Patent Application
20130163123
A voice coil motor is commonly used as a positioning actuator in the disk read-and-write head of computer hard disk drives. In general, the voice coil motor is driven by one of the following two different drivers depending on the amount of current necessary to drive the motor: (1) a first linear driver when the current applied to the voice coil motor is small and (2) a pulse-width modulation driver when the current applied to the voice coil motor is large. During operation of such hybrid drivers, mode shifts can occur in which the voice coil motor shifts from being driven by the linear driver to the pulse width modulation driver and vice versa. If, however, the output of the linear driver is not properly matched with the output of the pulse width modulation driver during the mode shift, additional time and power can be required to adjust the driver output, leading to poor operating efficiency.
The subject matter of this disclosure relates to feed-forward compensation of a linear-pulse width modulator hybrid power supply.
In general, one aspect of the subject matter described in this specification can be embodied in a circuit device that includes a linear driver circuit, a pulse-width modulation driver circuit, an oscillator circuit having an output coupled to the pulse-width modulation circuit, and a feed-forward circuit coupled to the oscillator circuit, the feed-forward circuit being configured to adjust an output of the oscillator circuit so that an output of the pulse-width modulation circuit substantially matches an output of the linear driver circuit.
The circuit device can include one or more of the following features. For example, in some implementations, the feed-forward circuit is configured to adjust the output of the oscillator circuit so that the output of the pulse-width modulation driver circuit matches the output of the linear driver circuit when a power source voltage deviates from a predetermined power source voltage.
In some implementations, the feed-forward circuit is configured to set both maximum and minimum amplitudes of an output of the oscillator circuit. The feed-forward circuit can be configured to set the maximum and minimum amplitudes based on a power source voltage. The feed-forward circuit can be configured to set a difference between the maximum and minimum amplitudes approximately equal to a ratio of a power source voltage to a gain of the linear driver circuit.
In some implementations, the feed-forward circuit includes multiple operational amplifiers. An output of a first operational amplifier of the feed-forward circuit can be coupled to a positive input of a second operational amplifier of the feed-forward circuit and to a positive input of a third operational amplifier of the feed-forward circuit. An output of the second operational amplifier of the feed-forward circuit can be coupled to the oscillator circuit to set a maximum output voltage of the oscillator circuit. An output of the third operational amplifier of the feed-forward circuit can be coupled to the oscillator circuit to set a minimum output voltage of the oscillator circuit.
In various implementations, the circuit device further includes a voice coil motor, in which an output of each of the linear driver circuit and the pulse-width modulation driver circuit is coupled to voice coil motor through a switch. The circuit device can further include an error detection circuit to detect a change in current in the voice coil motor, the error detection circuit being coupled to the pulse-width modulation driver circuit and to the linear driver circuit. The linear driver circuit can include first and second linear amplifiers, the output of the error detection circuit being coupled to an input of each linear amplifier. The pulse-width modulation driver circuit can include first and second operational amplifiers, the output of the error detection circuit being coupled to a positive input of the first operation amplifier and to a negative input of the second operation amplifier. The output of the oscillator circuit can be coupled to a negative input of the first operational amplifier and to a positive input of the second operational amplifier.
Another aspect of the subject matter described in this specification can be embodied in a method of adjusting a gain of a pulse-width modulation driver circuit, in which the method includes modifying, using feed-forward circuitry, an output of an oscillator circuit coupled to the pulse-width modulation driver circuit so that the gain of the pulse-width modulation driver circuit substantially matches a gain of a linear driver circuit.
Modifying the output of the oscillator circuit can include changing maximum and minimum output amplitudes of the oscillator circuit. Changing the maximum and minimum output amplitudes of the oscillator circuit can be based on a power source voltage coupled to the feed-forward circuitry. Modifying the output of the oscillator circuit can include adjusting a difference between maximum and minimum output amplitudes of the oscillator circuit. Adjusting the difference between the maximum and minimum output amplitudes can include setting the difference equal to a ratio of a power source voltage to the gain of the linear driver circuit.
In some implementations, the output of the oscillator circuit is coupled to a negative input of a first operational amplifier of the pulse-width modulation driver circuit and to a positive input of a second operational amplifier of the pulse-width modulation driver circuit. The method can further include coupling an output of an error detection circuit to a positive input of the first operational amplifier and to a negative input of the second operational amplifier. The method can further include coupling the output of the error detection circuit to an input of the linear driver circuit.
