DC-DC boost converter for driving a capacitive load

Abstract: A DC-DC boost converter for directly driving a capacitive load employs four switches for cyclically commuting the connection configuration of an energy storing inductor. First and second switches are driven at a relatively high frequency and provide an impulsive charge path of the inductor by connecting one or the other end to a power supply rail. Third and fourth switches are driven in phase opposition to each other at a relatively low frequency and provide a discharge path from one and the other end of the inductor, respectively, toward an output node of the circuit to which the capacitive load is connected.

Description: The present invention relates to an improved boost converter, particularly suited for directly driving a capacitive load at a relatively high voltage. The circuit is particularly suited for integration, being capable of delivering a voltage that is substantially twice the intrinsic breakdown voltage of the integrated structures that would compose the circuit. The necessity of driving or powering components such as electroluminescent diode displays (EL LAMPS) and more in general low power actuators or indicators functioning at relatively high voltage, for example at voltages comprised between 60V and 120V, is often encountered in electronic apparatuses. Reduction of energy consumption is an attendant requisite that becomes of fundamental importance in battery operated portable apparatuses, for example electronic watches, pocketable calculators, computers, etc.. The use of voltage multipliers for generating high voltages, starting from particularly low supply voltages, for example 1.2V battery voltage, is practically impossible because of a progressive increase of losses with the number of stages. In these cases, DC-DC converter circuits commonly referred to as boost converters, are employed. A boost converter is functionally constituted by an energy storing inductor, connected to the supply and momentarily driven toward ground by a switch, during a charging phase . The energy stored in the inductor is then discharged during a successive discharge phase through a diode to an output capacitor, across the terminals of which an incrementally increased voltage may be developed in function of the number of cycles of discharge of the reactive energy stored in the inductor. Normally a boost converter is completed by a control circuit that regulates the voltage developed across the charge storing capacitor by appropriately driving the switch. The charge capacitor or capacitors provide a high voltage supply to user circuits or to components that normally require a high voltage. In case of components that may be driven at a high voltage and which may be electrically assumed to be equivalent to a capacitance having a more less high loss-factor, as for example electroluminescent diodes, it is a common practice to directly drive the load element with the converter circuit, by inverting, by the use of pairs of swiches, driven in phase opposition by complementary driving signals at a relatively low frequency, the connections of the two terminals of the load (that may be considered as a capacitor) to the cathode of the discharging diode of the inductor and ground. As an example, such a direct driving circuit for an electroluminescent diode is disclosed in US-A-4,527,096. By fixing the number of switching pulses that are fed to the control terminal of the switch that is used to charge the inductor, during a phase of configuration of the connections of the electroluminescent diode, it is possible to predefine the maximum peak-to-peak voltage that is applied to the capacitive load at the end of each configuration phase. In practice, a succession of stepped ramps of voltage of alternating sign are applied to the load. The converter is capable of providing a high voltage boost ratio with small energy losses and therefore with a high deficiency, an aspect of fundamental importance in battery powered applications. Such a DC-DC boost converter circuit may be realized by employing discrete components and/or employing electromechanical relays for implementing the switches necessary for cyclically inverting the load connections. Alternatively, the converter circuit may be realized in the form of an integrated circuit, by employing electronic switches in the form of bipolar or field effect transistors with obvious advantages in term of miniaturization capabilities. This possibility is disclosed in US-A-4,527,096, wherein it is remarked the need of implementing the integrated circuit with a fabrication process for high voltage devices, in view of the fact that the relative integrated structures must be capable of withstanding a reverse voltage equal to the maximum-to-peak voltage that is generated by the boost converter across the capacitive load. In other words, the integrated circuit must have brakdown characteristics that are sufficiently higher than the value of the maximum peak-to-peak voltage produced by the circuit. WO 91/17537 discloses an integrated BIMOS circuit for driving an electroluminescent display from a low voltage source, wherein bipolar transistors are employed for enabling more efficient MOS transistors before being disabled at power-on. It is known that the architecture of integrated circuits may be adapted for achieving particularly high breakdown voltages as imposed by design needs, and which in the specific case of a boost converter circuit that can be used for directly driving a capacitive load, as the one represented by an electroluminescent diode, may reach exceptionally high values. This requirement, though not precluding the possibility of realizing the boost converter in the form of a monolithically integrated circuit, imposes the realization of sufficiently sturdy integrated structures for withstanding a reverse voltage that may reach up to about 120V. This imposes adequate sizes, thicknesses, depth of junctions and/or the implementation of particular structures and arrangements suitable to increase the breakdown voltages. In general these requisites determine limits to the miniaturization of the integrated circuit itself, which normally contains the whole functional circuit of the apparatus, beside the boost converter for driving an external load. Therefore there exists a need or utility for a boost converter, suitable for directly driving at a relatively high voltage a capacitive load, having a simplified circuit and which at the same time is readily integratable and, in such a monolithically integrated form, also permits a marked miniaturization of the whole integrated circuit. The boost converter circuit for driving a capacitive load object of the present invention employs four switches for delivering across the terminals of a capacitive load, an alternating sequence of positive and negative voltage ramps, at a first control frequency. Each voltage ramp is produced through a certain number of charge and discharge of an inductor on the capacitive load, at a second switching frequency. A control circuit generates the control signals at said first control frequency and at said second switching frequency. The first control frequency may be in the order of 20-100 hertz (Hz), while the second switching frequency may be in the order of kilohertz (kHz). In practice, each terminal of the inductor is connected to a charging switch and to a discharge path that comprises a switch and a diode in electrical series with each other, connected between the terminal of the inductor and the output node. The two discharge paths are alternately enabled by a pair of complementary signals at said first frequency, which close one or the other of the two switches that enable the respective discharge path toward the output node to which an external capacitive load is connected. Coherently, a charging switch of the inductor is driven to switch at a relatively high frequency (said second switching frequency) while another charging switch remains open during the entire semiperiod. During the following semiperiod, the circuit configuration is reversed, so as to produce a ramp of opposite polarity to the preceding ramp. The voltage ramps that are produced have a high frequency ripple, corresponding to the switching frequency of the respective charging switch of the inductor. The different aspects and advantages of the circuit of the invention will be more evident through the following description of an important embodiment and by referring to the attached drawings, wherein: Figure 1 is a circuit diagram of a boost comparator of the invention; Figure 2 is a timing diagram of the circuit of Fig. 1; Figure 3 shows the functional configuration of the circuit during a phase of production of a negative voltage ramp across a capacitive load; Figure 4 shows the functional configuration of the circuit during a phase of production of a positive voltage ramp across the load. With reference to Fig. 1, the L inductor is connectable through the charge switches SWA and SWB, respectively to the supply rail VDD and ground. Moreover, each of the two terminals of the inductor L may be alternately connected to the output node V OUT through the two distinct paths SWC-D1 and SWD-D2. The driving of the switchings SWA, SWB, SWC, SWD, may be implemented by suitable generators of control circuits IA, IB, IC and ID. By supposing, conventionally, that to a high logic value of the respective control signal corresponds a state of closing of the relative switch, the control signals of the circuit are shown in Fig. 2. Of course, the control signals may be generated by a suitable control circuit (not shown in the figure), which may produce the complementary pair of low frequency signals IC and ID, as well as the high frequency switching signals IA and IB that drive the charge switches SWA and SWB, by deriving them from a system clock signal, by the use of frequency dividing circuits. Operatively, during a phase (semiperiod) of the control signals IC and ID, the functional circuit will be configured as depicted in Fig. 3 and across the load C a negative voltage ramp will develop, as shown. During a subsequent phase (semiperiod), the functional circuit will be configured as depicted in Fig. 4, and a positive voltage ramp will develop across the load C. The whole circuit, with the exception of the storing inductor L and obviously of the external load C, can be integrated. Although the structure of the four switches SWA, SWB, SWC and SWD has been shown as composed substantially by a bipolar type transistor, it may be realized also by employing a field effect transistor, for example by employing integrated CMOS structures. Of course, as will be evident to a skilled person, the node V OUT of the circuit, will be subject to a negative voltage, in practice, in an integrated circuit designed for a single supply, for example for a positive supply, the node V OUT will be subject to a so-called "below ground voltage" , therefore the integrated structures that are directly connected to the output node V OUT , in particular the diodes D1 and D2, must be realized so as to withstand a certain negative voltage, as referred to the ground potential (substrate) of the integrated circuit. This may be easily obtained by employing for both D1 and D2, an integrated structure commonly known as a low leakage diode (LDD), which may be readily realized also in a basic CMOS fabrication process, as described in the Italian patent No. 1,188,609, of the same Applicant.

