PWM drive system in a current mode.

Abstract: In a current mode, PWM integrated drive system of an external load (VCM) a switched-capacitor amplifier with a Sample & Hold stage connected in cascade thereto as the current sensing amplifier of the control loop, overcomes the criticality of inevitable resistive mismatchings, thus permitting a scaling down of the integrated drive system while preserving a high precision.

Description: The present invention relates to integrated switching current mode drive systems of an external load and in particular to a PWM drive system, scaleable in terms of supply voltage and size of the integrated circuit while retaining a high accuracy. Rotation of mass memory components such as floppy discs, CD Rom, PC, CD readers and similar devices that are often present in battery powered portable apparatuses, takes place by the use of electric motors electronically switched by completely integrated driving systems. The document US-A-4 066 945 describes a linear driving circuit including a full bridge drive circuit across which a d.c. motor is connected. Each side of the bridge circuit has a feedback control loop wherein the currents through each half of the bridge circuit are compared to provide current feedback. A typical diagram of a current mode, pulse width modulation (PWM) switching drive system is shown in Fig. 1, for a generic actuator or external load, which in the illustrated example is a so-called Voice Coil Motor (VCM). The current control loop closes itself on the sum node A where the balance occurs of the currents imposed by the control voltage Vdac through the connecting resistor R1 to node A and by the voltage Vsense output by a sense amplifier of the current though the load, referred to as a whole as Current Sense, through the connecting resistor R2 to node A. Therefore, the loop comprises a differential error amplifier between the reference voltage Vref and the voltage on the node A. Therefore, the feedback voltage closing the control loop is derived from the voltage drop on the sense resistor R3, functionally connected in series to the load VCM. It is easily noticed that in such a system, if the differential voltage on the R3 resistor terminals, that is, Vin1-Vin2, is comparable to the level of the reference voltage Vref, in the resistive series Ra, Rb will flow a current of a relatively small value and therefore an eventual mismatching between the resistive ratios Ra/Ra', Rb/Rb' will not cause sensible inaccuracies in the control system. By contrast, if the differential voltage on the sensing resistor R3 is remarked markedly different from the reference voltage, a mismatching of the actual values of the above mentioned resistors will introduce inaccuracies. In a typical system like that depicted in Fig. 1, in which the motor's winding is driven through a single bridge with a supply voltage that usually may be of 12V, also the circuitry that realizes the control loop is supplied at 12V. The current sense amplifier, receives at input signals with a common mode ranging between 0 and 12V and its output is referred to a half of the supply voltage. Commonly, the current sense amplifier is realized using a properly feedback operational amplifier OP, as illustrated in the diagram of Fig. 1. It carries out a conversion of the differential signal into a "single-ended" output signal Vsense and as a consequence there exists the problem of the output offset in respect to the reference voltage Vref. Such an offset is caused by mismatching of resistances. By calling V + the voltage on the positive terminal, the current through Rb is given by I = (V + - Vref)/Rb Supposing an offset-free operational amplifier, the voltage on the negative terminal is the same. In absence of an input differential signal, the current that flows in the two branches is the same, hence the output voltage is given by Vo = V - - I * Rb' = V + - (V + - Vref) * (Rb'/Rb) If the matching of resistances is perfect (Rb = Rb'), the output voltage is exactly equal to the reference voltage. By supposing a resistive mismatching (Rb=R, Rb'=R+ΔR), the output voltage is equal to Vo = V - - I * Rb' = V + - (V + - Vrif) * (1 + ΔR/R) By considering, in the best of cases, a matching within 1% of the resistances and a difference of 6V between the common mode signal at the bridge terminals and the reference voltage, the voltage offset on Vsense will be of about 60mV. The need to reduce consumption and the size of such integrated systems, dictated by the spreading of battery powered apparatuses, imposes a scaling down of supply voltages as well as of the size of integrated devices. However, when because of such a scaling process the level difference between the reference voltage Vref and the differential voltage present at the inputs of the current sense amplifier, is