Abstract:
A current sensing circuit for sensing the current through a main switch, such as the PMOS or NMOS switches of a switching regulator, is disclosed. The circuit comprises a mirror switch, said mirror switch being substantially similar to said main switch but with a smaller aspect ratio, a difference amplifier for ensuring that the voltage across said first leg and across said second leg are substantially equal and thereby to derive from said mirror switch a sensing current nominally equal to a current flowing in said main switch divided by a sensing ratio, a current source for producing a quiescent current in said difference amplifier and a compensatory device for compensating for said quiescent current such that said current sensing circuit can sense currents in the main switch which are smaller than the quiescent current multiplied by the sensing ratio. The compensatory device may be one or two switches essentially similar to the mirror switch.

Description:
PRIOR APPLICATION DATA  
       [0001]     The present application claims priority from prior UK application GB 0521896.1, filed on Oct. 27, 2005, incorporated by reference herein in its entirety.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to current controlled switching mode regulator circuits and in particular to current sensing arrangements in such circuits.  
       BACKGROUND OF THE INVENTION  
       [0003]     Switching regulators are very commonly used in DC-DC conversion as they offer higher efficiency than linear regulators. They typically consist, in their most basic form, of an inductor, a first switch and a diode (or second switch), the latter two components switching the inductor alternately between charging and discharging states, in response to signals from a controller. These basic elements can be arranged to form a step-down (buck), step-up (boost) or inverting (buck-boost) regulator.  
         [0004]     It is well described in the literature that by sensing the current in the inductor (possibly via sensing the current in the switch), and using this sensed current in the control algorithm for the switch, certain benefits can be gained. The main advantage is that the control loop can be reduced from second order (2 pole), to approximately first order (1 pole). Other advantages are greater line rejection, and the instantaneous detection of peak current in the inductor. This control method is called “current mode control” 
         [0005]     One of the main difficulties with current mode control is accurately measuring the current in the inductor on a cycle by cycle basis. One way of doing this would be placing a resistor in series with the first switch on the supply side. This would have little common mode shift as the switch is turned on and off. Equally a resistor could be placed in series with the diode or second switch on the ground side to the same effect. A resistor in series with the inductor on the switch side would probably have large common mode shift and, for a low output voltage, could prove challenging to implement on the output side of the inductor necessitating a very wide common mode range on the amplifier sensing the voltage across the resistor. All these techniques also suffer from loss in the resistive component, and the necessity for a low value, but accurate resistor (which is difficult and expensive to achieve on silicon).  
         [0006]     One technique to avoid these problems is to mirror either the first switch or the second switch with another much smaller transistor with similar properties, having say 1:10000 ratio in size between them. This could be done by using a single cell of a multi-cell switch as the mirror.  
         [0007]     The examples shown will concentrate on the mirroring of the first switch. However it should be noted that the invention is equally applicable to circuits mirroring the second switch.  
         [0008]     A problem with many known mirror circuits is that they require a quiescent current to operate. While this quiescent current may be small by itself, it can result in much greater currents not being detected by the current sensing circuit. This is because the current sense output may be 10000 times smaller than the input and therefore unable to detect inductor currents 10000 times the quiescent current. Thus for light loads the current sensed will become zero and the converter may not work or may become unstable.  
       SUMMARY OF THE INVENTION  
       [0009]     It is an aim of the invention to address the above problem and to provide a current sensing circuit which can sense smaller inductor currents despite the quiescent current taken by the sensing circuit to operate.  
         [0010]     In a first aspect of the invention there is provided a current sensing circuit for sensing the current through a main switch, said circuit comprising: a mirror switch, said mirror switch being substantially similar to said main switch but of different dimensions, a difference amplifier having a first leg and a second leg connected respectively to the output electrodes of said main switch and said mirror switch, said difference amplifier ensuring that the voltage across said first leg and across said second leg are substantially equal and thereby to derive from said mirror switch a sensing current nominally equal to a current flowing in said main switch divided by a sensing ratio, and a current source for producing a quiescent current in said difference amplifier, wherein there is further provided a compensatory device for compensating for said quiescent current such that said current sensing circuit can sense currents in the main switch which are smaller than the quiescent current multiplied by the sensing ratio.  
