# Patent

## Title
Electronic drive circuit

## Abstract

An electronic circuit includes an input configured to receive an input signal and an output configured to be coupled to load, an output transistor including a load path and a control node, the load path being connected between the output and a first supply node, a drive transistor including a load path and a control node, the load path being connected to the control node of the output transistor, a first electronic switch connected in series with the load path of the drive transistor, a biasing circuit including an internal impedance and connected between the control node of the drive transistor and the first supply node, and a control circuit configured to receive the input signal and to drive the first electronic switch based on the input signal.

## Background
PRIORITY CLAIM
This application claims priority to German Patent Application No. 10 2015 102 878.6 filed on 27 Feb. 2015, the content of the application incorporated herein by reference in its entirety.
TECHNICAL FIELD
Embodiments of the present invention relate to an electronic circuit, in particular an electronic drive circuit for driving a capacitive load such as, for example, the gate of a transistor.
BACKGROUND
MOS transistors such as MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors) or IGBTs (Insulated Gate Bipolar Transistors) are widely used in automotive, industrial, or consumer electronic applications for driving loads, converting power, or the like. MOS transistors are voltage controlled devices that include an internal capacitance (often referred to as gate-source capacitance) formed by a gate electrode, a gate dielectric and body and source regions. The MOS transistor can be switched on and off by charging and discharging the internal capacitance, wherein switching on includes one of charging and discharging the internal capacitance, and switching off includes the other one of charging and discharging the internal capacitance. For example, in an enhancement type MOS transistor, switching on the MOS transistor includes charging the internal capacitance and switching off the MOS transistor includes discharging the internal capacitance.
Modern MOS transistors for switching electrical loads may have a voltage blocking capability of several 10V up to several 100V but may be designed to withstand voltages of only several volts, such as less than 5V or even less than 3V at their internal capacitance (at the gate node). Furthermore, it is desirable to switch those MOS transistors at high frequency, that is, to charge and discharge the internal capacitance at a high rate.
There is therefore a need for an electronic drive circuit that is capable of driving a capacitive load at a high frequency without exceeding a predefined voltage threshold across the capacitive load.
SUMMARY
One embodiment relates to an electronic circuit. The electronic circuit includes an input configured to receive an input signal and an output configured to be coupled to load, an output transistor having a load path and a control node, wherein the load path is connected between the output and a first supply node, and a drive transistor having a load path and a control node, wherein the load path is connected to the control node of the output transistor. A first electronic switch is connected in series with the load path of the drive transistor. A biasing circuit having an internal impedance is connected between the control node of the drive transistor and the first supply node. The electronic circuit further includes a control circuit configured to receive the input signal and to drive the first electronic switch based on the input signal.
Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
Examples are explained below with reference to the drawings. The drawings serve to illustrate certain principles, so that only aspects necessary for understanding these principles are illustrated. The drawings are not to scale. In the drawings the same reference characters denote like features.
FIG. 1 shows an electronic drive circuit according to one embodiment;
FIG. 2 shows timing diagrams of an input signal and of signals occurring in the electronic drive circuit shown in FIG. 1;
FIG. 3 shows timing diagrams illustrating one way of operation of the electronic drive circuit shown in FIG. 1;
FIG. 4 shows one embodiment of a biasing circuit in the electronic drive circuit;
FIG. 5 shows one embodiment of a voltage regulator in the biasing circuit shown in FIG. 4 in greater detail;
FIG. 6 shows an electronic drive circuit according to another embodiment;
FIG. 7 shows timing diagrams illustrating one way of operation of the electronic drive circuit shown in FIG. 1; and
FIG. 8 shows an electronic drive circuit according to yet another embodiment.



