Low distortion audio range class-AB full H-bridge pre-driver amplifier

• US 6,353,298 B1
• Filed: 04/11/2001
• Issued: 03/05/2002
• Est. Priority Date: 04/11/2001
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Jeffrey, Edward N.
• Primary Examiner(s): Ro, Bentsu
• Application Number:
  US09/681,459
• Time in Patent Office: 328 Days
• Field of Search: 318/280, 318/287, 318/291, 318/293, 318/294, 318/560, 318/619, 360/75, 369/43, 369/44.35, 369/44.36, 330/262, 330/269, 330/273, 330/274, 330/146
• US Class Current: 318/293
• CPC Class Codes: H03F 1/3217 in single ended push-pull amplifiers
• CPC Class Codes: H03F 3/3037 with asymmetric control, i.e. one control branch containing a supplementary phase inverting stage
• CPC Class Codes: H03F 3/3061 Bridge type, i.e. two complementary controlled SEPP output stages

Abstract

A circuit (40) and method for applying drive voltages to a voice coil motor (VCM) (22) of a mass data storage device (10) has two driver sets, each having a high side driver (HSD) (42,46) and a low side driver (LSD) (44,48) connected to the VCM (22). Each driver set has two SENSEFETs (50,52), each having a power FET and a sense FET. A circuit (106,104) is provided for sensing a sense current in the sense FET of the LSD, and a circuit (60,76,74) is provided for increasing the bias on the gates of the SENSEFET (52) in the LSD when the sense current falls below a predetermined level (VREF). Also, a circuit (113,114,110) is provided for driving a predetermined current in the SENSEFET of the HSD when the sense current falls below the predetermined level. Thus, a current at the predetermined level always flows in the SENSEFETs (50,52).

Claims

1. A circuit for providing drive voltages to a voice coil motor (VCM), said circuit having two driver sets, each driver set having a high side driver and a low side driver, each driver set for connection to respective opposite sides of said VCM, each driver set comprising:

first and second SENSEFET devices respectively in said high side driver and said low side driver, each SENSEFET device having a first portion for selectively connecting a side of said VCM to a VCM driving potential and a second portion for sensing a current in said first portion;

a circuit for sensing a sense current in said second portion of said SENSEFET device of said low side driver;

a circuit for increasing a bias on gates of said SENSEFET device in said low side driver when said sense current falls below a predetermined level; and

a circuit for driving a predetermined current in said SENSEFET device of said high side driver when said sense current falls below said predetermined level, wherein a current of at least about said predetermined level always flows in said SENSEFET devices.

2. The circuit of claim 1 wherein said first and second portions of said SENSEFET devices are two FET device with interconnected gates and drains.

3. The circuit of claim 2 wherein said two FET device are scaled with a ratio of about 200:

1.

4. The circuit of claim 1 wherein said circuit for sensing a current in said second portion of said SENSEFET device of said low side driver comprises:

a resistor between a source of said second portion of said SENSEFET device of said low side driver and a reference potential, wherein said sense current generates a sense voltage across said resistor; and

a transconductance amplifier connected to compare said sense voltage to a reference voltage to produce a current when said sense voltage exceeds said reference potential.

5. The circuit of claim 4 wherein said circuit for increasing a bias on a gate of said SENSEFET device in said low side driver when said sensed current falls below a predetermined level comprises an input amplifier to control said SENSEFET device of said low side driver, and wherein said input amplifier has an output controlled by said current produced by said transconductance amplifier.

6. The circuit of claim 1 wherein said circuit for driving a predetermined current in said second portion of said SENSEFET device of said high side driver when said sensed current falls below said predetermined level comprises:

a resistor connected at a first end to said second portion of said SENSEFET device of said high side driver;

a current source connected at a first end to a reference potential; and

a transconductance amplifier having first and second inputs, said first input being connected to a second end of said resistor and to a second end of said current source, and said second input being connected to said side of said VCM, said transconductance amplifier producing an output current related to a voltage difference between said first and second inputs to control a gate of said SENSEFET device of said high side driver.

7. The circuit of claim 1 wherein said VCM positions a head for at least reading data in a mass data storage device.

8. A circuit for maintaining a Class-AB mode of operation of an H-bridge voice coil motor (VCM) driver, comprising:

four SENSEFET devices connected in an H-bridge configuration with said VCM;

four gate biasing circuits connected to bias respective gates of said SENSEFET devices to selectively turn said SENSEFET devices on and off in response to positioning signals for said VCM;

sensing circuitry associated with respective said SENSEFET devices to operate said gate biasing circuits to maintain gate biases on said SENSEFET devices when currents in said SENSEFET devices fall below a predetermined level to maintain said circuit in Class-AB mode of operation.