         [0011]     It should be noted that the term “main switch” refers to the switch being mirrored, and may be the “first switch” or “second switch” of the above introduction.  
         [0012]     Said first mirror switch may be dimensioned to obtain a sensing ratio in the region of 10,000-100,000, although this sensing ratio conceivably could be 1000 or even 100. This may be achieved by it having an area or aspect ratio in the region of four or five orders of magnitude smaller than that of said main switch. It may comprise one cell isolated from the main switch, the main switch being comprised of a plurality of similar cells. Isolated in this case means electrically, not necessarily physically isolated, as it may be desirable for the single cell mirror switch to reside with the main switch so that ambient conditions are the same for both.  
         [0013]     Said compensatory device may comprise a resistive element located in the first leg of said difference amplifier. Said resistive element should have substantially the same characteristics than that of the first mirror switch. Said resistive element preferably comprises a device substantially similar to said mirror switch and arranged to be always on.  
         [0014]     Said compensatory device may purposely overcompensate for said quiescent current. This may be achieved by providing two resistive elements in series or a resistive element with greater resistance than said first mirror switch in the first leg of said difference amplifier. In one embodiment there are provided two devices substantially similar to said mirror switch in series arranged to be always on. In an alternative embodiment the overcompensation is achieved by having one device substantially similar to said mirror switch but passing a larger current (possibly two-times) than the quiescent current through it. This may be achieved by providing a further current source in series with a single mirror switch and in parallel with said difference amplifier.  
         [0015]     Said difference amplifier may comprise first and second difference amplifier transistors, said first and second difference amplifier transistors being substantially similar to one another and each having a control electrode and first and second main current-carrying electrodes, and arranged such that their control electrodes are tied together and that they are always on; and a further transistor, said further transistor arranged to control the current through the first difference amplifier transistor. As a result the voltages across the first leg and second leg of the difference amplifier are equal and the current ratio between the main switch and the mirror switch is maintained. Said transistors may be MOSFETS.  
         [0016]     Said difference amplifier may be further provided with dummy transistors. These may be arranged such that inputs of said difference amplifier are switched to these when said main switch and first mirror switch are off.  
         [0017]     Said switches may all comprise MOSFETS. The circuits may be arranged for either PMOS switches or NMOS switches depending on the main switch type.  
         [0018]     The above and other features and advantages of the invention will be understood from a consideration of the description of specific embodiments which follows. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which:  
         [0020]      FIG. 1  shows a basic known buck converter arrangement;  
         [0021]      FIG. 2  shows a known current sensing arrangement;  
         [0022]      FIG. 3  shows a current sensing arrangement according to a first embodiment of the invention;  
         [0023]      FIG. 4  shows a current sensing arrangement according to a second embodiment of the invention and an optional modification to make a third embodiment;  
         [0024]      FIG. 4   b  shows a variation on the current sensing arrangement  FIG. 4 , incorporating NMOS device mirroring of current sense output;  
         [0025]      FIG. 5  shows a current sensing arrangement according to a fourth embodiment of the invention; and  
         [0026]      FIG. 6  shows a current sensing arrangement according to a fifth embodiment of the invention suitable for sensing current in an NMOS switch. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0027]      FIG. 1  shows a basic, constant frequency, current mode control buck converter (although the invention is equally applicable in use with other types of converters e.g. boost or buck-boost). The converter consists of a PMOS switch  10  in series with a NMOS switch  20  (or possibly a diode) between a voltage source V BAT  and ground GND. In parallel with the NMOS switch  20  (also in series with the PMOS switch) is an inductor  30  and a capacitor  40 . Converter output V OUT  is taken from the node between inductor  30  and capacitor  40 . The output voltage is also fed into an error amplifier  50 .  