accentuated a typical control system as that shown in Fig. 1 tends to become inaccurate because of the increase of the difference between the level of the common mode signal on the bridge terminals and the reference voltage level. PURPOSE AND SUMMARY OF THE INVENTION Confronted with these difficulties, an effective solution has now been found to the increasingly critical aspect of mismatching when going through the scaling down of a switching current mode integrated drive system, as well as providing for other important advantages in terms of enhanced performances of the whole driving system. According to the present invention, these important results are obtained by employing as a current sense amplifier a switched-capacitor amplifier composed of an operational amplifier alternately configured as a buffer and as a charge transfer circuit among capacitors by dedicated switches controlled by a pair of complementary control signals and provided with a Sample & Hold output stage. The system is more easily controlled, and it is possible to command the sampling of the differential voltage (V1-V2) at an instant conveniently distant from the switching transients. In this manner the control loop is practically made insensitive to the common mode signal of the voltage drop monitored on the current sensing resistor functionally connected in series to the external load of the integrated driving system. A system made according to the present invention may be scaled down almost unlimitedly without any loss of accuracy caused by an increased difference between the levels of the compared voltages. The current sense amplifier, realized in a switched-capacitor technique ensures a theoretically infinite common mode rejection and hence a total cancellation of the offset due to inevitable mismatchings of integrated resistances. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 depicts the typical circuit diagram of a current mode switching drive system according to the known practice, as discussed above. Figure 2 shows the circuit diagram of a current mode switching drive system made according to the present invention. DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION By referring to the diagram of Fig. 2, during phase Φ1, with the switches swA, swB and sw1 closed, the sampling of the voltage on the sensing resistor R3 is performed and the operational amplifier OP is configured as a buffer (unity gain). In this phase the output, that is, the voltage Vsense, reaches the voltage Vref reference, C1 is charged to Vin1-Vref and C2 is charged to Vin2-Vref. During phase Φ2, the two capacitances C1 and C2 are connected in series and find instantaneously at their terminals the voltage that existed on the sensing resistor R3. The charge instantaneous stored by C1 and C2 is equal to ΔQ = (Vin1 - Vin2) * (1/C1 + 1/C2) The feedback of the operational amplifier OP tends to being the negative terminal to Vref and the charge stored in C1 and C2 transfers itself into C3. In this phase the output Vsense' undergoes a voltage shift by ΔV = ΔQ/C3 = [(Vin1 - Vin2) * (1/C1 + 1/C2)] / C3 By choosing C1=C2=C and C3=C/2, the output voltage (Vsense) of the operational amplifier OP charges by exactly the voltage existing on the sensing resistor R3. In order to retain this value of Vsense on the output of the current sense amplifier, a sampling and storing stage implementing a so-called Sample & Hold function is added in cascade of the operational amplifier OP.

Claims: An integrated current mode switching drive system of an external load (VCM) comprising a control loop closed on an input node (A) of a differential error amplifier (Error Amplifier) comparing a reference voltage (Vref) with a control voltage (Vdac), on which node (A) add, balancing each other, a first current forced by said control voltage (Vdac) through a first resistor (R1) and a second current forced by the voltage (Vsense) output by a current sense amplifier (Current Sense) of the voltage drop on a current sensing resistor (R3), connected in series to said external load (VCM) through a second resistor (R2), wherein the level of said reference voltage (Vref) is relatively smaller than the level of the differential voltage signal existing at the inputs of said current sense amplifier, and characterized in that said current sense amplifier is a switched-capacitor amplifier composed of an operational amplifier (OP) configured alternately as buffer and as a charge transfer circuit by switches (swA, swB, swC, sw1) controlled by a pair of complementary control signals (Φ1, Φ2) and of a sampling and storing output stage (Sample & Hold).

Inventors: BRAMBILLA DONATELLA, SALINA ALBERTO, BRAMBILLA, DONATELLA, SALINA, ALBERTO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1997
Main classification: H02P7
IPC codes: H02P25, H02P7