Driver circuit for P-channel MOS switches

Abstract: The invention relates to a driver circuit for P-channel MOS switches, of the type comprising: a power transistor (2) having a control terminal (G) and having first (D) and second (S) conduction terminals; a controlled current generator (ION) connected to said control terminal (G) for turning on the power transistor (2); a control circuit (5) for controlling the turning on of said current generator (ION); and a protection circuit (3) coupled to said control terminal (G). The driver circuit (1) further comprises a second current generator (IOFF) connected to the control terminal (G) of the power transistor (2) and in turn driven by the control circuit (5) to control the transistor turn-off. Advantageously, the control circuit (5) also receives a control signal (STOP) from the protection circuit (3) at the end of the latter's action.

Description: This invention relates to a driver circuit for a power switch incorporated into a switching voltage regulator. The starting point of this invention is to be found in the development of two discrete devices, that is, a step-down switching voltage regulator, and a current loop for a battery charger, and reference will be made to this field of application in the ensuing description for convenience of explanation. Switching voltage regulators are extensively employed in many applications because of their effectiveness and accuracy. The basic components of these regulators include: a power switch, a loop-back diode, an LC output filter, and an optional current sensor for the battery charger. Prior Art To compete favorably, new generation regulators are to exhibit ever higher switching frequencies, so that smaller external components can be used, both for economy of circuit area and of cost. In particular, regulators of this type are to have the smallest possible number of external components. An increased frequency reflects in the regulator efficiency being mainly limited by the commutation losses of the power switch. Accordingly, the most critical design aspect of such regulators concerns the driver circuit portion of the power switch, since commutation losses are dependent on that portion. Commercially available regulators mostly use an N-channel MOS transistor for a power switch, by reason of the simple way this can be driven. However, to ensure adequate overdrive for the driver, a MOS power switch requires a higher drive voltage than the device own supply voltage. A boosted voltage can be provided basically in one of two ways: by using a charge pump, or by using a bootstrap capacitor. Either solutions have drawbacks. In particular, the first-mentioned solution requires substantial silicon area for its implementation, in order for the charge pump to be able to deliver the large currents needed to drive the power switch. In addition, one or more external capacitors are usually required. The second solution additionally requires an external component, represented by a bootstrap capacitor, and circuitry to monitor the charging of said component, in order to ensure full operability of the system in all running conditions. Both solutions involve increased manufacturing cost for the switching regulator. In an attempt at overcoming this drawback and lowering manufacturing costs, P-channel MOS transistors have been used as power switches in late generation regulators. Unlike N-channel switches, these switches require no boost drive voltage and, therefore, no additional external components for the drive circuitry. A driver circuit adapted for this type of PMOS switches should exhibit the following features: ensure fast turn-on/off edges, but without straining the loop-back diode and the switch itself; minimize electromagnetic noise; prevent false operation of any current limiters active at turning on; optimize the regulator efficiency and exhibit low static consumption. In the state of the art, the above features have been optimized for driving switches of the NMOS type. For example, US Patent No. 5,883,505 to this Applicant discloses a driver circuit for an NMOS switch which can manage a loop-back diode in a smart style with good resultant drive. For completeness' sake, it should also be recalled that the application of a higher drive voltage than a predetermined safe threshold, across the gate and source terminals of the PMOS transistor, should be avoided as the switch is turned on. This specific problem has been the subject of extensive research and implementations directed to solve it in essentially three different ways. In a first of such prior solutions, external switches are used which can bear, between their gate and source, a drive voltage same as the supply voltage to the device. In a second of such prior solutions, the maximum supply voltage (VALmax) is limited to a value lower than or equal to the maximum Vgsmax that the PMOS transistor can bear without deterioration of its performance. In the third of such prior solutions, protection circuits are used incidental on the maximum supply voltage VALmax still exceeding the gate-source voltage drop that the transistor can endure. Shown in Figure 3 is an electric diagram of a conventional switching regulator which incorporates a PMOS power transistor equipped with a driver circuit including a protection circuit. The operation of the protection circuit occasions very high consumption that markedly depresses the regulator efficiency. Consider the instance