DETAILED DESCRIPTION
In the following detailed description, reference is made to the accompanying drawings. The drawings form a part of the description and by way of illustration show specific embodiments in which the invention may be practiced. It is to be understood that the features of the various embodiments described herein may be combined with each other, unless specifically noted otherwise.
FIG. 1 shows an electronic drive circuit according to one embodiment. This electronic drive circuit is configured to drive a load. In particular, the electronic drive circuit is configured to drive a capacitive load. For example, the capacitive load is an MOS transistor. An MOS transistor, such as a MOSFET or an IGBT, is a voltage controlled semiconductor device that switches on or off dependent on a charging state of an internal gate-source capacitance. Just for the purpose of illustration, FIG. 1 shows a capacitive load Z implemented as a MOSFET, in particular, an n-type enhancement MOSFET. However, this is only an example. Any other type of MOS-transistor may be driven by the electronic drive circuit as well. The internal gate-source capacitance of this MOSFET is represented by a capacitor CGS connected between a gate node G and a source node S of the MOSFET Z.
Referring to FIG. 1, the electronic drive circuit includes an input 11 configured to receive an input signal SIN and an output 12 configured to be coupled to a load Z. In case the load Z is a MOSFET (as shown in FIG. 1) the output 12 of the electronic drive circuit is connected to the gate node of the MOSFET.
The electronic drive circuit further includes an output transistor 11, a drive transistor 21, and a first electronic switch 31. The output transistor 11 includes a load path connected between the output 12 and a first supply node 131, and a control node. The drive transistor 21 includes a control node, and a load path connected to the control node of the output transistor 11 and connected in series with the first electronic switch 31. The first electronic switch 31 is connected between a second supply node 141 and the load path of the drive transistor 21.
The electronic circuit may include a second electronic switch 61 connected between the control node of the output transistor 11 and the first supply node 131. Optionally, a resistor 71 (illustrated in dashed lines in FIG. 1) is connected in parallel with the second electronic switch 61 and, therefore, between the control node of the output transistor 11 and the first supply node 131.
A biasing circuit 41 is connected between the control node of the drive transistor 21 and the first supply node 131. The biasing circuit 41 includes a voltage source 411 configured to provide a biasing voltage V41, and an internal impedance 421. The biasing circuit 41 is explained in greater detail herein below. A control circuit 51 is configured to receive the input signal SIN and to drive the first electronic switch 31 and the second electronic switch 61 based on the input signal SIN.
The circuit elements explained above, that is, the output transistor 11, the drive transistor 21, the first electronic switch 31, the biasing circuit 41, the control circuit 51, the second electronic switch 61, and the optional resistor 71 are part of a first drive circuit 101, which will briefly be referred to as first driver or low-side driver in the following. This first driver 101 is configured to discharge a capacitive load coupled to the output 12. If, as shown in FIG. 1, the load Z is an enhancement MOSFET and has its gate node coupled to the output 12 the capacitive load is the internal gate-source capacitance CGS of the MOSFET. Discharging this internal capacitance CGS by the first driver 101 is equivalent to switching of the MOSFET Z. Thus, if the load Z driven by the electronic drive circuit is an MOS transistor the first driver 101 is configured to switch off the MOS transistor. Besides the first driver 101 configured to discharge a capacitive load (switch off an MOS transistor) the electronic circuit may include a second driver (not shown in FIG. 1) configured to charge the capacitive load (switch on the MOS transistor).
The first driver 101 discharges the capacitive load CGS (switches off the MOS transistor Z) when the output transistor 11 is in an on-state (is switched on). In the on-state, the output transistor 11 electrically connects the output 12 with the first supply node 131. In operation of the first driver 101, the first supply 131 may be connected to a load terminal of the capacitive load CGS facing away from the output 12 so that the load path of the output transistor 101 is connected in parallel with the capacitive load CGS.
In general, the operation state of the output transistor 11 is dependent on operation states of the first electronic switch 31 and the second electronic switch 61, respectively. The output transistor 11 is driven in the on-state when the first electronic switch 31 switches on and the second electronic switch 61 switches off, and the output transistor 11 is driven in the off-state when the first electronic switch 31 switches off and the second electronic switch 61 switches on. The control circuit 81 is configured to switch the first electronic switch 31 and the second electronic switch 61 based on the input signal SIN such that only one of the first electronic switch 31 and the second electronic switch 61 is switched on at the same time. In order to prevent a current shoot through, that is, an electrically conducting path between the first supply node 131 and the second supply node 141, the control circuit 81 may be configured to drive the first electronic switch 31 and the second electronic switch 61 such that there is a delay time (dead time) between switching off one of the first electronic switch 31 and the second electronic switch 61 and switching on the other one of the first electronic switch and the second electronic switch 61.
One way of operation of the control circuit 81 is shown in FIG. 2. FIG. 2 shows timing diagrams of the input signal SIN, the drive signal S31 of the first electronic switch 31 and the drive signal S61 of the second electronic switch 61. The input signal SIN defines the desired operation state of the load Z driven by the electronic switch 10. For the purpose of illustration, it is assumed that the input signal SIN can have one of two different signal levels, namely a first signal LE1 and a second signal level LE2. The first signal level LE1 indicates that it is desired to switch off the output transistor 11, so as to prevent the capacitive load CGS from being discharged, and the second signal level LE2 indicates that it is desired to switch on the output transistor 11, so as to discharge the capacitive load CGS. Just for the purpose of illustration, the first level LE1 is a high-level and the second level LE2 is a low-level in the example shown in FIG. 2.
Each of the drive signals S31, S61 can have one of an on-level, which switches on the respective electronic switch 31, 61, and an off-level, which switches off the respective switch 31, 61. When the input signal SIN has the first level LE1 the control circuit 81 switches off the first electronic switch 31 by generating an off-level of the drive signal S31 and switches on the second electronic switch 61 by generating an on-level of the drive signal S61. In this operation mode of the first driver 101 the output transistor 11 is switched off. When the signal level of the input signal SIN changes from the first level LE1 to the second level LE2 the control circuit 81 switches off the second electronic switch 61 by generating an off-level of the drive signal S61 and, after an optional delay time TD, switches on the first electronic switch 31, by generating an on-level of the drive signal S31. In this operation mode of the first driver 101 the output transistor 11 is switched on.
