Bias voltage control circuit for a floating well in a semiconductor integrated circuit

Abstract: A control circuit comprising a plurality of input terminals (HSTRAP,HSRC) and at least one output terminal (POLEPI) for biasing a floating well (EPI) in a semiconductor integrated circuit structure, and comprising a first transistor (NCH1) which has its conduction terminals connected between a first input terminal (HSTRAP) and an output terminal (POLEPI), and a second transistor (PCH1) which has its conduction terminals connected between a second input terminal (HSTRAP) and the output terminal (POLEPI), wherein the output terminal (POLEPI) is coupled to each of the control terminals of said first and second transistors through a regulator (Dz).

Description: Field of the Invention This invention relates to a control circuit for controlling the bias voltage of a floating well in a semiconductor integrated circuit structure. More particularly, the invention relates to a control circuit comprising a plurality of input terminals and at least one output terminal for biasing a floating well in a semiconductor integrated circuit structure, and comprising a first transistor having its conduction terminals connected between a first input terminal and an output terminal, and a second transistor having its conduction terminals connected between a second input terminal and the output terminal. The invention relates, in particular but not exclusively, to a control circuit for controlling the bias voltage of a floating well, in order to form transistors for use in switching regulators, and the ensuing description will cover this specific field of application for convenience of illustration. Prior Art As is known, there are many electrical applications wherein the value of a current flowed through an electric load requires to be regulated. In most cases, the current through the electric load has been regulated by means of a power transistor which may either be of the integrated or the discrete type. The power transistor, in turn, is driven by a an integrated drive circuit commonly referred to as the high side driver. This power transistor is usually a MOS transistor having gate, source, and drain terminals. To charge the gate terminal of this transistor, a second voltage supply, higher than that to be applied to the drain terminal, must be made available. To produce this second voltage supply, a bootstrap capacitor is employed which can be re-charged during the conduction phase of a second power transistor, for example. This transistor is itself driven by means of an integrated drive circuit referred to as the low side driver. However, the supply voltage to the bootstrap capacitor must be high, if the efficiency of switching circuits is to be enhanced. Thus, the MOS power transistor is driven with gate-source voltages selected to have the smallest possible switch-on resistance RDSon. A possible construction of transistors using MOS technology is illustrated by Figure 1. An epitaxial well II of the N type is grown over a substrate I of the P type. Body regions III of the P type and IV of the N type are then formed to respectively provide N-channel and P-channel transistors. For example, two regions of the N+ type are formed in the region BODY III of the P type to provide the source and drain regions of an N-channel transistor. The source region and the body region III are conventionally connected together by a common terminal HSRC. By using a conventional process of manufacturing structures such as that shown in Figure 1, e.g. with BCDIII technology, the operation of MOS transistors at relatively high working voltages can be ensured. In particular, for circuits employing voltage bootstrap structures, working voltages may be provided whose values equal the voltage drop across the bootstrap capacitor. However, as the bootstrap voltage is increased, a bias voltage of the epitaxial well II cannot be ensured to equal the working voltage, because the breakdown voltage of the junction created between this well II and the regions III of the P type is smaller than the voltage drop across the bootstrap capacitor. The underlying technical problem of this invention is to provide a control circuit for controlling the bias voltage of a floating well, which circuit has structural and functional features such that relatively high bias voltages can be used, and a breakdown of the junction created between the floating well and strongly biased regions effectively prevented, thereby overcoming the limitations and/or drawbacks with which prior art devices are beset. Summary of the Invention The principle behind this invention is one of providing a control circuit for the bias voltage of the floating well during operation of switching regulators such that the well voltage becomes variable proportionally to the voltages of contiguous regions, instead of being a fixed voltage. Based on this principle, the technical problem is solved by a circuit as previously indicated and defined in the characterising portion of Claim 1. The features and advantages of a device according to the 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 In the drawings: Figure 1 shows a portion of a semiconductor device formed with a floating epitaxial well; Figure 2 shows a control circuit according to the invention; Figure 3 illustrates an application of the control circuit according to the invention; Figures 4a, 4b are plots of voltage signals in the control circuit of this invention. Detailed Description Referring to the drawing figures, generally shown at 6 is a control circuit according to the invention. The circuit 6 comprising two input terminals HSTRAP and HSRC, and an output terminal POLEPI. A first transistor NCH1 has a first conduction terminal 1 connected to the terminal HSTRAP and a second conduction terminal 2 connected to the terminal POLEPI. A second transistor PCH1 has a first conduction terminal 10 connected to the input terminal HSRC, a second conduction terminal 2 connected to the output terminal POLEPI, and a third control terminal 30 connected to the third terminal 3 of the transistor NCH1 at a node G. Advantageously, the body terminals of said transistors are connected to their respective second conduction terminals. The control terminals 3, 30 of the two transistors NCH1, PCH1 are connected to a common node G. The terminal HSRC controls the voltage at the node G by means of a regulator Dz. This regulator may be a reverse biased Zener diode D Z . However, the regulator could be obtained in an another way known to the skilled ones in the art. Advantageously, a capacitor C Z is connected in parallel with the diode Dz. The bias for the diode Dz may be provided by a first current mirror 4 connected between the terminal HSTRAP and the node G. The mirror 4 comprises first M1 and second M2 mirror transistors which are both of the PMOS type in the example considered. The first mirror transistor M1 has its respective source and drain conduction terminals connected to the terminal HSTRAP and the node G, and has a control terminal connected to the control terminal of the second mirror transistor M2, which is diode-connected and has its drain terminal connected to the ground GND through a second current mirror 5. The second current mirror 5 comprises a pair of mirror transistors M3 and M4 of the NMOS type having their respective control terminals connected together. The transistor M3 is diode-connected. The control terminal of the transistor M3 has a capacitor C1 toward ground. As the skilled ones in art know well, the Zener diode Dz could also be biased in other ways. Shown in Figure 3 is an integrated circuit 7 which includes an output stage of switching regulators with power transistors, and incorporates a conventional bootstrap capacitor. This capacitor could be external of the integrated circuit 7. The control circuit of this invention may be associated to advantage with the integrated circuit 7. In this integrated circuit 7, a first power transistor T1 operates as a switch function and has a first conduction terminal connected to a voltage Vin, and has a second conduction terminal connected to the node HSTRAP and connected to a ground reference through a second power transistor T2. The control terminal of the first transistor T1 is connected to the output of a first drive circuit Driver Hside. The control terminal of the second transistor T2 is connected to the output of a second drive circuit Driver Lside. A bootstrap capacitor Cboot is connected between the terminals HSTRAP and HSRC of the drive circuit Driver Hside, and is powered from a voltage generator Vdriver having a diode Dboot in series therewith. The control circuit of this invention is connected to the terminals HSTRAP and HSRC. Figure 1 shows a portion of a semiconductor device wherein the input terminal HSRC', in a embodiment of the invention, is connected to the body region III of the P type, and the output terminal POLEPI is connected to the epitaxial well II. A diode Cepi-sub represents the junction between an epitaxial well II and the substrate I where the integrated circuit is formed. The operation of the control circuit according to this invention will now be described. With the transistor T1 in the off state and the transistor T2 in the on state, the bootstrap capacitor is charged, and the terminal HSTRAP is at the drive voltage of the drivers, so that: V HSTRAP = Vdriver - Vbe where, Vbe is the voltage drop across the diode Dboot. The voltage V HSRC at the terminal HSRC is equal to zero. When the transistor T1 is turned on, and the transistor T2 turned off, the terminal HSRC goes to a voltage value V HSRC =Vin, and the terminal HSTRAP to a voltage value given as V HSTRAP =Vin+Vdrive-Vbe. This rising edge of the signal Vin applied to the terminal HSRC is sensed by the Zener diode Dz, which will cause the transistor NCH1 to conduct. The terminal POLEPI will then attain a voltage value given as: V EPI = V zener - V gs (NCH1) ÷ V HSRC . Advantageously, the capacitor Cz provided holds the voltage constant across the Zener diode Dz. When the transistor T1 is turned off, and the transistor T2 turned on, the terminals HSRC and HSTRAP attain respectively V HSRC =0 and V HSTRAP =Vdriver-Vbe. In this condition, the transistor NCH1 is turned off and the transistor PCH1 turned on. Thus, the voltage at the terminal POLEPI is controlled to a value given by: V EPI = V zener - V gs(PCH1) . Shown schematically in Figure 4a is a voltage vs. time plot of the voltages V HSTRAP , V HSRC , and V EPI , on a common time base. This first plot brings out the fact that the voltage V EPI follows the patterns of the voltages V HSTRAP , V HSRC . Figure 4b shows a plot illustrating the difference between the epitaxial well voltage and that of the regions BODY where the N-channel transistors of the integrated circuit 7, the drive circuit Driver Hside, the circuit 6 of this invention, and the transistor T1 are all formed. The use of the control circuit according to the invention ensures that the voltage across the epitaxial well and the regions BODY will not exceed the breakdown voltage of the resulting junction. To summarise, the circuit of this invention affords control of the bias voltage of a floating well as an input voltage varies.