Claims: A DC-DC boost converter for directly driving a capacitive load (C), comprising an energy storing inductor (L), a first switch (swB) connected between a first terminal of the inductor (L) and ground, a discharge path of the inductor (L) from said first terminal thereof through said load (C) connected during operation between an output node (V OUT ) and ground, comprising a first diode (D2) and a second switch (swD), functionally connected in series, and characterized by comprising a third switch (swA) connected between a second terminal of the inductor (L) and a supply rail (Vdd); a second discharge path from said second terminal of the inductor (L) and said output node (V OUT ), comprising a second diode (D1) and a fourth switch (swC), functionally connected in series; control circuitry for driving said first and third switches (swB, swA) in phase opposition at a switching frequency essentially higher than the frequency at which said second and fourth switches (swD, swC) are driven in phase opposition, respectively, during a semiperiod of closure of said fourth switch (swC) and during a semiperiod of closure of said second switch (swD). A boost converter as defined in claim 1, characterized by the fact that said capacitive load (C) is an electroluminescent diode. A boost converter according to claim 1, wherein said high switching frequency is comprised between 20 and 100 Hz and said lower switching frequency is higher than 1 kHz. A boost converter according to claim 1, wherein said switches (swA, swB, swC, swD) are bipolar transistors. A boost converter according to claim 1, wherein said switches (swA, swB, swC, swD) are field effect transistors.

Inventors: ROSSI DOMENICO, TANAKA KAZUYUKI, ROSSI, DOMENICO, TANAKA, KAZUYUKI
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1993
Main classification: H02M1
IPC codes: H02M3