Charge pump driving circuit for a power transistor

Abstract: The invention relates to a charge pump control circuit for a power transistor, comprising a driver circuit (1) connected to a first supply voltage reference (Vcc) through a diode (D) and to a second voltage reference (GND) through a load (RL). The driver circuit (1) is connected to the power transistor (MP), which power transistor, linked to a pump capacitor (Cb), is connected between a further supply voltage reference (VH) and the load (RL). The control circuit of the invention further comprises a control logic (2) connected between the first supply voltage reference (Vcc) and the second voltage reference (GND), and connected to the driver circuit (1) and a switch (3), which switch is connected between the power transistor (MP) and the load (RL) and connected, in turn, to the driver circuit (1), and a circuit (4) for checking the charged state of the pump capacitor (Cb) connected between the switch (3) and the second voltage reference (GND), downstream to the control logic (2).

Description: Field of the Invention This invention relates to a power transistor control circuit of the charge pump type which includes a device for checking the charged state of the pump capacitor. More specifically, the invention concerns a charge pump control circuit for a power transistor which comprises a driver circuit connected to a first supply voltage reference through a diode and to a second voltage reference through a load, said driver circuit being connected to the power transistor, and the power transistor, linked to a pump capacitor, is connected between a further supply voltage reference and the load. Background Art As is well known, control circuits of the charge pump type are generally used in high-voltage applications that employ power transistors of the MOS type. Such circuits can provide, across the pump capacitor, a higher voltage than the supply voltage. This allows the driver circuit, which drives the power transistor, to be powered. A basic requirement for proper operation of the circuit is that the pump capacitor Cb should be sufficiently charged before the driver circuit is turned on. Known solutions may all be reduced to the scheme of operation depicted in figure 1. Shown in this Figure 1 is a prior art control circuit which comprises a driver circuit PIL connected to a first supply voltage Vcc through a diode D' and to a ground reference voltage GND through a load resistance RL'. The driver circuit PIL is input with a control signal Si, and has an output connected to a power transistor MP'. The power transistor MP', in turn, is connected between a second supply voltage VH and the load resistance RL', in parallel with a pump capacitor Cb'. By providing the load resistance RL', a low impedance toward ground can be achieved. This resistance RL', therefore, performs its charging function by virtue of a current, designated Ic' in the Figure, being flowed therethrough. While being in many ways advantageous, this solution still has a few drawbacks, most serious among which is that, with this type of circuit, current is absorbed through the load resistance RL' continually. Furthermore, the circuit of Figure 1 cannot provide for the capacitor Cb' to become sufficiently charged before any enable signal is applied to the input of the driver circuit PIL. A further prior art solution is disclosed in the EP application No. 0 351 898 in the name of the same applicant. This documents relates to a charge pump circuit for driving a power MOS transistor half-bridge including two power transistors M1 and M2 allowing to obtain a voltage Vp variable between to values higher than the supply voltage Vs. A charge pump circuit CPC, including a charge pump capacitor, supplies the required voltage to two driving stages of said power transistors M1 and M2. However this solution doesn't provide any control of the charge of the charge capacitor during the switching phase turning on the driving stages. The underlying technical problem of this invention is to provide a control circuit for power transistors, which has appropriate structural and functional features to measure the charged state of the capacitor Cb' and correctly drive the driver circuit, thereby overcoming the limitations and drawbacks with which prior art circuits are beset. Summary of the Invention The solutive idea on which this invention stands is one of making the charge signal of the pump capacitor dominant over any other circuit enable signals. Based on this idea, the technical problem is solved by a circuit as indicated above and defined in the characterizing portion of Claim 1. The features and advantages of a circuit according to this invention will be apparent from the following detailed description of an embodiment thereof, given by way of non-limitative example with reference to the accompanying drawings. Brief Description of the Drawings In the drawings: Figure 1 shows in schematic form a charge pump circuit according to the prior art; Figure 2 shows in schematic form the control circuit of this invention; Figure 3 shows an equivalent circuit of the circuit in Figure 2; and Figure 4 shows schematically a modified embodiment of the inventive circuit. Detailed Description With reference in particular to the example shown in Figure 2, generally denoted by the reference CC is a control circuit according to the invention. The control circuit CC comprises a driver stage 1 connected between a first supply voltage Vcc and a second voltage reference, such as a signal ground