Claims: A control circuit comprising a plurality of input terminals (HSTRAP,HSRC) and at least one output terminal (POLEPI) for biasing a floating well (EPI) in a semiconductor integrated circuit structure, and comprising a first transistor (NCH1) having its conduction terminals connected between a first input terminal (HSTRAP) and an output terminal (POLEPI), and a second transistor (PCH1) having its conduction terminals connected between a second input terminal (HSRC) and the output terminal (POLEPI), characterised in that the second input terminal (HSRC) is coupled, through a regulator (Dz), to the control terminals (3, 30) of said first and second transistors (NCH1, PCH1), said control terminals (3, 30) being connected to a common node (G), said first and second transistor (PCH1, NCH1) being of opposite conduction type so that the second transistor (PCH1) is conductive if said second input terminal (HSRC) goes to a first voltage value (V HSRC =0) and the first transistor (NCH1) is conductive if said second input terminal (HSRC) goes to a second voltage value (V HSRC =Vin). A control circuit according to Claim 1, characterised in that said regulator (Dz) is a Zener diode. A control circuit according to Claim 2, characterised in that a capacitor (Cz) is connected in parallel with said Zener diode (Dz). A control circuit according to Claim 1, characterised in that said first transistor (NCH1) is a MOS transistor of the N-channel type. A control circuit according to Claim 1, characterised in that said second transistor (PCH1) is a MOS transistor of the P-channel type. A control circuit according to Claim 2, characterised in that a current mirror (4) is provided for biasing said Zener diode (Dz). A control circuit according to Claim 2, characterised in that said Zener diode is reverse biased to the control terminals of said first and second transistors (NCH1, PCH1).

Inventors: MARINO FILIPPO, MARINO, FILIPPO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1998
Main classification: G05F3
IPC codes: G05F3, G05F1