System for increasing the definition in converting a digital datum in a PWM signal for driving a full-bridge output stage

Abstract: The definition of conversion of a digital value in a PWM signal using an N-bit up/down counter is improved by increasing the dimension of the input datum to N+2 bits, using the two lsb of the N+2 input datum for selecting one among three intermediate levels between two consecutive values of an N-bit dynamic, according to a predefined table of combinations. The converter may still use an N-bit comparator.

Description: FIELD OF APPLICATION OF THE INVENTION The present invention relates to techniques for driving a generic R-L actuator through an output power stage having a so-called bridge configuration and more in particular to a technique for driving an output bridge stage in a PWM mode thus partitioning the voltage delivered to the actuator and thereby controlling the current flowing through it. TECHNOLOGICAL BACKGROUND The control signal of the PWM driving system may, according to recently developed techniques, be generated by a circuit that transforms a predefined N-bit digital value permanently stored in a nonvolatile memory that can be scanned at a variable clock speed, in a digital signal whose amplitude is compatible with the input requisites of the output power stage, the duty-cycle of which is proportional to the N-bit digital value read from the memory. A conversion system of this nature is described in the European patent application EP-A-0 809 349, of May 22, 1996, in the name of the same applicant where N was 8. A conversion system as those mentioned above and shown in Fig. 1 is based on comparing the input BYTE (N=8) containing the value to be converted with the state of an 8-bit timer functioning in a continuous up/down mode. DISCUSSION OF THE PRIOR ART The above referred conversion circuit is depicted in Fig. 2. By referring to Figures 1 and 2, the sample (N=8 BYTE) to be converted is synchronously loaded in the SL register in order to prevent sample updating during its conversion. The comparator COMP generates a clock impulse for the toggle bistable circuit FF2 each time the CNT counter state equals the value of the sample to be converted. This generates a PWMOUT signal whose duty-cycle varies proportionally to the input sample value and symmetrically in respect to the maximum CNT counting value. However, as it may be observed, the unitary increment (highlighted in an exaggerated manner in Fig. 2) of the input sample value to be converted results in a double and symmetric decrement of the output duty-cycle. For example, going from a sample value 188 to a sample value 189 results in a duty-cycle decrement as depicted by the dashed line of Fig. 2. In driving an output bridge stage by controlling the current in a Phase Shift Modulation mode, according to the method disclosed in the European patent application EP-A-0 760 552, of November 15, 1995, of the same applicant, two digital values must be converted, one for each half-bridge, having a symmetric value about to 2 N /2. In a driving system of this kind, where to a unit increment of the digital signal forced in a half-bridge corresponds a unit decrement of the digital signal that is simultaneously forced in the other half-bridge to maintain symmetry, a double duty-cycle differential increment is produced if compared to the case of a half-bridge output stage. The system of the present invention avoids such augmented duty-cycle increments for unitary variations of the value of input samples in driving a full-bridge. SCOPE AND SUMMARY OF THE INVENTION An object of the present patent application is a circuit that improves the definition of the above mentioned conversion process by increasing from N to N+2 bits the dimension of the input datum, thereby improving the control of the current in the actuator without incrementing the size (number of bits) of the digital comparator. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a BYTE/PWM converter (N=8) as described in the above mentioned patent application. Figure 2 show the signals generated by the circuit of Fig. 1. Figures 3a, 3b, 3c and 3d show respectively the output bridge stage and the drive signals originating from the NBITS/PWM converter. Figure 4 shows the duty-cycle variation that is obtained with a unit variation of the digital value to be converted in a PWM signal. Figure 5 shows the block diagram of the N+2 BIT DATA / PWM converter object of the present invention. Figures 6a, 6b, 6c and 6d show the way the conversion of an N+2 bit input datum takes place for different configurations of the 2 lsb of the original datum. DESCRIPTION OF AN EMBODIMENT OF THE INVENTION As shown in Figures 3a, 3b, 3c and 3d, relative to a PWM driving system of a generic R-L actuator through a bridge output stage according to a Phase Shift Modulation mode as described in the preceding European patent application EP-A-0 760 552 of November 15, 1995, of the same applicant, the two signals IN1 and IN2 are obtained by converting into two PWM signals an original digital datum DATAIN which is considered stored in N+2 bits. To analyze the behavior of such a known system, the N msb of DATAIN will be referred to as VAL whereas the complementary of VAL, in respect to 2 N /2, will be referred to as *VAL+1. In the present contest reference is made to systems where N=8 even though theoretically N may be of any integer value. The condition of null current through the load is obtained when the two input signals IN1 and IN2 are perfectly in phase with each other and with a duty-cycle equal to 50%. This corresponds to input the BYTE/PWM block with two identical values of VAL and *VAL+1 both equal to 128 decimal (2 N /2 in the case on a generic N value). To a unitary increment of VAL corresponds a unitary decrement of *VAL+1 in order to maintain symmetry about the value corresponding to a null current through the load (2 N /2 for the case of a generic N value). As noticed in Fig. 4, in a whole period of the up/down counter there exist four instants of comparison among the values VAL, *VAL+1 and the state or content of the counter. In the example of Fig. 4, the value VAL is incremented from 145 to 146 and, correspondingly, the value of *VAL+1, symmetric about the value 128, is decremented from 111 to 110. This corresponds to a duty-cycle decrement by an amount A+C for the signal N1 and an increment by an amount B+D for the IN2 signal, respectively. The differential duty-cycle DIFF is therefore incremented by A+B+C+D pursuant a unitary increment/decrement of VAL and *VAL+1. Thus, the system permits to have a definition of 1/255 (( 1 / 2 N -1) in the case of a generic N value) for a differential duty-cycle ranging from 0 to 100%. For the case of the known system, the four comparisons take place without any further processing, contrarily, according to the present invention, adequate actions are forced depending on the condition of the 2 lsb of the DATAIN signal to be converted. Advantageously, the system of the present invention permits to obtain a greater definition without increasing the number of bits of the up/down counter (comparator). More precisely, if N is the number of bits of the up/down counter, the system herein disclosed allows for the conversion of an input datum (original datum) mapped on N+2 bits. Fig. 5 shows a block diagram of the system of the invention. The DATAIN value to be converted is mapped on N+2 bits in the memory. Among these, the N msb are indicated with VAL. From the VAL value, by means of a simple logic processing, the complementary value *VAL is obtained and the two digital values are loaded in two registers of N-bits each. Two additional N-bit registers contain the same values incremented by 1. The two lsb of the input value, together with the up/down signal of the counter, are used by the multiplexing block to select the datum to be compared. This selection occurs according to the following table. UP-DOWN BIT 1 BIT 2 DATUM 1 DATUM 2 0 0 0 VAL * VAL+1 0 0 1 VAL * VAL+1 0 1 0 VAL * VAL+1 0 1 1 VAL+1 * VAL+1 1 0 0 VAL * VAL+1 1 0 1 VAL+1 * VAL+1 1 1 0 VAL+1 * VAL 1 1 1 VAL+1 * VAL As shown in Fig. 5 and in TABLE 1, the two least significative bits of the input datum are used by a digital circuit to identify three intermediate levels between two consecutive values representable by an N-bit dynamic. In the case of N=8, VAL equal to 145 and, consequently, *VAL+1 equal to 111, the following situation exists: 8msb bit1 bit0 effective value Resulting Dcxxx 145 0 0 145 DC145 145 0 1 145.25 DC145+A 145 1 0 145.5 DC145+A+B 145 1 1 145.75 DC145+A+B+C Thus, the two supplementary lsb are used by a dedicated digital circuit to decide of "how much" the base duty-cycle (identified by DC145 and corresponding to the two Isb equal to zero) should be incremented or decremented. Naturally the following relationship applies: DC145+A+B+C+D = DC146 Hence, the