One way of operation of the first driver 101 shown in FIG. 1 is explained in the following. For the purpose of explanation, it is assumed that each of the output transistor 11 and the drive transistor 21 is an n-type MOSFET, in particular, an n-type enhancement MOSFET. In this case, the control node of the output transistor 11 and the drive transistor 21, respectively, is the gate node of the MOSFET forming the respective transistor, and the load path is the drain-source path of the MOSFET forming the respective transistor. Referring to FIG. 1, the drain node of the MOSFET forming the output transistor 11 (briefly referred to as the drain node of the output transistor 11 in the following) is connected to the output 12, and the source node is connected to the first supply node 131. The source node of the MOSFET forming the drive transistor 21 (briefly referred to as the source node of the drive transistor 21 in the following) is connected to the gate node of the output transistor 11, and the drain node of the drive transistor 21 is connected to the first electronic switch 31.
For the purpose of explanation, it is further assumed that the first electronic switch 31 and the second electronic switch 61 are transistors of complementary conductivity types. In the embodiment shown in FIG. 1, the first electronic switch 31 is a p-type MOSFET, in particular, a p-type enhancement MOSFET, and the second electronic switch 61 is an n-type MOSFET, in particular, an n-type enhancement MOSFET. The MOSFET forming the first electronic switch 31 receives the drive signal S31 from the control circuit 81 at its gate node. The drain node of this MOSFET forming the first electronic switch 31 is connected to the drain node of the drive transistor 21, and the source node of the MOSFET forming the first electronic switch 31 is connected to the second supply node 141. The MOSFET forming the second electronic switch 61 receives the drive signal S61 from the control circuit 81 at its gate node. The drain source path of this MOSFET forming the second electronic switch 61 is connected between the gate node of the output transistor 11 and the first supply node 131. That is, the drain-source path of the MOSFET forming the second electronic switch 61 is connected in parallel with an internal gate-source capacitance CGS11 of the output transistor 11. The output transistor 11 further includes an internal gate-train capacitance. However, this capacitance is not explicitly illustrated in FIG. 1.
The drive transistor 21 includes an internal gate-source capacitance CGS21 and an internal gate-drain capacitance CGD21. In FIG. 1, these capacitances are represented by capacitors connected between the gate node and the source node and the drain node, respectively. VGS21 and VGD21 denote voltages across these capacitances CGS21, CGD21.
If the first electronic switch 31 is implemented as a p-type MOSFET, then, the off-level of the drive signal S31 may correspond to the level of the electrically potential V21 at the second supply node 141, while the on-level may be a signal level that is less than the electrically potential V21 at the second supply node 141 minus the threshold voltage of this p-type MOSFET 31. Those signal levels are shown in FIG. 2. If the second electronic switch 61 is an n-type MOSFET, then the off-level of the drive signal S61 may correspond to the level of the electrical potential V11 at the first supply node 131, while the on-level may be a signal level that is higher than the electrical potential V11 at the first supply node 131 plus the threshold voltage of this n-type MOSFET. These signal levels are also shown in FIG. 2.
An operation mode of the first driver 101 in which the output transistor 11 is switched off will be referred to as off-state of the first driver 101, and an operation mode in which the output transistor 11 is switched on will be referred to as on-state of the first driver 101. There are applications in which it is desirable for the first driver 101 to rapidly switch from the off-state to the on-state, that is, to rapidly switch on the output transistor 11. Switching on the output transistor 11 includes charging the internal gate-source capacitance CGS11 such that the gate-source voltage VGS11 across this internal capacitance CGS11 rises above the threshold voltage of the output transistor 11. In order to rapidly switch on the output transistor 11 it is desirable to rapidly charge the internal gate source capacitance CGS11 without causing the voltage VGS11 to exceed a predefined voltage threshold. The drive transistor 21, which has its gate node biased by the biasing circuit 41, is capable of rapidly charging the gate-source capacitance CGS11 of the output transistor 11. This is explained below.
When the first electronic switch 11 is in the off-state and the gate-source capacitance CGS11 of the output transistor 11 has been discharged, then the electrical potential at the source node S21 of the drive transistor 21 corresponds to the electrical potential V11 at the first supply node 131 so that the gate-source voltage VGS21 of the drive transistor 21 corresponds to the biasing voltage V41 provided by the biasing circuit 41. This biasing voltage V41 is such that it is higher than the threshold voltage of the drive transistor 21 so that the drive transistor 21 is in the on-state. However, a current IDS21 through the drive transistor 21 is zero until the first electronic switch 31 switches on. Before the first electronic switch 31 switches on, the electrical potential at the drain node D21 of the drive transistor 21, which is a circuit node between the drive transistor 21 and the first electronic switch 31, substantially corresponds to the electrical potential V11 at the first supply node 131. Thus, a gate-drain-voltage VGD21 of the drive transistor 21 also equals the biasing voltage V41 provided by the biasing circuit 41. In FIG. 1, VG21 denotes a voltage between the gate node G21 of the drive transistor 21 and the first supply node 131. This voltage will be referred to as gate voltage in the following. In the steady state, before the first electronic switch 31 switches on, the gate voltage VG21 substantially equals the biasing voltage V41.
FIG. 3 shows timing diagrams of the drive signal S31 of the first electronic switch 31, the gate voltage VG21 of the drive transistor 21, the gate-source voltage VGS11 of the output transistor 11, and the current IDS11 through the output transistor 11. The timing diagrams shown in FIG. 3 begin at a time shortly before the electronic switch 31 switches on. Before the first electronic switch 31 switches on, the gate-source voltage VGS11 of the output transistor 11 is zero, the current IDS11 (the drain-source current) through the output transistor 11 is zero, and each of the gate voltage VG21, the gate-source voltage VGS21, and the gate-drain voltage VGD21 of the drive transistor 21 substantially equal the biasing voltage V41.
When the control circuit 81 switches on the first electronic switch 31 based on the input signal SIN the gate-source capacitance CGS11 of the output transistor 11 is rapidly charged because the drive transistor 21 is already conducting when the first electronic switch 31 switches on. The first electronic switch 31 switches on as soon as the drive signal S31 reaches the threshold voltage of the MOSFET forming the first electronic switch 31. As soon as the drive signal S31 reaches the threshold voltage a current IDS21 with a current level defined by the drive transistor 21 flows through the drive transistor 21 and into a gate-source capacitance CGS11 of the output transistor 11. A further increase of the signal level between the gate node and the source node of the first electronic switch 31 to above the threshold voltage may reduce the losses occurring in the first electronic switch but does not change the current IDS21. This is by virtue of the drive transistor 21 being pre-biased by the biasing source 41. The level of the current IGS21 through the drive transistor 21 is substantially defined by the gate-source voltage VGS21 of the drive transistor 21.
The current IGD21 which flows right after the first electronic switch 31 switches on, rapidly charges the gate-source capacitance CGS11 of the output transistor 11, thus causing a rapidly increasing current IDS11 through the output transistor 11. This current IDS11 decreases as the capacitive load CGS is discharged.
Referring to FIG. 3, the gate voltage VG21 of the drive transistor 21—which corresponds to the gate-source voltage VGS21 before the gate-source capacitance CGS11 of the output transistor is charged—increases to a voltage level which is above the level of the biasing voltage V41 when the electronic switch 31 switches on. That is,