9. The circuit of claim 8 wherein:

each of said gate biasing circuits comprises a fixed current source and a variable current source connected in series between a supply voltage and a reference potential;

interconnecting nodes between respective said fixed and said variable current sources are connected to a gate of a respective one of said SENSEFET devices; and

said variable current source is connected to be controlled by said positioning signals.

10. The circuit of claim 8 wherein said VCM positions a head for at least reading data in a mass data storage device.

11. A circuit for providing drive voltages to a voice coil motor (VCM), said circuit having two driver sets, each driver set including a high and a low side driver, each driver set for connection to respective opposite sides of said VCM, each driver set comprising:

first and second SENSEFETs respectively in said high and low side drivers, each SENSEFET having a power FET portion for selectively connecting a side of said VCM to a driving voltage potential and a sense FET portion for sensing a sense current in said power FET portion;

means for developing a sense voltage proportional to said sense current sensed by said sense FET portion of said SENSEFET of said low side driver;

means for increasing a gate bias on gates of said SENSEFET of said low side driver when said sense current falls below a predetermined level; and

means for driving a predetermined current in said power FET portion of said SENSEFET of said high side driver when said sense current falls below said predetermined level, wherein at least about said predetermined current flows in said SENSEFETs at least during a switchover from a high side driver to a low side driver in one of said driver sets.

12. The circuit of claim 11 wherein said power FET and said sense FET portions are scaled with a ratio of about 200:

1.

13. The circuit of claim 11 wherein said VCM positions a head for at least reading data in a mass data storage device.

14. A method for providing drive voltages to a voice coil motor (VCM), comprising:

configuring four SENSEFET devices in an H-bridge configuration with said VCM to provide two driver sets, each driver set including a high side driver and a low side driver; and

biasing said SENSEFET devices to conduct a minimum quiescent current during a switchover between a high side driver and a low side driver in one of said driver sets.

15. The method of claim 14 wherein said biasing said SENSEFET devices to conduct a minimum quiescent current during switchover comprises:

developing sense voltages in said low side drivers proportional to currents sensed by said SENSEFET devices of said low side drivers;

increasing a gate bias on gates of said SENSEFET devices of said low side drivers when currents in said SENSEFET devices fall below a predetermined level; and

driving a predetermined current in said SENSEFET devices of said high side drivers when a currents in said SENSEFET devices fall below said predetermined level, wherein at least said predetermined current flows in said SENSEFET devices of said high and low side drivers at least during a switchover from a high side driver to a low side driver in a driver set.

16. A mass data storage device, comprising:

a circuit for providing drive voltages to a voice coil motor (VCM), said circuit having two driver sets, each having a high and a low side driver, each driver set for connection to respective opposite sides of said VCM, each driver set comprising;

first and second SENSEFET devices respectively in said high and low side drivers, each SENSEFET device having a first portion for selectively connecting a side of said VCM to a VCM driving potential and a second portion for sensing a current in said first portion;

a circuit for developing a sense voltage proportional to a sense current sensed by said second portion of said SENSEFET device of said low side driver;

a circuit for increasing a gate bias on gates of said SENSEFET device of said low side driver when said sense current falls below a predetermined level; and

a circuit for driving a predetermined current in said SENSEFET device of said high side driver when said sense current falls below said predetermined level, wherein a current of at least about said predetermined level always flows in said SENSEFET devices.

17. The mass data storage device of claim 16 wherein said first and second portions of said SENSEFET devices are two FET device with interconnected gates and drains.

18. The mass data storage device of claim 17 wherein said two FET devices are scaled with a ratio of about 200:

1.

19. The mass data storage device of claim 16 wherein said circuit for sensing a current in said second portion of said SENSEFET device of said low side driver comprises:

a resistor between a source of said second portion of said SENSEFET device of said low side driver and a reference potential, wherein said sense current in said second portion of said SENSEFET device of said low side driver generates a voltage across said resistor; and

a transconductance amplifier connected to compare said voltage across said resistor to a reference voltage to produce a current when said voltage across said resistor exceeds said reference potential.

20. The mass data storage device of claim 19 wherein said circuit for increasing a bias on a gate of said SENSEFET device in said low side driver when said sensed current falls below a predetermined level comprises an input amplifier to control said SENSEFET device of said low side driver, and wherein said input amplifier has an output controlled by said current produced by said transconductance amplifier.

21. The mass data storage device of claim 16 wherein said circuit for driving a predetermined current in said second portion of said SENSEFET device of said high side driver when said sensed current falls below said predetermined level comprises:

a resistor connected at a first end to said second portion of said SENSEFET device of said high side driver;

a current source connected at a first end to a reference potential; and

a transconductance amplifier having first and second inputs, said first input being connected to a second end of said resistor and to a second end of said current source, and said second input being connected to said side of said VCM, said transconductance amplifier producing an output current related to a voltage difference between said first and second inputs to control a gate of said SENSEFET device of said high side driver.

22. The mass data storage device of claim 16 wherein said VCM positions a head for at least reading data in a mass data storage device.