GND. The driver stage 1 is connected to the first supply voltage reference Vcc through a diode D, and to ground GND through a load represented by a resistance RL. The driver stage 1 is also connected to a power transistor MP of the MOS type. The power transistor MP, being linked to a pump capacitor Cb, is connected between a second supply voltage VH and the load resistance RL. Advantageously, in accordance with the invention, a switch 3, linked to a control logic 2, is connected between the driver circuit 1 and the load resistance RL. This control logic 2 is connected between the first supply voltage Vcc and ground GND, and linked to the driver circuit 1 and to a circuit 4 for checking the charged state of the pump capacitor Cb, to be described. Advantageously, in accordance with the invention, the switch 3 comprises a transistor M1 of the MOS type. The transistor M1 has its source terminal S1 connected to the driver circuit 1 and to a common node X between the power transistor MP and the pump capacitor Cb. The drain terminal D1 of the transistor M1 is connected to ground GND through the load resistance RL, while the gate terminal G1 thereof is connected to an output 9u of the control logic 2. The control logic 2 comprises a first logic gate 5 of the AND type having its output connected to the driver circuit 1. The first logic gate 5 receives, on a first input 6, a control signal Si for the driver circuit 1. As depicted in Figure 2, this control signal Si is basically a square wave. The control logic 2 further comprises a flip-flop 9, said flip-flop having its output terminal Q connected to a second input terminal 7 of the first logic gate 5 and to the gate terminal G1. The input terminals S and R of the flip-flop 9 are connected to respective outputs 14 and 14' of first and second comparators A1 and A2. The first A1 and second comparators A2 have respective first input terminals 12 and 12' connected to the first supply voltage Vcc, and respective second input terminals 13 and 13' connected to respective generators of reference voltage VR1 and VR2. The comparator A2 is connected to the input terminal R of the flip-flop 9 through a second logic gate 8 of the AND type. Specifically, the comparator A2 is connected to a first input terminal 11 of the second logic gate 8 connected, in turn, with its output to the input terminal R of the flip-flop 9. The second input terminal 10 of the logic gate 8 is connected to an additional comparator A3 which is incorporated to the circuit 4 for checking the charged state of the capacitor Cb. The comparator A3 has a first input terminal 14 connected to the drain terminal D1 of the transistor M1 and a second input terminal 15 connected to a generator of a reference voltage VR3. The operation of the control circuit of this invention will now be described. In the preferred embodiment, the reference voltage VR1 of the first comparator A1 is less than the reference voltage VR2 of the second comparator A2. With this assumption, during the switching of the supply voltage Vcc, as the voltage Vcc drops below VR1, the output of the first comparator A1 goes to a high logic value LL1. In a dual manner, the output of the second comparator A2 goes to a low logic value LL0. This low logic value is applied to the input terminal 11 of the AND logic gate 8 and holds a low logic value LL0 at the output terminal of that gate 8. Accordingly, the comparator A2 is called an under-voltage lockout comparator. The high logic value LL1 is applied to the input terminal S (SET input) of the flip-flop 9, while the low logic value LL0 is applied to the input terminal R (RESET input) of that flip-flop 9. Under these first conditions, the output terminal Q of the flip-flop 9 is in a high logic state LL1, and the MOS transistor M1, being driven by that high logic value LL1, closes the charging circuit of the pump capacitor Cb. Throughout the capacitor Cb charging cycle, the power transistor MP is held in the off state. In fact, the output of the AND logic gate 5 which controls the driver stage 1 has a low logic value LL0, due to the output Q of the flip-flop 9 being inverted to the input 7 of the AND logic gate 5. The device operation under these conditions can be represented schematically by means of the equivalent circuit shown in Figure 3. In Figure 3, the path of a charge current Ic through the capacitor Cb is shown by a dash-and-dot line. The current Ic flows through said capacitor Cb and a series of an equivalent resistance Re and the load resistance RL. Therefore, the value of the charge current Ic will obey the following relationship: I c = Vcc - V D R *e -t/RCb where: V D is the voltage drop across the diode D, and R is the series of the resistances Re and RL. Thus, a charge voltage Vc exists across the capacitor Cb which is given by: Vc = Vcc - V D * (1 - e -t/RCb ) The value of the reference voltage VR3 for the comparator A3 is the lowest limit at which the capacitor Cb can be considered to be charged. In fact, as the voltage across the load resistance RL decreases until the level of the voltage VR3, the output of the comparator A3 switches to a high logic value LL1 and enables the AND logic gate 8. As the supply voltage Vcc exceeds the reference voltage VR1, the logic signal to the input terminal S of the flip-flop 9 goes to a low logic level LL0. As a result, the output signal 11 from the comparator A2 switches to a high logic level LL1. Through the AND logic gate 8, the terminal R of the flip-flop 9 goes to a high logic value LL1. Under these conditions, the