difference between the common mode voltage and a second supply voltage level coupled to the source of the NMOS current source transistor.

5. The current driver circuit of claim 4, wherein the common mode voltage monitoring circuit comprises:

a plurality of resistors that are connected in series to each other between the first and second supply voltage levels;

a current source for supplying a current through the serial connection of the resistors; and

a plurality of comparators, each said comparator comparing the common mode voltage to a terminal voltage of associated one of the resistors.

6. The current driver circuit of claim 5, wherein each said comparator exhibits hysteresis between input and output thereof.

7. The current driver circuit of claim 5, wherein the current supplied by the current source changes its amount according to current drivability of each of the PMOS and NMOS current source transistors.

8. The current driver circuit of claim 4, further comprising:

a supply voltage monitoring circuit for monitoring the first supply voltage level; and

a bias regulator for regulating gate bias voltages of the PMOS and NMOS current source transistors in such a manner as to compensate for the output current of the current driver in accordance with a result of monitoring performed by the supply voltage monitoring circuit.

9. The current driver circuit of claim 1, further comprising means for decoupling the current compensator from the twisted pair cable in transmitting or receiving a signal with the common mode voltage changed.

10. The current driver circuit of claim 9, wherein an interval during which the common mode voltage is changed is included in an interval during which the current compensator is decoupled from the twisted pair cable.

11. A method for transmitting a signal by supplying a current with constant amplitude through a twisted pair cable, the cable being made up of two signal lines that are coupled to a terminal bias voltage through respective terminal resistors, the method comprising the steps of:

monitoring a difference between a common mode voltage of the twisted pair cable and a supply voltage level for a current driver that drives the twisted pair cable; and

compensating stepwise for an output current of the current driver in accordance with a result of the monitoring step so that an amount of the current flowing through the twisted pair cable falls within a predetermined range.