system functions in a dynamic manner generating a different duty-cycle depending on the value of the two lsb of the datum to be converted and of the counting direction of the up/down counter. This results in a definition which is four times higher than that in the case of the known system taken as a comparative example, while using a comparator of the same number of bits. Thereby the comparator thereby generates the two IN1 and IN2 signals that drive the output power stage. From TABLE 1 and Figures 6a, 6b, 6c and 6d, the way the intermediate levels between two consecutive VAL values are obtained may be observed. For example, by assuming VAL=145, then: VAL=145 VAL+1=146 *VAL=110 *VAL+1 = 111 In the case of converting 145 (Fig. 6), the fractionary term is null because BIT0=0 and BIT1=0. From TABLE 1 and from Fig. 6a it may be noticed that DATO1 and DATO2 are forced respectively to VAL and to *VAL+1, during the timer "UP COUNTING" as well as during "DOWN COUNTING". The result of this conversion is observable in Fig. 6a. If 145.25 (Fig. 6b) must be converted, then BIT0=1 and BIT1=0. In this case there exists a modulation of DATO 1 resulting in the conversion of VAL+1 when the timer is counting "UP" and the conversion of VAL when the timer is counting "DOWN". The result of this conversion is illustrated in Fig. 6b. In case of converting 145.50 (Fig. 6c), BIT0=0 and BIT1=1. In this case there is a modulation of both input data namely of DATO1 and DATO2, which implies the conversion of VAL+1 and *VAL when the timer is counting "UP" and the conversion of VAL and *VAL+1 when the timer is counting "DOWN". The result of this conversion is depicted in Fig. 6c. In case of converting 145.75 (Fig. 6d), BIT0=1 and BIT1=1. In this case there is a modulation of DATO2 resulting in the conversion of *VAL when the timer is counting "UP" and the conversion of *VAL+1 when the timer is counting "DOWN". By contrast, DATO1 is always equal to VAL+1. The result of the conversion is depicted in Fig. 6d.

Claims: Method for increasing the definition of conversion of a PWM signal using an up/down N-bit counter, characterized in that it comprises a) increasing a dimension of an N-bit digital value to an N+2 bit digital value, the N+2 bit digital value having two least significant bits and N most significant bits; b) using the two least significative bits to select one among three intermediate levels between two consecutive values representable by an N-bit STRING according to a predefined table of combinations; c) employing the N most significative bits to store four values in respective ones of four registers, the four stored values comprising the N-bit input digital value, a complement of the N-bit digital value, the N-bit digital value incremented by 1, and a complement of the N-bit digital value incremented by 1, respectively; and d) employing said two least significative bits and an up/down output signal of said up/down N-bit counter to select a pair of the four stored values to be compared during each up-counting phase and down-counting phase of the up/down N-bit counter, as a function of said predefined table of combinations. A system for converting an N-bit digital value into a signal for a power ouput stage driving PWM an inductive load, said system comprising a register of the N-bit digital value , an N-bit comparator, an N-bit up/down counter, reset means, enabling means of at least a bistable circuit generating said PWM signal as a function of the output status of said N-bit comparator, characterized in that said register of the input digital value has a capacity of N+2 bits, and in that the system further comprises four additional registers for storing four values comprising the N-bit digital value, a complement of the N-bit digital value, the N-bit digital value incremented by 1, and a complement of the N-bit digital value incremented by 1, respectively; and a multiplexer for selecting a pair of said four stored values in said four additional registers to be compared in said N-bit comparator when said up-down N-bit counter is counting up and is counting down based upon a combination of values of the two least significative bits of said N+2 bit digital valve and by an up/down signal of said up/down N-bit counter.

Inventors: GALBIATI EZIO, MAIOCCHI GIUSEPPE, GALBIATI, EZIO, MAIOCCHI, GIUSEPPE
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1997
Main classification: H03M1
IPC codes: H02P6, H03M1, H02P7, H02M7