of Figure 4, where different current signals appearing in a conventional voltage regulator with a PMOS switch are plotted vs. time. Upon the leading edge of the turn-on signal DRIVE, three other signals ON1, ON2 and ON3 of predetermined duration are activated. These signals enable the power transistor to be turned on at a fast rate, while also overcoming a problem of recovery time with the loop-back diode. More particularly, the signal ON1 sets the current value I1 which makes the power transistor conductive, and the signal ON2 further charges the gate of the power transistor with a current I2, over a time interval T2, during the reversal period of the loop-back diode. Upon the leading edge of the signal DRAIN, the signal ON3 also becomes activated to bring the current on the gate terminal of the power transistor to a value I3 effective to quicken the transistor turn-on. Turning off presents no special problems, since a desired rate of turn-off can be simply achieved by suitable selection of a current Ioff magnitude. At the end of the turn-on transient, the current being delivered to the gate terminal of the power transistor should be larger than the current Ioff and the current supplied to the protection circuit. Designating T4 the turn-on time interval, it is: I4 = ION > Ioff + Iclamp. This current is on the order of a few tens of milliAmpere and represents substantial consumption. It could not be considered of reducing this consumption by limiting the drive currents, because the on/off transients would then be slowed and commutation losses increased. Furthermore, during the time interval T3 (to be selected at the designing stage, and being liable to process spread and power supply variability), the power transistor gate is pulled down quite rapidly (T3 being on the order of a few tens of ns), and the protection circuit will have a trigger time ΔT when the gate keeps dropping to values well beyond the threshold Vgsmax. This has two drawbacks: 1) a behavior such as this may result in short term problems of reliability, because the power transistor gate is subjected to a higher voltage than is acceptable during the periods ΔT; 2) to ensure a fast recovery, the protection circuit delivers a large amount of current, which further increases consumption. A further prior art solution is disclosed in the US patent No. 4,859,927 concerning a driver circuit for a P-channel MOS power transistor according to the preamble of the enclosed claim 1. Document US-A-4 853 563 teaches to reduce the turn-on control current to a constant holding current which is greater than the leakage current. However, even this solution doesn't allow to reduce the overall power consumption during the switching phases. The technical problem underlying the present invention is one of providing a novel type of driver circuit for a PMOS power transistor with such structural and functional features as to overcome the drawbacks with which the prior art is beset. Summary of the Invention The solution idea on which the present invention is based is that one of varying the current levels of the generators, used for turning on/off the PMOS switch, to suit the states of the protection circuit, the power transistor gate, and the transistor itself and reducing the current drawn from a first current generator upon receipt of a STOP signal. Based on this solution idea, the technical problem is solved by a driver circuit as previously indicated and defined in Claim 1. The features and advantages of the protection circuit of this invention will be apparent from the following description of an embodiment thereof, given by way of non-limitative example with reference to the accompanying drawings. Brief Description of the Drawings Figures 1A and 1B show respective schematic diagrams of conventional driver circuits for a power transistor of the N-channel MOS type. Figure 2 shows diagramatically an embodiment of a conventional driver circuit for a P-channel MOS transistor. Figure 3 is a more detailed diagram of a conventional driver circuit for a P-channel MOS transistor. Figure 4 is a graph vs. time of a series of current signal waveforms appearing in the conventional circuit of Figure 3. Figure 5 shows a schematic diagram of a driver circuit for a P-channel MOS transistor, according to the invention. Figures 6A and 6B are respective diagramatic detail views of the driver circuit according to the invention. Figure 7 is a graph vs. time of a series of current signal waveforms appearing in the inventive circuit of Figure 5. Figure 8 is a graph vs. time of a series of current signal waveforms appearing in a particular portion of the circuit of Figure 6B. Figure 9 is a more detailed diagram of the driver circuit of this invention. Detailed Description Referring to the drawing figures, a driver circuit for driving a P-channel MOS transistor, embodying this invention, is generally shown at 1 in schematic form. More particularly, though not exclusively, the circuit 1 is intended for driving a PMOS power transistor incorporated to a switching type of voltage regulator. The PMOS transistor is denoted by the reference numeral 2 in Figure 5, and has a control or gate terminal G, as well as first and second conduction terminals S and D. The first or source terminal S is connected to a first supply voltage reference VAL, and the second terminal D is connected to a second voltage reference GND which may be a ground reference, through a loop-back diode