VG21=V41+ΔV  (1),

where ΔV is the increase of the gate potential relative to the biasing voltage V41. This increase ΔV of the gate voltage VG21, which is equal VGS21 at the time of switching on the first electronic switch, results in an increase of the current IDS21 as compared to a scenario in which the drive transistor 21 is only biased by the biasing voltage 411 The reason for this increase ΔV in the gate voltage VG21 is as follows.

Basically, there are two effects that cause the gate voltage VG21 to increase. The first effect is based on the fact that the gate node G21 is capacitively coupled to the drain node D21 through the internal gate-drain capacitance CGD. When the first electronic switch 31 switches on, the electrical potential VD21 at the drain node D21 of the drive transistor 21 rises from the first supply potential V11 to the second supply potential V21. By virtue of the capacitive coupling of the gate node G21 to the drain node D21 the electrical potential at the gate node G21 increases as the electrical potential VD21 at the drain node D21 increases. The internal impedance 421 of the biasing circuit 41 prevents the biasing circuit 41 from instantaneously balancing such increase ΔV of the electrical potential at the gate node G21 of the drive transistor 21.
According to one embodiment, shown in FIG. 4, the internal impedance 421 of the biasing circuit 41 includes a parallel circuit with a resistor 4221 and a capacitor 4231 between the gate node G21 of the drive transistor 21 and the first supply node 131. The capacitor 4321 of the biasing circuit 41 and the gate-drain capacitance CGD21 of the drive transistor 21 form a capacitive voltage divider. In the steady state, before the first electronic switch 31 switches on, the capacitor 4231 of the biasing circuit 41 has been charged to the biasing voltage V41 and the gate-drain capacitance CGD21 has been charged to the biasing voltage V41. When the electrical potential VD21 at the drain node increases the gate voltage VG21 of the drive transistor 21 increases. It can be shown that right after switching on the first electronic switch 31 an increase ΔV′ of the gate voltage VG21, as a first approximation, is as follows:




  
    
      
        
          
            Δ
            ⁢
            
              
            
            ⁢
            
              V
              ′
            
          
          =
          
            
              
                CGD
                ⁢
                
                  
                
                ⁢
                
                  2
                  1
                
              
              
                
                  CGD
                  ⁢
                  
                    
                  
                  ⁢
                  
                    2
                    1
                  
                
                +
                
                  C
                  ⁢
                  
                    
                  
                  ⁢
                  
                    423
                    1
                  
                
              
            
            ·
            
              (
              
                
                  V
                  ⁢
                  
                    
                  
                  ⁢
                  
                    2
                    1
                  
                
                -
                
                  V
                  ⁢
                  
                    
                  
