Methods and systems for trimming a PWM signal

Details Methods and systems for trimming a PWM signal Methods and systems for trimming a PWM signal

1997-12-17

2001-05-01

Pham, Chi (Department: 2631)

Pulse or digital communications

Pulse width modulation

C332S109000

Reexamination Certificate

active

06226324

ABSTRACT:

TECHNICAL FIELD
The present invention relates, in general, to pulse width modulation (“PWM”) circuitry. More specifically, the present invention relates to methods and systems for trimming a PWM signal.
BACKGROUND OF THE INVENTION
Many circuits use PWM techniques for a variety of purposes. In one example, a PWM output provides a signal source for receipt by another device. By varying, for example, the frequency and duty cycle of the PWM signal, control of the other device can be affected. One particular example is in the field of motor control. A PWM signal can be used to drive (through appropriate current-drive circuitry) a motor. By varying the duty cycle of the PWM signal, the amount of energy transferred to the motor is changed. This is useful in regulating motor torque and speed. In another example, the PWM signal is passed through a low-pass filter, thereby producing a filtered analog voltage output that varies as a function of the duty cycle of the PWM signal.
By way of definition and background, a PWM (pulse width modulated) signal is one in which the information is contained in the width of each pulse, typically of a repeating string of pulses. It is considered a form of analog signal in that the information is contained in the time duration of a pulse, which is varied continuously, or in such small steps as to be effectively continuous. That is to be contrasted with a digital signal in which information is contained in discrete steps (such as two steps for binary) and in which values are assigned to the various discrete combinational possibilities.
Generally, a set of digital data stored in registers controls the PWM signal parameters such as, for example, duty cycle and frequency. Thus, to alter the duty cycle of the PWM signal, the data value in the appropriate “duty cycle” register is altered. However, it may be desirable to “trim”, or adjust (i.e., lengthen or shorten) the PWM duty cycle without altering the main stored digital duty cycle data value. For example, it may be desirable to trim the duty cycle as a function that is independent of the function which controls the main duty cycle data value.
The present invention is directed toward solutions to the above-identified problem.
SUMMARY OF THE INVENTION
Briefly described, in a first aspect, the present invention includes a PWM generation circuit that includes a main PWM signal generator, a trimming signal general and a summer. The main PWM signal generator includes a main PWM output, while the trimming signal generator includes a trimming output which produces pulses. The summer includes a first input connected to the main PWM output, a second input connected to the trimming output of the trimming signal generator, and a trimmed PWM output. In particular, the trimmed PWM output has a duty cycle of the main PWM output modified as a function of the trimming output.
As an enhancement, the trimming signal generator may include a trimming value input for receiving a trimming value that controls a frequency of the pulses produced by the trimming signal generator. The trimming value may be a function of a measured parameter comprising, for example, temperature.
The circuit may also include a lookup table having an index input and a table data output. In particular, the table data output is coupled to the trimming value input of the trimming signal generator. The lookup table thereby controls the trimming signal generator as a function of the data on the index input and the contents the lookup table. Further, a buffer may couple the lookup table to the trimming signal generator to facilitate table reloading without affecting the trimming signal generator.
As a further enhancement, the PWM generation circuit may extent, preserve, or shorten the duty cycle of the main PWM output in generating the trimmed PWM output. Accordingly, and in particular regard to the summer, it may include at least on counter logically coupled to the trimming output, the main PWM output, and the trimmed PWM output such that the counter records the pulses during an on time of the main PWM output. The summer thereby controls the extending, preserving or shortening of the trimmed PWM output. The at least one counter may be, for example, a synchronous up/down counter or a pair of asynchronous up counters in alternate embodiments.
In the embodiment where the counter is a pair of asynchronous up counters, the circuit also includes steering logic coupled to the trimming value input of the trimming signal generator. The steering logic selectively directs the pulses to one of the pair of asynchronous up counters depending on at least a sign bit of the trimming value presented on the trimming value input.
The circuit may also include a comparator coupled to compare a majority of bits of the pair of asynchronous up counters to each other. An equal output of the comparator is logically coupled to the trimmed PWM output.
More specifically, the main PWM signal generator includes a delayed output for generating a pulse a predetermined time after a rising edge of the main PWM output. Also, the PWM generation circuit includes a set-reset flip-flop having a set input, a reset input, and flip-flop output. The set input is coupled to the delayed output, the reset input is coupled to the equal output of the comparator and the flip-flop output comprises the trimmed output.
As a further enhancement to the PWM generation circuit, the main PWM signal generator, the trimming signal generator and the summer may be within a single integrated circuit, for example, an ASIC.
In accordance with another embodiment, the PWM generation circuit is in combination with a low-pass filter coupled to the trimmed PWM output. Accordingly, the low-pass filter provides an analog voltage output. Further, a voltage to current converter is attached to the analog voltage output of the low-pass filter such that a current output is produced which corresponds to the analog voltage output.
As yet another enhancement, the PWM generation circuit may be included within a process variable transmitter. This transmitter may include an analog output, at least one sensor, conditioning circuitry and a microprocessor. The microprocessor is coupled to the PWM generation circuit and the sensor. The trimmed PWM output of the PWM generation circuit is coupled through the conditioning circuitry to the analog output of the process variable transmitter. The conditioning circuitry may include a low-pass filter and a voltage-to-current converter such that the process variable transmitter may produce a current output.
Methods corresponding to the above-disclosed apparatus are also disclosed herein.
To briefly summarize, the present invention has several advantages and features associated with it. A PWM signal is selectively trimmed in accordance with a trimming value. Trimming may include shortening, lengthening, or preserving the original PWM signal. Thus, precise control of the PWM signal is facilitated, without requiring adjustment of the source of the PWM signal. Compensation of a PWM signal for various factors is thus facilitated, without the requirement to modify the original PWM value itself. This can be utilized in many applications including, for example, temperature compensation of a circuit used to generate an analog voltage output based upon a PWM signal. Advantageously, the microprocessor involved in such a circuit does not carry the burden of computing the temperature compensation function, as it is performable within the circuitry disclosed herein which may be implemented in, for example, an ASIC.


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