DR. The terminal D is further connected to an electric load Load via an inductor L. A capacitor, along with its Equivalent Series Resistance (ESR), is placed in parallel with the load Load. The gate terminal G of the PMOS transistor receives a drive voltage Vgs through a protection circuit 3 schematically represented in the form of a block CLAMP connected to the supply reference VAL. A bias circuit portion 8 enables the protection circuit 3 to be supplied. Advantageously, the driver circuit 1 of this invention includes a first turn-on current generator ION, connected between the second voltage reference GND and the gate G of the power transistor 2. Connected in series to the first generator ION is a controlled switch 4 which is controlled by a signal ON from a control circuit to be described. The switch 4 is normally open. A second, turn-off current generator IOFF is connected between the first supply voltage reference VAL and the gate of the power transistor 2. The second generator IOFF is connected in series with a controlled switch 7 which is controlled by a signal OFF from the aforesaid control circuit. The switch 7 is normally closed. Also provided is a logic control circuit 5 which is input two discrete signals DRIVE and DRAIN and has a pair of outputs ON and OFF for driving the current generators ION and IOFF, respectively. A third input of the logic control circuit 5 is connected to an output of the protection circuit 3 to receive a signal STOP therefrom indicative of the end of the protection circuit action, as well as a related request to have the power transistor 2 turned off. The current from the generator ION is given the waveform shown in Figure 7 by the logic control circuit 5. The construction of the generator ION is illustrated in greater detail by the diagram of Figure 6A and comprises a set of at least four switches placed in parallel and being each connected in series with a respective one of the current generators. The sequence of the signals ON1, ON2, ON3 is unchanged from prior art driver circuits, this ensuring proper management of the loop back diode recovery time. Contrary to prior circuits, however, the levels of the currents I1 and I2 in the driver circuit of this invention change over the time intervals T1 and T2 of the turn-on transient. During that time period, the control circuit 5 would disconnect the generator IOFF, thereby avoiding problems from cross-conduction (signals OFF1 and OFF2 low). Thus, lower values of the currents I1 and I2 can suffice to provide the same turn-on rate for a given power switch. Possible variations in the duration of the signal ON3 (time interval T3) are accommodated by arranging for the current from the generator ION to be reduced at once (from about ten milliAmperes to a few tens of microAmperes) through the signal STOP upon operation of the protection circuit 3. To minimize the trigger time ΔT of the protection circuit 3, and thus minimize the consumption and strain on the gate terminal which affect known solutions, it is arranged for the signal STOP to cross the zero one ΔT ahead of the gate reaching the threshold Vgsmax. The information about the issuing of the signal STOP and the related operation of the protection circuit 3 should be stored in order to prevent possible oscillations from causing noise problems and raising consumption. For the purpose, a storage element 6 is incorporated to the protection circuit 3 as described hereinafter. Turning off is quite straightforward: it only requires that a current level IOFF be provided, as needed for a fast turn-off edge and to prevent cross-conduction, again in order to keep consumption low. For the purpose, the generator IOFF is formed as a parallel of two generators I5 and I6, as shown in Figure 6B. Referring to Figure 8 as well, as the turn-off signal DRIVE goes low, the signal OFF1 is generated to turn off the power transistor 2 by a large current supplied by the generators I5 and I6. This current has a duration of one time interval T5 and is sufficient to provide the fast turn-off rate sought. Thereafter, only the generator I6 will be held on which is much smaller than the generator I5. In this way, cross-conduction at the next turn-on, due to that the signals ON may arrive before the signals OFF go to zero, can be prevented. The duration of the time interval T4 is dependent on the signal DRIVE, and should the next turn-on edge arrive before the end of the time interval T4 (as a high duty cycle could allow), the signal OFF2 is immediately brought to zero. The circuit construction is rather complicated, but for the purpose of this specification, only the various functional blocks implementing the inventive principle need to be discussed. In this respect, the logic circuit 5 is operative to generate the signals ON and OFF with the timing relationships shown in Figure 7. The interior construction of the circuit 5 is not part of this invention. The first current generator ION consists of a set of switches X4, X5, X6 and X7 in the forms of MOS transistors which are driven by respective signals ON1, ON2, ON3 and DRIVE, timed as shown in Figure 7. The source terminals of the transistors X4, X5 and X6 are connected to respective current generators I1, I2 and I4. The drain current of the transistor X7 represents a generator I4. This transistor X7 has its voltage threshold Vgs as the leading edge of the signal ON3. The second current generator IOFF consists of P-channel MOS transistors