                  ⁢
                  
                    1
                    1
                  
                
              
              )
            
          
        
        ,
      
    
    
      
        (
        2
        )
      
    
  




where C4231 is the capacitance of the capacitor, CGD21 is the capacitance value of the gate-drain capacitance. This first approximation neglects the gate-source capacitance CGS21 of the drive transistor, that is, it is based on the assumption that the capacitive voltage divider between the drain node D21 and the first supply node only includes the gate-drain capacitance CGD21 and the capacitor 4231. If, however, the capacitance of the capacitor 4231 is significantly higher than the gate-drain capacitance CGD21 this assumption is valid. If the gate-source capacitance CGS21 is taken into account additionally, the voltage increase ΔV′ is less than the value obtained by applying equation (2). Referring to equation (2), the voltage difference ΔV′ can be adjusted by suitably designing the capacitance C4231 of the capacitor 4231 relative to the capacitance value of the gate-drain capacitance CGD21.

After switching on the first electronic switch 31, an increase of the drive transistor's gate potential VG21 to above the level of the biasing voltage V41 is not only caused by the increase of the drain potential VG21 of the drive transistor 21, but is also caused by an increase of the gate-source voltage VGS11 of the output transistor 11. This is a second effect that causes an increase of the gate voltage VG21. The gate node of the output transistor 11 is capacitively coupled to the gate node G21 of the drive transistor 21 via the gate-source capacitance CGS21 of the drive transistor 21, so that an increase of the gate-source voltage VGS11 of the output transistor 11 causes an increase of the gate potential VG21 of the drive transistor 21. As a first approximation, that neglects the gate-drain capacitance CGD21, an increase ΔV′′ of the gate potential VG21 resulting from this effect is given as follows:




  
    
      
        
          
            Δ
            ⁢
            
              
            
            ⁢
            
              V
              ′′
            
          
          =
          
            
              
                
                  CGS
                  ⁢
                  
                    
                  
                  ⁢
                  
                    2
                    1
                  
                
                
                  
                    CGS
                    ⁢
                    
                      
                    
                    ⁢
                    
                      2
                      1
                    
                  
                  +
                  
                    C
                    ⁢
                    
                      
                    
                    ⁢
                    
                      423
                      1
                    
                  
                
              
              ·
              VGS
            
            ⁢
            
              
            
            ⁢
            
              1
              1
            
          
        
        ,
      
    
    
      
        (
        3
        )
      
    
  




where VGS11 denotes the voltage level of the output transistors gate-source voltage, CGS21 denotes the capacitance value of the drive transistor's 21 gate-source capacitance, and C4231 denotes the capacitance of the capacitor 4231 in the biasing circuit 41. Based on equation (3) it can be seen that by suitably designing the capacitance of the capacitor 4231 relative to the capacitance value of the gate-source capacitance CGS21 of the drive transistor 21 the increase ΔV′′ of the gate potential VG21 can be limited.

