Power switching output stage with dead zone minimization and possibility of parallel synchronous connection

[0002] The present invention relates to the field of power switching output stages, and more particularly, to amplifiers operating in Pulse Width Modulation (PWM). The invention is particularly useful for multichannel audio amplifiers that are configurable in different modes of operation by connecting their respective output power stages in parallel according to different schemes.

[0003] In many applications, particularly in the field of power amplifiers for low and medium frequency signals, and primarily in the field of high efficiency audio amplifiers, a Pulse Width Modulation with switching frequencies of up to several hundreds of KHz is often a preferred approach. In these applications, the output power stage includes at least a power device switching the output to the supply line (HIGH SIDE DRIVER) and at least a complementary power device, switching the output to the ground line (Low SIDE DRIVER), driven by a digital PWM signal that may have a switching frequency of several hundreds of KHz.

[0004] PWM power stages are often realized monolithically on the same chip that contains the drive and control circuitry, and it is necessary to ensure that the turn-on of one or the other of the two power switching devices, respectively of one and the other switching branch of the output node, never happens before the other device has already started the turn-off. If so, this would increase the power dissipation in the superposition intervals and, because of the high switching frequency, could lead to the failure of the power devices.

[0005] To prevent such a destructive condition from occurring, a small time interval must be established, during which both power transistors of the output stage are turned-off. Such an interval is commonly referred to as “dead zone” or “dead time”. In all the applications where it is necessary to ensure a high accuracy (such as in low distortion audio applications), the dead time should be minimized for obvious reasons.

[0006] Commonly, to avoid a simultaneous turn-on of the two branches of a output power stage, a dead zone for prevention of accidental overlapping of the turn-on commands for the two complementary devices of the stage is determined, by suitable delay circuits, based on RC networks dimensioned to ensure a certain delay, that is established by considering the worst condition of operation. In practice it is very difficult or almost impossible to fully optimize the delay value introduced in the turn-on commands because of the impossibility of effective electrical values of the manufacturing process of the integrated circuit, of their temperature dependence as well as of the variability of the load and other parameters that may affect it in a more or less marked manner.

[0007] The known solutions do not allow an optimization for minimizing the dead time value to be introduced in the turn-on signal of the complementary pair of power transistors of the output stage. moreover, in many applications, as in high efficiency audio amplifiers, in order to ensure most efficient driving conditions, switching from a bridge to single ended driving modes and vice versa is often implemented.

[0008] The present invention offers an effective approach in the form of a particularly efficient and versatile power switching output stage in order to optimize the requirements set forth above.

[0009] The circuit of the invention provides a power stage which is exempt of the cross-conduction phenomenon (simultaneous conduction of the two power switching devices) and able to automatically adapt itself to any intervening variation of electrical parameters due to the spread of the manufacturing process of temperature and of the load. The circuit of the invention allows a plurality of output stages to be externally connected in parallel, according to a selected scheme in a simple and programmable manner, while ensuring a perfectly effective control of the switching conditions of the power devices from an off state to a conduction state, in any configuration.

[0010] Different paralleling schemes of the output stages of different amplification channels, according to a certain number of admitted configurations, are made possible or rendered easier by the present invention. For example, to obtain high output power from audio reproduction apparatuses installed in vehicles, loudspeakers of a relatively low resistance typically on the range of 1-0.5 Ohm can be used, while in other applications of lower output power and a larger number of channels, 4 Ohm loudspeakers can be used.

[0011] The different aspects and advantages of the invention will become more evident through the following description of several embodiments and by referring to the attached drawings, in which:

[0012] FIGS. 1, 2, 3 and 4 illustrate different configurations in which a high efficiency monolithic amplifier can be set;

[0013] FIG. 5 is a basic circuit diagram of a power switching output stage according to the present invention;

[0014] FIGS. 6aand 6bshow the mechanism by which the turn-on commands of the two complementary devices of the power stage are conditioned by the circuit of the invention;

[0015] FIGS. 7aand 7bshow a detailed basic scheme of detection of the turn-off or turn-on state of the respective power switching device of the output stage and the relative block diagram of the integrated stage;

[0016] FIG. 8 is a scheme of a switching output stage of the invention suitable to be paralleled with two other stages;

[0017] FIG. 9 is a connection scheme for three output power stages of FIG. 8;

[0018] FIG. 10 is a basic diagram of an output power stage of the invention in which the supply voltage of the output stage is higher than the supply voltage of the control logic circuitry; and

[0019] FIG. 11 is a generalized block diagram of the power output stage of the invention.

[0020] Considering the case of a high efficiency switching audio amplifier as an example of an application of the invention, FIGS. 1, 2, 3, and 4 illustrate the block diagram of a monolithic high efficiency amplifier, highlighting four different configurations made possible by the present invention for adapting the amplifier to different types of loads represented by one or more loudspeakers in the examples.

[0021] The need to ensure a safeguarding turn-on control of all the pairs of complementary power devices of the different output stages, considering the several possible different operating configurations of the output stages of the amplifier, requires certain characteristics for any effort of optimization because the worst operating condition expectable in any one of the different configurations that the amplifier can assume must be accounted for. The circuit of the invention overcomes these evident difficulties securing an effective minimization of the dead zone or turn-off dead time of the power devices that is preserved by a self-adapting ability to the variations of the important parameters.