Another aspect of the subject matter described in this specification can be embodied in a method of driving a voice coil motor, in which the method includes driving the voice coil motor with a linear driver circuit over a first operating range, driving the voice coil motor with a pulse width modulation driver circuit over a second operating range, and modifying, using feed-forward circuitry, an output of an oscillator circuit coupled to the pulse-width modulation driver circuit so that a gain of the pulse-width modulation driver circuit matches a gain of the linear driver circuit.
Another aspect of the subject matter described in this specification can be embodied in a hard disk drive that includes a voice coil motor, a linear driver circuit, a pulse-width modulation driver circuit, in which an output of each of the linear driver circuit and the pulse-width modulation driver circuit is coupled to the voice coil motor through a switch, an oscillator circuit having an output coupled to the pulse-width modulation driver circuit, and a feed-forward circuit coupled to the oscillator circuit, the feed-forward circuit being operable to adjust an output of the oscillator circuit so that a gain of the pulse-width modulation driver circuit substantially matches a gain of the linear driver circuit.
Another aspect of the subject matter described in this specification can be embodied in a device that includes a linear driver circuit, a pulse-width modulation driver circuit, an oscillator circuit having an output coupled to the pulse-width modulation driver circuit, and a feed-forward circuit coupled to the oscillator circuit, the feed-forward circuit being operable to adjust an output of the oscillator circuit so that output currents of the linear driver circuit and the pulse-width modulation driver circuit become the same when the same input voltages are provided to the linear driver circuit and the pulse-width modulation driver circuit.
The different aspects of the subject matter can include various advantages. For example, in some implementations, adjusting the output of a pulse-width modulation driver can be used to match the output of a linear driver such that reductions in amplifier circuit operating efficiencies due to gain mismatch between drivers are reduced or eliminated. In addition, by using a feed-forward circuit to adjust the output of a pulse-width modulation driver, further inefficiencies due to delays in correcting driver output can be reduced.
Other potential aspects, features and advantages will be apparent from the description, the drawings, and the claims.
The linear driver 110 and PWM driver 120 are arranged such that either the output of the first power amplifier 112 or the output of the first PWM amplifier 124 is coupled to the VCM 200 at a first node (“OUT_A”). For example, in some implementations, the output of the first power amplifier 112 and the first PWM amplifier 124 are coupled to the first node through a switch. Similarly, the output of the second power amplifier 114 and the second PWM amplifier 126 also are coupled to the VCM 200 at a second node (“OUT_B”).
The amplifier circuit 100 also includes a current sensing amplifier 130 coupled to a series resistor Rs that is in series with the VCM 200. The current sensing amplifier 130 is operable to detect the current flowing through the VCM 200 based on the voltage measured across the series resistor Rs. The output of the current sensing amplifier 130 is coupled through a feedback resistor Rf to a negative input of error amplifier 140. The positive input of error amplifier 140 is coupled to a reference voltage VRef.
A digital-to-analog converter (DAC) 128 is electrically coupled through resistor Ri to the negative input of the error amplifier 140. The DAC 128 is operable to alter the voltage applied to the error amplifier 140 so that appropriate changes in current to the motor can be obtained. For example, during periods when a change in torque provided by the motor requires a corresponding change in current to the motor, the DAC 128 is operable to adjust the voltage at the negative of the error amplifier 140, as appropriate.
The output of error amplifier 140 is coupled to an input of the PWM driver 120 and to an input of the linear driver 110. In particular, the output of error amplifier 140 is provided as an input voltage Vin both to the positive input of the first PWM amplifier 124 and to the negative input of the second PWM amplifier 126. The VCM 200 is represented in the example of
During operation of the amplifier circuit 100, the linear driver 110 is used to provide current to the VCM 200 over a first range, whereas the PWM driver 120 is used to provide current to the VCM 200 over a second range. The absolute value of the first range of current is smaller than the absolute value of the second range of current. For example, the linear driver 110 can be configured to drive the VCM 200 between 0.5 A and −0.5 A. The PWM driver 120, on the other hand, can be configured to drive the VCM 200 over a range corresponding to larger amount of current, such as currents greater than 0.5 A and less than −0.5 A. The larger current levels may be required, for example, when a disk read-and-write head in a hard disk drive coupled to the VCM 200 moves at high speeds suddenly. Thus, when the current required to drive the VCM 200 is within the first range of current, the linear driver 110 drives the VCM 200. When the magnitude of the current level required to drive the VCM 200 increases from the first range into the second range of current, the circuit 100 switches driving operation to the PWM driver 120.