The overall increase ΔV of the gate voltage VG21 referred to in equation (1) takes into account both of the effects explained with reference to equations (2) and (3). According to one embodiment, the capacitance C4231 is adapted to capacitance values of the gate-drain capacitance CGD21 and the gate-source capacitance CGS21 and to the voltage swings at the drain node D21 and the source node S21 of the drive transistor such that overall increase ΔV of the gate voltage VG21 is between 5% and 25%, in particular between 10% and 20% of the biasing voltage V41. According to one embodiment, the capacitance C4231 of the capacitor 4231 is at least 10 times, in particular at least 50 times the maximum capacitance value of the gate-drain capacitance CGD21. According to one embodiment, the capacitance C4231 of the capacitor 4231 is at least 5 times, in particular at least 10 times the maximum capacitance value of the gate-source capacitance CGS21.
The supply voltage V21−V11 between the second supply node 141 and the first supply node 131 is higher than the biasing voltage V41. According to one embodiment, the supply voltage is at least 2 times, at least 3 times, or even at least 5 times the biasing voltage. According to one embodiment, the biasing voltage V41 is between 2.5V and 3.5V while the supply voltage is 10V or higher.
Referring to FIG. 3 the gate-source voltage VGS11 of the output transistor 11 is below the electrical potential VG21 at the gate node of the drive transistor 21 because the drive transistor 21 switches off when the gate-source voltage VGS11 of the output transistor 11 reaches a voltage level that equals the electrical potential VG21 at the gate node G21 of the drive transistor 21 minus the threshold voltage of the drive transistor 21. Thus, the gate-source voltage VGS11 of the output transistor 11 is always lower than the biasing voltage V41 plus the voltage difference ΔV so that the maximum gate-source voltage VGS11 of the output transistor can be adjusted by suitably designing the biasing circuit 411 in particular, by suitably selecting the biasing voltage V41 and the capacitor 4231. In the biasing circuit 41, the resistor 4221 connected in parallel with the capacitor 4231 discharges the capacitor 4231 so that a voltage level of the gate voltage VG21 gradually decrease to the level of the biasing voltage V41. The rate at which the gate potential VG21 decreases is dependent on the time constant of the parallel circuit with the resistor 4221 and the capacitor 4231. In general, the rate at which the gate potential VG21 decreases is the higher the lower a resistance R4221 of the resistor 4221 is, and vice versa.
Referring to FIG. 4, the internal impedance 421 includes a further resistor 4211 connected between the voltage source 411 and the gate node G21 of the drive transistor 21. This further resistor 4211 defines the biasing circuit's 41 capability to counteract a decrease of the gate potential VG21 to below the biasing voltage V41. The lower a resistance R4211 of this resistor 4211 is the faster the biasing circuit 41 regulates the gate potential V21 from a level of below the biasing voltage V41 to a level corresponding to the biasing voltage V41.
FIG. 5 shows one embodiment of the voltage source 411 in the biasing circuit 41 in greater detail. In this embodiment, the voltage source 411 is implemented as a linear voltage regulator connected between the first supply node 131 and the second supply node 141. In this embodiment, the voltage regulator includes an output transistor 4111 that has a load path connected between the second supply node 141 and the parallel circuit with the resistor 4221 and the capacitor 4231. The further resistor 4211 shown in FIG. 4 is defined by the characteristic of the output transistor 4111 in the embodiment shown in FIG. 5. In particular, the resistor is defined by the transconductance, often referred to as gm, of the output transistor 4221. In the embodiment shown in FIG. 5, the output transistor 4111 is implemented as a MOSFET, in particular as an n-type MOSFET. In this voltage regulator 411, the output transistor 4111 is driven such that, in the steady state, the voltage across the parallel circuit with the resistor 4221 and the capacitor 4231 corresponds to the biasing voltage V41. This biasing voltage V41 is defined by a reference current source 4121 and a further resistor 4141. The reference current source 4121 drives a reference current I4121 through the resistor 4141 and a MOSFET 4151 that has its gate node connected to its drain node. A series circuit with the further resistor 4141 and the further transistor 4151 is connected between the gate node of the output transistor 4111 and the first supply node 131. According to one embodiment a resistance of the further resistor 4141 equals a resistance of the resistor 4221 and the output transistor 4111 and the further transistor 4151 are transistors of the same type, in particular, of the same channel width, the same channel length and the same threshold voltage. In this case, the gate-source voltage of the further transistor 4151 and the gate-source voltage of the output transistor 4111 are equal, and a voltage V4141 across the further resistor corresponds to the biasing voltage V41. That is, the biasing voltage V41 is defined by the level of the reference current I4121 multiplied with a resistance R4141 of the further resistor 4141:

V4141=V41=I4121·R4141  (4).