[0022] A basic diagram of the circuit of the power switching output stage of the invention is shown in FIG. 5. Generally, each one of the pair of power switching complementary devices constituting the HIGH SIDE DRIVER and Low SIDE DRIVER stages, respectively, is provided with a circuit capable of monitoring the turn-on or the turn-off state, producing a respective logic signal, sens-H and sens-L, and indicating the state of the switching device. The pair of logic signals is combined with the digital driving input signal (INPUT) by a combinatory logic circuit, realized in the example by an AND gate switching the Low SIDE DRIVER device to ground potential and an OR gate switching the HIGH SIDE DRIVER device to the supply line. This ensures the switching from a cut-off state to a conduction state of one or the other power-switching device only if it is enabled to do so by a positive verification of the substantially attained turn-off state of the switching device complementary to the switching device commanded to turn-on.

[0023] FIGS. 6aand 6brespectively show how the turn-on enabling logic signal pair, sens-H and sens-L, is generated as a function of the verification of the respective driving signal reaching a level appropriate to acknowledge that a cut-off phase of the complementary device has already started. Such a detection may be effectively implemented for example in the way illustrated in FIGS. 7aand 7b.

[0024] The detailed circuit diagram of FIG. 7a, highlights a preferred embodiment of the devices employed for detecting the state of the power devices Qp1 and Qp2 constituting the output stage. Each power device has an associated transistor of the same kind, respectively Qs1 and Qs2, but having fractional dimensions, which may be between 1/100^thand 1/100^thof the dimensions of the respective power device Qp1 and Qp2. Each detecting transistor has an associated bias load current generator Iref-H and Iref-L. The output node of the stage so constituted is coupled to the input of a logic inverter I1 and I2.

[0025] The detection transistor, Qs1 and Qs2, is driven in parallel with the respective power transistor, Qp1 and Qp2, and when the current exceeds the current established by the respective bias current generator, Iref-H and Iref-L, the respective inverter I1 and I2, switches to generate an output logic signal, respectively sens-H and sens-L, as illustrated in the diagrams of FIGS. 6aand 6b.

[0026] Establishing a certain current level, definable by dimensioning the respective current generators Iref-H and Iref-L, suitable to consider as practically off the respective power switching device Qp1 and Qp2, according to the schemes indicated in FIGS. 6aand 6b, a pair of logic signals is obtained. Sens-H and sens-L reliably confirm a substantially attained cut-off phase of one or the other of the two power switching devices of the stage. The two logic signals, sens-H and sens-L, combined to the driving digital signal INPUT, ensure the absence of overlapping of turn-on states of the two power devices of the stage, under any operating condition, being substantially unaffected by process spread, change of configuration, variations of temperature and load conditions.

[0027] In the case of several power output stages, configurable in different modes to adapt the output of the amplifier to a different number of driven loads or to loads of different values, the combinatory logic circuit can be easily adapted depending on the number of output stages to support different paralleling schemes. Details of the combinatory logic circuit for each power stage of a pair of output power stages that may be connected in parallel to drive in a synchronous mode a single load, are depicted in FIG. 8.

[0028] FIG. 9 shows a paralleling scheme of three final stages. For sake of simplicity, in FIGS. 5, 7 and 8 it is assumed that the combinatory logic circuit and the inverters I1 and I2 generating the pair of logic signal sens-H and sens-L, be powered at the same supply voltage Vcc of the output stage power. In many instances this may not be true, being relatively common to supply the output power stages with a voltage advantageously higher than the supply voltage of the control logic circuits and in general of the modulation and drive circuits.

[0029] In these cases it is often necessary to make the combinatory logic circuitry of the output power stage of the invention compatible with high voltage signals by the use of appropriate level shifting stages capable of withstanding the relatively high supply voltage Vcc of the output stage. In general, according to a common technique, when a logic signal must propagate from a high voltage section to low voltage section, it is necessary to interpose level shifting stages capable of withstanding the full supply voltage.

[0030] A modified diagram of the output power stage of the invention, provided with level shifters: LEVEL SHIFTER 1, LEVEL SHIFTER 2 and LEVEL SHIFTER 3, is shown in FIG. 10. As depicted, the logic gates driving the Low SIDE DRIVER are supplied between the node VLOGIC and ground while the sections driving the HIGH SIDE DRIVER are supplied between the Vcc linr and the VCC-VLOGIC node.

[0031] Of course, the combinatory logic circuit for conditioning the turn-on of the power output stage complementary devices may be realized using different logic circuits equivalent to each other according to the boolean algebra rules, and the signs of the outputs and of the inputs sensing the cut-off state or the turn-on state of the power switching devices can be complementary to those illustrated in the figures.

[0032] A more general functional block diagram of a power switching output stage of the invention is shown in FIG. 11, wherein the blocks, HIGH SIDE LOGIC and Low SIDE LOGIC, are respectfully provided with a plurality of logic inputs for as many pairs of logic signals representing the state of the respective power switching devices of a plurality of output power stages, eventually paralleled among themselves.