         [0028]     The output of the error amplifier  50  is fed into one input of a comparator  60 . A current monitor  80  generates a signal representative of the current in inductor  30 , and this is fed to the inverting input of comparator  60 . The output of the comparator  60  is fed to the reset input of a latch  70  which controls switches  10  and  20  via gate  90 .  
         [0029]     Control of the switch  10  has been achieved previously by techniques such as “voltage mode control” and “current mode control”. Typically, the PMOS switch  10  is connected to an input voltage and is closed at the beginning of a clock cycle. Closing the switch  10  causes the current in the inductor  30  connected between the switch and the output of the converter to rise. When the output of the inductor current monitor  80  exceeds the output of the error amplifier  50 , the comparator  60  resets latch  70 . This causes the PMOS switch  10  to be turned off, and not turned on again until the beginning of the next clock cycle while the NMOS switch  20  is driven in anti-phase with the PMOS switch  10 . In this way the output voltage is controlled to the required value.  
         [0030]      FIG. 2  shows a preferred form of current monitor  80  using the current mirror principle for sensing the current in the PMOS switch  100  of  FIG. 1 . The main converter components of  FIG. 1  are not shown. This shows the main PMOS switch  100  and, in parallel with it, mirror switch  105 . The mirror switch  105  is substantially identical to the main PMOS switch  100 , except for its dimensions. The main PMOS switch  100  and the mirror switch  105  have common source, gate and bulk connections. The main PMOS switch  100 , as before, is connected between voltage source V BAT  and the inductor (not shown), while the mirror switch  105  is connected between V BAT  and a sense leg  110  which forms part of the current monitor. A difference amplifier  125  is provided by two PMOS devices  115 ,  120 . The first of these devices  115  has its source connected to the inductor side of the PMOS switch  100  and the second device  120  has its source connected to the sense side of the mirror switch  105 . A further PMOS device  130  provides the output of the amplifier  125  and is provided in the sense leg  110 . Device  130  has its gate tied to the drain of PMOS device  115   
         [0031]     In the case of MOSFETs, the aspect ratio of the mirror switch  105  compared to the main PMOS switch  100  determines the sensing ratio. Typically the width (W) of the main PMOS switch  100  is very large, say 10 mm, and therefore the width of the mirror device may be 10 μm to scale by 1000, for the same length (L) (say 0.5 μm). In this case the channel area, and the total area of the mirror device, will end up smaller. Conceivably L might also be increased, to say 5 μm, to give a further 10 times scaling of current without making the width too small. In this case the aspect ratio would reduce, but the area would in fact increase. This contrasts with bipolar transistors, where the sensing ratio is given approximately by the ratio of their emitter areas. In the examples below the sensing ratio will be 1:10000.  
         [0032]     In operation differential amplifier  125  keeps the drain voltage of the mirror switch  105  the same as that of the main switch  100 , such that the voltage across them matches precisely. Any difference in source voltage of the two common gate PMOS devices  115 ,  120  will cause the voltage on the drain of PMOS device  115  to rise or fall and thus pull the gate of the device  130  up or down, altering the current therein until the sources are more equal.  
         [0033]     Current from the mirror switch  105  passes through the sense leg  110 , through PMOS device  130 , and is used to sense the current in the main PMOS switch  100 . The ratio of this sense current I sense  to the actual current being measured is the same as that of the size of the mirror switch  105  to main PMOS switch  100 , i.e.1:10000. Note that the main PMOS switch  100  and its PMOS mirror switch  105  will typically both be operating in linear or triode region, with the other PMOS devices  115 ,  120 ,  130  in saturation.  