Referring to FIG. 5, a capacitor 4131 may be connected in parallel with the series circuit including the resistor 4141 and the transistor 4151. Referring to the above, the voltage across the series circuit with the resistor 4141 and the transistor 4151 is a reference voltage that drives the transistor and defines the biasing voltage V41. The gate node of the output transistor 4111 is capacitively coupled with the output of the voltage regulator 411, that is, with the parallel circuit including the resistor 4221 and the capacitor 4231. Thus, rapid changes of the biasing voltage V41, such as the increase of the biasing voltage at the time of switching on the first electronic switch 31, may change the electrical potential at the gate node of the output transistor 4111 and, therefore, the reference voltage if no additional measures are taken. The capacitor 4131 filters such changes of the electrical potential at the gate node of the output transistor 4111 and, therefore, stabilizes the reference voltage.
The electronic circuit 1 with the first driver 101 explained above is configured to discharge a capacitive load CGS connected to output 12. FIG. 6 shows one embodiment of an electronic drive circuit that is configured to charge a capacitive load CGS connected to the output 12 based on an input signal SIN. Like in the embodiment shown in FIG. 1, the capacitive load is a gate-source capacitance of a MOSFET Z.
The electronic circuit shown in FIG. 6 includes a driver 102, which will be referred to as second driver or high-side driver in the following. The tropology of the driver 102 corresponds to the tropology of the driver 101 shown in FIG. 1 to which reference is made. In the drivers 101, 102 shown in FIGS. 1 and 6 like features have the same reference characters that are only different by a subscript index “1” added to the reference characters in FIG. 1 and a subscript index “2” added to the reference characters in FIG. 6. In particular, the driver 102 shown in FIG. 6 an output transistor 12, a drive transistor 22, and a first electronic switch 32. The output transistor 12 includes a control node and a load path connected between the output 12 and a first supply node 132. The drive transistor 21 includes a control node, and a load path connected to the control node of the output transistor 12 and connected in series with the first electronic switch 32. The first electronic switch 32 is connected between a second supply node 141 and the load path of the drive transistor 21.
The driver 102 may include a second electronic switch 62 connected between the control node of the output transistor 12 and the first supply node 132. Optionally, a resistor 72 (illustrated in dashed lines in FIG. 6) is connected in parallel with the second electronic switch 62 and, therefore, between the control node of the output transistor 12 and the first supply node 132.
A biasing circuit 42 is connected between the control node of the drive transistor 22 and the first supply node 132. The biasing circuit 42 includes a voltage source 412 configured to provide a biasing voltage V42, and an internal impedance 422. A control circuit 52 is configured to receive the input signal SIN and to drive the first electronic switch 32 and the second electronic switch 62 based on the input signal SIN.
Referring to FIG. 7, which shows timing diagrams of the input Signal SIN, a drive signal S32 of the first electronic switch 32, and a drive signal S62 of the second electronic switch 62. The input signal SIN defines the desired operation state of the load Z driven by the electronic switch 10. For the purpose of illustration, it is assumed that the input signal SIN can have one of two different signal levels, namely a first signal LE1 and a second signal level LE2. The first signal level LE1 indicates that it is desired to switch on the output transistor 12, so as to charge the capacitive load CGS, and the second signal level LE2 indicates that it is desired to switch off the output transistor 12. Just for the purpose of illustration, the first level LE1 is a high-level and the second level LE2 is a low-level in the example shown in FIG. 7.
Each of the drive signals S32, S62 can have one of an on-level, which switches on the respective electronic switch 32, 62, and an off-level, which switches off the respective switch 32, 62. When the input signal SIN has the second level LE2 the control circuit 82 switches off the first electronic switch 32 by generating an off-level of the drive signal S32 and switches on the second electronic switch 62 by generating an on-level of the drive signal S62. In this operation mode of the first driver 102 the output transistor 12 is switched off. When the signal level of the input signal SIN changes from the second level LE2 to the first level LE1 the control circuit 82 switches off the second electronic switch 62 by generating an off-level of the drive signal S62 and, after an optional delay time TD, switches on the first electronic switch 32, by generating an on-level of the drive signal S32. In this operation mode of the first driver 102 the output transistor 12 is switched on.
The driver 102 shown in FIG. 6 is different from the driver 101 shown in FIG. 1 in that the output transistor 12 and the drive transistor 22 are p-type MOSFETs, wherein the drain node of the output transistor 12 is connected to the output 12, and the source node of the drive transistor 22 is connected to the gate of the output transistor 12. Further, the first electronic switch 32 is an n-type MOSFET and the second electronic switch 62 is a p-type MOSFET, wherein a source node of the MOSFET forming the first electronic switch 32 is connected to the second supply node 142. Furthermore, an electrical potential V12 at the first supply node 132 is higher than an electrical potential V22 at the second supply node 142.
If, in the driver 102 shown in FIG. 6, the first electronic switch 32 is implemented as an n-type MOSFET, then the off-level of the drive signal S31 may correspond to the level of the electrically potential V22 at the second supply node 142, while the on-level may be a signal level that is higher than the electrically potential V22 at the second supply node 142 plus the threshold voltage of this n-type MOSFET 32. Those signal levels are shown in FIG. 7. If the second electronic switch 62 is a p-type MOSFET, then the off-level of the drive signal S61 may correspond to the level of the electrical potential V11 at the first supply node 131, while the on-level may be a signal level that is lower than the electrical potential V12 at the first supply node 132 minus the threshold voltage of this p-type MOSFET. These signal levels are also shown in FIG. 7.
Everything that has been explained with regard to the functionality of the first driver 101 shown in FIG. 1 applies to the second driver 102 shown in FIG. 6 as well. In particular, at the time of switching on the first switch 31 the magnitude of the gate voltage VG22 of the drive transistor 22 increases to above the magnitude of the biasing voltage by virtue of the capacitive coupling of the gate node G22 to the drain node D22 and the source node S22, respectively, and by virtue of the internal impedance 422 of the biasing source 42. The biasing source may be implemented as explained with reference to FIGS. 4 and 5 herein below, wherein the output transistor (not shown) in the biasing source 42 may be implemented as a p-type transistor instead of the n-type transistor 4111 shown in FIG. 5.
FIG. 8 shows one embodiment of an electronic switch that includes a low side driver 101 as shown in FIG. 1 and a high side driver 102 as shown in FIG. 6. This electronic circuit is configured to charge the capacitive load CGS or discharge the capacitive load based on the input signal. In this embodiment, the second supply node 141 of the low side driver 101 is connected to the first supply node 132 of the high side driver 102 so that the second supply potential V21 of the low-side driver 101 equals the first supply potential V12 of the high-side driver 102, that is, V21=V12=V2. Furthermore, the second supply node 142 of the high side driver 102 is connected to the first supply node 131 of the low side driver 101 so that the second supply potential V22 of the high-side driver 102 equals the first supply potential V11 of the low-side driver, that is, V22=V11=V1. V1 denotes of an electrical potential received at the first supply node 131 (the second supply node 142) of the low side driver 101 (the high side driver 102), and V2 denotes an electrical potential received at the second supply node 141 (the first supply node 132) of the low side driver 101 (the high side driver 102). The electrical potential V1 may correspond to the electrical potential at a load terminal of the capacitive load CGS facing away from the output 12.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.

