Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Slope control of leading or trailing edge of rectangular or...
Reexamination Certificate
2000-03-31
2003-07-01
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Slope control of leading or trailing edge of rectangular or...
C327S132000, C327S134000
Reexamination Certificate
active
06586980
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to drive circuits for a power device of a power driving stage, and, more particularly, to a driver circuit operating from a supply voltage (e.g. a charge pump voltage) that is higher than the supply voltage of the power device.
2. Description of Related Art
A driver circuit is generally a relatively low power circuit that drives, or controls, a higher power device. The power device may be part of a power driving stage for a load. An example is a load that is a motor, such as a brushless motor, that provides the motive force for a spindle of a hard disk drive. Similar driver circuits are applied elsewhere, such as in voice coil motor (VCM) systems.
One of the most widely used types of driver circuits in such applications uses a three-phase brushless motor in a configuration in which current energizes respective motor coils using a full wave bridge configuration. The bridge includes two power stages for each phase, so typically there are six power stages, each with a power device. Three of the power stages, and their power devices, are referred as being “low side” stages and devices because they are connected between a motor coil and ground. The other three of the power stages, and their power devices, are referred to as “high side” stages and devices because they are connected between a power supply and a motor coil.
The power devices are operated as switches in a sequence that allows pulses of current to flow from the power supply through a high side power device, a coil of a first of the three stages, a coil of a second of the three stages, and then through a low side power device to ground. This process is repeated in a generally well known manner for the other power devices and coil pairs to achieve three phase energization from a single, direct current, power supply. The switching, or commutation, characteristics of the power devices are very important in achieving good performance from the motor and other favorable characteristics.
Control of the switching of the power devices is performed by a driver circuit for each power device. In the typical use described above with six power stages, there ate three low side drivers and three high side drivers. The power devices may be of a variety of electronic switch devices and the driver circuits are configured suitably for the power devices. Power devices of general application to hard disk drivers, and the like, are each often an MOS (metal-oxide-semiconductor) FET (field effect transistor). One type of such transistors of considerable interest is referred to as a DMOS transistor (double diffusion MOS).
DMOS devices can be readily integrated in chips with other circuitry, including power control circuitry. So it is attractive to have an entire set of drive stages, including all the power devices and all the driver circuits for the power devices, in one chip.
Even where all the power devices are alike, e.g. N channel DMOS devices, it is generally the case that the high side drivers differ from the low side drivers because high side drivers for such power devices often require a voltage, referred to as a charge pump voltage or boost voltage, at a higher voltage level than that supplied by the, power supply for the power stages. By known techniques, a charge pump voltage may be generated from the supply voltage and used by all of the high side drivers. Such an auxiliary supply if present, however, is power limited; the desired voltage can be supplied but at a modest current level.
The field of motion control using integrated signal and power components, the respective requirements of low and high side drivers, and the characteristics sought in applications of motor drivers are described more fully in
Smart Power
ICs, by B. Murari et al;, Eds., 1995, particularly Chapter 5, “Motion Control” by R. Gariboldi, at pp. 225-283, which is herein incorporated by reference for its description of background to the present invention.
As is disclosed, for example from the above-mentioned Gariboldi publication, for applications such as hard disk drives it is of utmost importance to control the output voltage slope in order to reduce electromagnetic interference (EMI). Generally, the slope is desired to be steep, but not so abrupt as to cause any appreciable noise. Drive circuits have therefore generally included slew rate control circuits to achieve fast, smooth transitions.
In a typical slew rate control system, a capacitor is charged and discharged by two current generators. Preferably, one wishes to have the same smooth, linear commutation both in going off-to-on and on-to-off. Also, one wishes to have the gate voltage change over a range from ground, or zero, to the maximum supply voltage, or at least a voltage that assures full turn-on of the power device. The circuitry for doing so is referred to as a voltage ramp generator. It can be achieved, by typical integration techniques, using basic current mirrors, one of a pair of matched PNP bipolar transistors on the high side of the drive and the other of a pair of matched NPN bipolar transistors on the low side of the drive. Each pair of the transistor structures has one with a base-collector connection so the device acts as a diode. The diode is connected to the base of the other matched transistor. In some applications, this can produce good linearity for much of the supply voltage range, but is limited by collector-emitter saturation voltages near ground and near the positive voltage. Generally, problems in achieving the desired linearity increase as the supply voltage is increased. The greater precision with which linearity is achieved means that less noise can occur to affect the driver or its load.
An approach for attaining linearity at higher voltages than that for which the basic current mirror is suitable would be to use cascoded current mirrors. A description of basic cascoded current mirrors and their use in constant-current stages is contained, for example, in
Bipolar and MOS Analog Integrated Circuit Design
by Alan B. Grebene, Sec. 4.1, pp. 170-183, which is herein incorporated by reference. However, a single cascoded solution is not effective because it is not capable of ramping down to zero volts. The PN junction effects of the cascode-connected transistors mean an inherent higher lower voltage limit. The inability to go to zero volts is unacceptable for a high performance drive.
Similar problems are encountered with current mirrors or cascoded current mirrors made up of MOSFET (metal-oxide-semiconductor-field-effect-transistor) devices. Basic MOSFET current mirrors are also limited as far as providing good linearity in ramping with voltages encountered in integrated, circuit charge pump supplies. Cascode connected MOSFETs provide a better degree of linearity but lack the ability to ramp down to a zero level to ensure turn off of a power device. There are inherent gate to source voltage drops of MOSFETs that prevent a satisfactory reduction in voltage. The book of Grebene cited above also describes the nature and use of MOSFET current mirrors, at Sec. 6.2, pp. 271-277, and is herein incorporated by reference.
Referring now to
FIG. 1
, a circuit schematic diagram of a representative prior art voltage ramp generator is shown. A DC supply of a voltage Vcc is applied across a combination of current generators and a capacitor C, which may, for example, be a slew rate control capacitor of a driver circuit. The current generators, also referred to as constant current sources, include a first current source Ic that is connected between the supply and the capacitor C for charging the capacitor. A second current source Id for discharging the capacitor C is connected to the capacitor's high side or charge terminal, as is source Ic, and to ground. A switch Sw
1
is connected in a position to make or break a connection between source Ic and the capacitor C. Switch Sw
1
is activated by command logic signals (COM) applied from other circuitry. A switch Sw
2
is connected to make or break
Cunningham Terry D.
Jorgenson Lisa K.
Larson R. Michelle
Nguyen Long
STMicroelectronics Inc.
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