When the amplifier circuit 100 operates in the second range of current, the PWM driver 120 drives the VCM 200 by outputting a pulse train signal (e.g., a series of square pulses). The average value of voltage (or current) fed to the VCM load depends on the duty cycle and amplitude of the pulse train signal. That is, as the duty cycle of the PWM signal increases from 0 to 100%, the average voltage (or current) supplied by the PWM driver also increases. The duty cycle of the PWM driver output can be controlled by adjusting the value of the reference voltage supplied to each PWM amplifier 124, 126.
In some implementations, the power supply voltage VDD is under a state of burden (e.g., as a result of a change in the current required to drive the VCM 200). When this occurs, the power supply voltage VDD deviates either above or below a nominal value. Such changes in supply voltage typically do not alter the operation of the linear driver 110, which has a constant linear gain curve. In contrast, the slope of the gain curve for the PWM driver 120 typically decreases when the supply voltage falls below the nominal value or increases when the supply voltage rises above the nominal value. As a result, the gain curves of the linear driver 110 and the PWM driver 120 may not be properly matched when there are fluctuations in supply voltage. Such mismatch in gain curves can lead to inefficiencies in power consumption and operating speed of the amplifier circuit 100 particularly when the amplifier circuit 100 switches driver operation from the linear driver 110 to the PWM driver 120 or vice-versa.
For instance,
The change in slope of the PWM driver gain curves occurs when the supply voltage VDD fluctuates. That is, when the power supply voltage VDD changes under a state of burden, the slope of the PWM driver gain curve also changes. To correct for the variation in PWM driver gain curve, a feed-forward circuit can be added to the amplifier circuit 100. Because the variation in the PWM driver circuit gain curve is based on changes in the supply voltage, the feed-forward circuit is configured to detect a change in the power supply voltage and automatically adjust the output of the PWM driver so that the output matches the gain curve of the linear driver.
During operation of the amplifier circuit 300, the feed-forward circuit 400 adjusts, based on the power supply voltage, the difference between maximum and minimum values of a triangle waveform signal produced by the oscillator circuit 122. By adjusting the difference between the maximum and minimum values of the waveform signal, it is possible to cause the current output of the PWM driver 120 to match the current output supplied by the linear driver 110, regardless of the variation in power supply voltage.
The difference between the maximum and minimum values of the triangle waveform signal that cause the output of the PWM driver 120 to match the output of the linear driver 110 can be determined from the gain equations for each driver. The current output, Iout, of the linear driver 110 is given by
where ALin is the gain of each power amplifier in the linear driver 110, RM corresponds to the value of the VCM resistor, and VRef is a reference voltage supplied to each power amplifier. The current output of the PWM driver 120 is given by
I
PWM
□PWM
Duty(VDD/RM), (2)
where
PWM
Duty=2Iin/(VH−VL). (3)
VH and VL correspond, respectively, to the maximum output voltage and minimum output voltage supplied by the oscillator circuit 122. From equations (2) and (3), the gain of one of the amplifiers of the PWM driver 120 is given by
A
PWM□2VDD/2(VH−VL). (4)
From equation (4), the difference between the maximum and minimum output of the oscillator circuit 122 is
(VH−VL)=2VDD/2APWM. (5)
As shown in
Using the example resistor values set forth in Table I below, as well as a reference voltage VRef of 0.75 V, Table II gives examples of the voltages that can be obtained at the different nodes shown in the circuit of
Thus, as shown in TABLE II, the feed-forward circuit 400 is capable of changing the maximum output voltage of the oscillator circuit 122 from 1.25 V (when VDD equals 10 V) to 1.45 V (when VDD equals 14 V). Similarly, the feed-forward circuit can change the minimum output voltage of the oscillator circuit 122 from 0.25 V (when VDD equals 10 V) to 0.05 V (when VDD equals 14 V). Feed-forward circuit 400 is not limited to the example resistor values and reference voltage specified in Tables I and II. Rather, any suitable resistor values and reference voltages that enable the feed-forward circuit 400 to obtain a specified between voltage difference between VH and VL can be used.
An example operation of the amplifier circuit 300 in which the power supply voltage VDD is low (e.g., 10 V) relative to a nominal power supply value (e.g., 12 V), will now be described. For the present example, it is assumed that each of the power amplifiers 112, 114 has a gain ALin approximately equal to ten, the reference voltage VRef supplied to the power amplifiers is about 0.75 V, and the VCM resistor RM has a value of about 10Ω. In addition, each of the power amplifiers 112, 114 has a neutral operating voltage equal to one-half the power supply voltage VDD. That is, when the difference between Vin and VRef at the input to either power amplifier is zero, each amplifier has an output voltage approximately equal to VDD/2.