## Claims

1. An electronic circuit, comprising:
an input configured to receive an input signal and an output configured to be coupled to a load;
an output transistor comprising a load path and a control node, the load path being connected between the output and a first supply node;
a drive transistor comprising a load path and a control node, the load path of the drive transistor being connected to the control node of the output transistor;
a first electronic switch connected in series with the load path of the drive transistor;
a biasing circuit comprising an internal impedance and connected between the control node of the drive transistor and the first supply node; and
a control circuit configured to receive the input signal and to generate a first drive signal for driving the first electronic switch based on the input signal, the first drive signal having an on-level and an off-level which depend on a state of the input signal.

2. The electronic circuit of claim 1, wherein the first electronic switch is connected between the load path of the drive transistor and a second supply node.

3. The electronic circuit of claim 1, further comprising:
a second electronic switch connected between the control node of the output transistor and the first supply node,
wherein the control circuit is configured to generate a second drive signal for driving the second electronic switch based on the input signal, the second drive signal having an on-level and an off-level which depend on the state of the input signal.

4. The electronic circuit of claim 3, wherein the control circuit is configured to drive the first electronic switch and the second electronic switch such that at most one of the first electronic switch and the second electronic switch is driven in an on-state at the same time.

5. The electronic circuit of claim 1, further comprising:
a resistor connected between the control node of the output transistor and the first supply node.

6. The electronic circuit of claim 1, wherein the internal impedance of the biasing circuit comprises at least one of a resistor and a capacitor connected between the control node of the drive transistor and the first supply node.

7. The electronic circuit of claim 6, wherein the drive transistor has an internal gate-drain capacitance, and wherein a capacitance of the capacitor is at least 10 times a capacitance value of the gate-drain capacitance.

8. The electronic circuit of claim 6, wherein the drive transistor has an internal gate-source capacitance, and wherein a capacitance of the capacitor is at least 5 times a capacitance value of the gate-source capacitance.

9. The electronic circuit of claim 1, wherein the output transistor and the drive transistor have the same conductivity type.

10. The electronic circuit of claim 9, wherein the first electronic switch is implemented as a transistor of a conductivity type complementary to the conductivity type of the output transistor and the drive transistor.

11. The electronic circuit of claim 1, wherein the biasing circuit further comprises a voltage regulator having a supply input connected between the first supply node and a second supply node, and having an output coupled to the control node of the drive transistor.

12. The electronic circuit of claim 2, wherein the first supply node is configured to receive a first electrical potential and the second supply node is configured to receive a second electrical potential higher than the first electrical potential, wherein each of the output transistor and the drive transistor is an n-type MOSFET, and wherein the first electronic switch is a p-type MOSFET.

13. The electronic circuit of claim 2, wherein the first supply node is configured to receive a first electrical potential and the second supply node is configured to receive a second electrical potential lower than the first electrical potential, wherein each of the output transistor and the drive transistor is a p-type MOSFET, and wherein the first electronic switch is an n-type MOSFET.

14. The electronic circuit of claim 2, further comprising:
a further output transistor comprising a load path and a control node, the load path being connected between the output and a third supply node;
a further drive transistor comprising a load path and a control node, the load path being connected to the control node of the further output transistor;
a further first electronic switch connected in series with the load path of the further drive transistor;
a further biasing circuit comprising an internal impedance and connected between the control node of the further drive transistor and the third supply node; and
a further control circuit configured to receive the input signal and to drive the further first electronic switch based on the input signal.

15. The electronic circuit of claim 14, wherein the further first electronic switch is connected between the load path of the further drive transistor and a fourth supply node.

16. The electronic circuit of claim 15, wherein the first supply node and the fourth supply node are connected and the second supply node and the third supply node are connected.

17. The electronic circuit of claim 14, further comprising:
a further second electronic switch connected between the control node of the output transistor and the third supply node,
wherein the further control circuit is configured to drive the second electronic switch based on the input signal.

18. The electronic circuit of claim 17, wherein the control circuit is configured to drive the further first electronic switch and the further second electronic switch such that at most one of the further first electronic switch and the further second electronic switch is driven in an on-state at the same time.

19. The electronic circuit of claim 14, further comprising:
a further resistor connected between the control node of the further output transistor and the third supply node.

20. An electronic circuit, comprising:
an input configured to receive an input signal and an output configured to be coupled to a load;
an output transistor comprising a load path and a control node, the load path being connected between the output and a first supply node;
a drive transistor comprising a load path and a control node, the load path of the drive transistor being connected to the control node of the output transistor;
a first electronic switch connected in series with the load path of the drive transistor;
a biasing circuit comprising an internal impedance and connected between the control node of the drive transistor and the first supply node; and
a control circuit configured to receive the input signal and to drive the first electronic switch based on the input signal,
wherein the biasing circuit further comprises a voltage regulator having a supply input connected between the first supply node and a second supply node, and having an output coupled to the control node of the drive transistor.