signal at the output Q of the flip-flop 9 achieves a low logic level LL0. This low logic level LL0 is inverted at the input 7 of the AND logic gate 5 to re-enable the driver circuit 1 and disable the transistor M1. The circuit of this invention may also be used with supply voltages Vcc higher than 500V. High supply voltage values are suitable to drive inductive loads that would require, however, a mechanism for preventing that parasitic bipolar transistors are turned on. To prevent turning on such parasitic transistors, the circuit of this invention can be modified as shown in Figure 4. Advantageously, in this second embodiment of the inventive control circuit, a second diode D2 is connected between the driver circuit 1 and the source terminal S1 of the transistor M1 included in the switch 3. Also shown in phantom lines in Figure 4 is a parasitic transistor Tp bound to the load RL. The parasitic transistor Tp is of the NPN type, and has its emitter terminal Ep connected to the source terminal S1 of the transistor M1, and its base terminal Bp connected to ground GND. The diode D2 inhibits current backflow through that parasitic transistor Tp, thereby preventing that it is turned on. In summary, the control circuit of this invention allows the charging of the pump capacitor Cb to be completed before the driver circuit 1 is enabled. The charged state of the pump capacitor Cb is, in fact, dominant over any other enable signals. Thus, the use of the control circuit of this invention will ensure that the power transistor MP is properly powered at all times, by preventing that it is turned on whenever the operating conditions are not correct (e.g., an underthreshold condition). Advantageously, the circuit of this invention also prevents, in an alternative embodiment thereof, that any parasitic transistors in parallel with the load are turned on.

Claims: A charge pump control circuit for a power transistor, comprising a driver circuit (1) connected to a first supply voltage reference (Vcc) through a diode (D) and to a second voltage reference (GND) through a load (RL), with said driver circuit (1) being connected to the power transistor (MP), and the power transistor, linked to a pump capacitor (Cb), is connected between a further supply voltage reference (VH) and the load (RL), and comprising a control logic (2) connected between the first supply voltage reference (Vcc) and the second voltage reference (GND) and connected to said driver circuit (1), characterised by comprising a switch (3) connected between said power transistor (MP) and said load (RL) and further connected to the driver circuit (1), and a circuit portion (4) for checking the charged state of said pump capacitor (Cb) with an input terminal (14) connected to the load (RL) and an output terminal connected to the control logic (2) allowing the charging of the pump capacitor (Cb) to be completed before said driver circuit (1) being enabled. A control circuit according to Claim 1, characterized in that said control logic (2) comprises at least one first logic gate (5) of the AND type adapted to receive, on a first input terminal (6), a control signal (Si) for the driver circuit (1). A control circuit according to Claim 2, characterized in that said control signal (Si) is a square wave. A control circuit according to Claim 2, characterized in that said control logic (2) comprises first (A1) and second comparators (A2) in parallel, a second logic gate (8) of the AND type, and a flip-flop (9), said flip-flop (9) having an output terminal (Q) connected to a second input terminal (7) of said first logic gate (5). A control circuit according to Claim 4, characterized in that said flip-flop (9) has first (S) and second (R) input terminals, said first input terminal (S) being connected to the output of said first comparator (A1) and said second input terminal (R) being connected to the output of the second comparator (A2) through said second logic gate (8). A control circuit according to Claim 4, characterized in that said second logic gate (8) has a first input terminal (10) connected to said charged state checking circuit (4) and a second input terminal (11) connected to said second comparator (A2). A control circuit according to Claim 6, characterized in that said first (A1) and second comparators (A2) have respective first input terminals (12, 12') connected to said first supply voltage (Vcc) and respective second input terminals (13, 13') connected to corresponding generators of reference voltage (VR1, VR2). A control circuit according to Claim 1, characterized in that said switch (3) comprises a transistor (M1) of the MOS type having a source terminal (S1) connected to said driver circuit (1) and to a common node (X) between the power transistor (MP) and the pump capacitor (Cb), a gate terminal (G1) connected to said control logic (1), and a drain terminal (D1) connected to said load (RL). A control circuit according to Claim 1, characterized in that said charged state checking circuit (4) comprises a comparator (A3) having a first input terminal (14) connected to the load (RL) and a second input terminal (15) connected to a generator of a reference voltage (VR3). A control circuit according to Claim 8, characterized in that said switch (3) further comprises a diode (D2) connected between said MOS transistor (M1) and said driver circuit (1).

Inventors: ALZATI ANGELO, NOVELLI ALDO, ALZATI, ANGELO, NOVELLI, ALDO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1995
Main classification: H03K17
IPC codes: H03K17, H02M3