M5 and M6 having respective drain currents I5 and I6 that correspond to the generators described in relation to Figure 6B. These generators are turned on/off by means of switches X2 and X3 which are driven by the logic signals OFF1 and OFF2, as shown in Figure 7. The protection circuit 3 comprises a sensing and clamping portion, and a latch 6 adapted to generate the signal STOP. More particularly, a reference voltage Vgsmax is obtained at a node A through a Zener diode D4 and a generator Ib4. A set of bipolar transistors Q1, Q2, Q3 and Q4 provide the clamping function, with the transistors Q3 and Q4 being connected together in a Darlington configuration and arranging for connection of the sense resistor Rsense to the collector of the transistor Q4. This resistor sets the threshold of operation for the signal STOP that allows any variations of the time interval T3 and ΔT of the protection circuit 3 to be recovered. More particularly, the signal STOP output from the latch 6 is set at a logic high by the signals DRIVE and OFF1 (the latter representing the negated signal DRIVE) and reset to zero as a PMOS transistor MM carries a drain current Is larger than that supplied by a generator Ib3. This would occur upon the voltage drop across the resistor Rsense being large enough to ensure, for the transistor MM, a sufficient Vgs to present said current Is on its drain. Thus, by suitably selecting the resistance of Rense, the time delays can be reduced to a minimum and consumption optimized. The bias block BIAs is operative to supply the whole driver circuit 1 with the necessary voltages and currents for it to operate as expected. A transistor M1, driven by the signal ON2, strengthen the circuit construction from the standpoint of resistance to noise. In fact, upon turn-on, as the gate G of the power transistor 2 is quickly pulled down, the generators might enter the "on" state for a short time -- thus increasing consumption by cross-conduction -- even with the generators I5, I6 in the off state, due to the effect of a parasitic capacitance which would always be present between the drains and gates of the transistors M5 and M6. The action of the transistor M1 causes the current on the diode M2 to increase and hold the transistors M5 and M6 "off" throughout the time interval T2 (via the transistors M3, M4). The driver circuit of this invention does solve the technical problem, and affords a number of advantages, foremost among which is that the same rates of commutation can be provided at a reduced overall consumption from the circuit. In addition, the protection for the gate terminal of the power transistor can be more effective and poses less problems of reliability. The consumption of the driver circuit, moreover, is unrelated to the switch size; barring the gate charge required to turn on the power transistor, the consumption from the stage is, in fact, under constant control by the logic control circuit, preventing inessential consumption.

Claims: A driver circuit for a P-channel MOS power transistor (2) having a control terminal (G) and having first (D) and second (S) conduction terminals; said driver circuit comprising a first controlled current generator (ION) connected to said control terminal (G) for turning on the power transistor (2); a control circuit (5) for controlling the turning on of said first current generator (ION); a protection circuit (3) coupled to said control terminal (G) for sensing and clamping a drive voltage (Vgs) of said control terminal (G); and a second current generator (IOFF) connected to said control terminal (G) of the power transistor (2) and in turn driven by said control circuit (5) to control the turning off of said transistor, characterized in that said control circuit (5) is arranged to reduce the current from the first current generator (ION) upon receipt of a control signal (STOP) from said protection circuit (3) indicative of said protection circuit (3) sensing a clamping current level. A circuit according to Claim 1, characterized in that said second current generator (IOFF) is connected in series to a controlled switch (7), and that said control circuit (5) is connected to act on said controlled switch (7). A circuit according to Claim 1, characterized in that said second current generator (IOFF) comprises at least one pair of generators (I5,I6) connected in parallel together and individually connected in series to a corresponding controlled switch. A circuit according to Claim 1, characterized in that a storage element (6) is arranged in said protection circuit (3) to generate said control signal (STOP) at the end of the protection circuit (3) action. A circuit according to Claim 4, characterized in that said control signal (STOP) is issued as a predetermined threshold of operation, defined by a sense resistor (Rsense), of the protection circuit (3) is exceeded. A circuit according to Claim 3, characterized in that one generator (I5) in said pair (I5,I6) is much smaller than the other. A circuit according to Claim 6, characterized in that said generators (I5,I6) are each connected in series to a corresponding MOS transistor. A switching type of voltage regulator, characterized in that it includes at least one driver circuit as claimed in Claim 1.

Inventors: CAPICI SALVATORE VINCENZO, MARINO FILIPPO, CAPICI, SALVATORE VINCENZO, MARINO, FILIPPO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1999
Main classification: H03K17
IPC codes: H03K17