Assuming, for the purposes of the present example, that the current, Iout, required to drive the VCM 200 is about 1.0 A, equation (1) can be used to solve for the voltage VII, applied to the linear driver 110 from the error amplifier 140. That is, Vin=(RM*IOUT)/(2ALin)+VRef=1.25 V. Equation (1) also can be rearranged to determine the voltage output of the first power amplifier 112. In particular, the power amplifier voltage output for amplifier 112 is given by:
V
OUT1=(Vin−VRef)ALin+VDD/2=5 V+5 V=10 V (6).
In contrast, the polarities of Vin and VRef are reversed for the second power amplifier 114. Thus, the voltage output for amplifier 114 is given by:
V
OUT2=(VRef−Vin)ALin+VDD/2=−5 V+5 V=0 V (7).
The difference between VOUT1 and VOUT2 gives the overall voltage (10 V) supplied by the linear driver 110 across the VCM 200.
When the amplifier circuit 300 switches from the linear driver 110 to the PWM driver 120 at low VDD, the feed-forward circuit 400 is operable to modify the output of the PWM driver 120 so that the gain curves for both the linear driver 110 and PWM driver 120 match.
In particular, the feed-forward circuit 400 automatically adjusts, based on the supply voltage VDD, the maximum and minimum output voltages (VH, VL) of the triangle wave signal produced by oscillator circuit 122. The values of VH and VL, in turn, establish the duty cycle of the pulse train signal output by the PWM driver 120. In the present example, the feed-forward circuit 400 automatically sets the maximum voltage VH to 1.25 V and sets the minimum voltage VL to 0.25 V based on the supply voltage (VDD=10 V), resulting in a 100% duty cycle for the pulse train signal produced by the first PWM amplifier 124 and 0% duty cycle for the second PWM amplifier 126.
For example,
When the operating current of the VCM 200 changes, the error amplifier 140 automatically adjusts Vin and thus changes the duty cycle of the pulse train signals produced by the amplifiers of PWM driver 120. For example, Table III below shows the duty cycles for the first PWM amplifier 124 and for the second PWM amplifier 126 at different currents through the VCM 200.
0
0
An example operation of the amplifier circuit 300 in which the power supply voltage VDD is high (e.g., 14 V) relative to a nominal power supply value (e.g., 12 V), will now be described. For the present example, it is again assumed that each of the power amplifiers 112, 114 has a gain ALin approximately equal to ten, the reference voltage VRef supplied to the power amplifiers is about 0.75 V, and the VCM resistor RM has a value of about 10Ω. The neutral operating voltage for each of the power amplifiers 112, 114 is VDD/2=7 V.
Assuming, for the purposes of the present example, that the current, Iout, required to drive the VCM 200 is about 1.0 A, the voltage Vin is determined using equation (1) to be 1.25 V. The power amplifier voltage output for amplifier 112 is given by:
V
OUT1=(Vin−VRef)ALin+VDD/2=5 V+7 V=12 V (8).
In contrast, the polarities of Vin and VRef are reversed for the second power amplifier 114. Thus, the voltage output for amplifier 114 is given by:
V
OUT2=(VRef−Vin)ALin+VDD/2=−5 V+7 V=2 V (9).
The difference between VOUT1 and VOUT2 gives the overall voltage (10 V) supplied by the linear driver 110 across the VCM 200.
When the amplifier circuit 300 switches from the linear driver 110 to the PWM driver 120 at high VDD, the feed-forward circuit 400 is operable to modify the output of the PWM driver 120 so that the gain curves for both the linear driver 110 and PWM driver 120 match.
In particular, the feed-forward circuit 400 automatically adjusts, based on the supply voltage VDD, the maximum and minimum output voltages (VH, VL) of the triangle wave signal produced by oscillator circuit 122. The values of VH and VL, in turn, establish the duty cycle of the pulse train signal output by the PWM driver 120. In the present example, the feed-forward circuit 400 automatically sets the maximum voltage VH to 1.45 V and sets the minimum voltage VL to 0.05 V based on the supply voltage (VDD=14 V), resulting in an approximately 85.7% duty cycle for the pulse train signal produced by the first PWM amplifier 124 and approximately 14.2% duty cycle for the second PWM amplifier 126.
For example,
When the operating current of the VCM 200 changes, the error amplifier 140 automatically adjusts Vin and thus changes the duty cycle of the pulse train signals produced by the amplifiers of PWM driver 120. For example, Table IV below shows the duty cycles for the first PWM amplifier 124 and for the second PWM amplifier 126 at different currents through the VCM 200 when VDD=14 V.
0
A number of implementations have been described. Nevertheless, various modifications may be made without departing from the spirit and scope of the invention. Other implementations are within the scope of the claims.