         [0034]     A problem with this circuit is that the 10 μA quiescent taken by the amplifier  125  means that in ideal conditions, no current is measured (I sense =0) until the main PMOS switch  100  supplies 100 mA (10000*10 μA). This is because, if we assume that the main PMOS switch  100  has on-resistance (R onPMOS ) of 0.1 ohm, the mirror switch  105  will have an on-resistance of 1 kohm (R onMIRROR ). If input current I in  is 100 mA then this 100 mA through the main PMOS switch  100  results in 10 mV being dropped across it. 10 μA through the PMOS mirror  105  also results in a 10 mV drop. Therefore the circuit is balanced (the same voltage being dropped across each leg of the differential amplifier  125 ) and the current in the sense leg  110 , I sense , equals zero. Similarly a 200 mA input means that there is 20 μA through the mirror switch resulting in only 10 μA for I sense . Therefore I sense =I/10000−10 μA=(I=100 mA)/10000(for I&gt;100 mA)or=0 otherwise. Thus for light loads the current in the inductor is measured as zero and the control mechanism of the converter may not work or could be unstable.  
         [0035]      FIG. 3  shows a circuit similar to that of  FIG. 2  adapted according to an embodiment of the invention. The circuit is essentially similar but with the addition of a copy device  150  similar to mirror switch  105  between the main PMOS switch  100  and the difference amplifier transistor  115 . The device  150  is arranged to be permanently on with a similar gate voltage as  105  is connected to when “on”.  
         [0036]     Analysing this circuit using the same example component values as the previous drawing, and the same input current I in  of 100 mA, this current in the main PMOS switch  100  again results in a drop of 10 mV across it. The copy device  150  induces a further drop of 10 μA*R onMIRROR  (1 kohm in this example) which equals 10 mV. As the copy device  150  drops a further 10 mV, the mirror device  105  sees 20 mV across main PMOS switch  100  and the copy of the PMOS mirror switch and, to remain in equilibrium, delivers 20 μA. 10 μA of this is delivered down the left-hand leg, leaving 10 μA (I sense ) to go down the right-hand (sense) leg  110 , and through PMOS device  130 . As I sense  is 1/10000 of the input current I in  (that is the inductor current being measured), it can be seen that I sense  is now correct and current is now sensed, in the ideal case, as soon as any current flows through the main PMOS switch  100 . In principle, copy device  150  is acting as a simple resistor. Because it is a copy of mirror switch  105 , and because copy device  150  will see very close to the same gate-source voltage V gs  as the mirror device it will be a resistor with a very similar on-resistance (R on ) to that of mirror switch  105 .  
         [0037]     One remaining problem, however, is the case of offset in the amplifier (for example random manufacturing offset, or second order effects due to different drain voltages across the differential amplifier). An adverse offset could mean that current is still not sensed until greater than a certain threshold.  
         [0038]      FIG. 4  shows two alternatives for addressing the offset problem. In one alternative a second copy device  160  is added to the main PMOS sensing leg in series with the first copy device  150 . The other alternative shown (by dotted line) has only the one copy device  150  (device  160  should be ignored in this case) and a further 10 μA current source  170 .  
         [0039]     Both of these alternatives result in the sense circuit seeing (again using the component values of the previous example and input current of 100 mA) the equivalent of 10000*10 μA=100 mA in the main PMOS switch  100  even when there is no input, and makes the circuit immune to offsets equal to 100 mA*R ONPMOS =10 mV. Of course with both of these approaches, there is now a static error of 100 mA in the current measurement (0 to 200 mA in the worst case), but this is not important for stability since it is only a DC shift.  
         [0040]      FIG. 4   b  shows a variation which allows for multiple outputs I sense  as well as allowing for further flexibility in the sensing ratio. In this variation the differential amplifier  125  is reversed and PMOS device  130  is replaced with NMOS device  180  which is mirrored with further NMOS device  181 . If the NMOS devices  180  and  181  are identical then the sensing ratio will depend on the aspect ratios of main PMOS switch  100  and mirror device  105  as before, but if different, then the aspect ratio is further dependent on the aspect ratios  6   f  the NMOS devices  180 ,  181 . Further copies of I sense  are also easily obtained by adding further NMOS devices to mirror NMOS device  180 . Each of these outputs can have its sensing ratio set independently depending on the aspect ratio of the mirroring NMOS.  
         [0041]     It is also possible to mirror the PMOS device  130 . Simply adding a further PMOS device in parallel with PMOS device  130  with common gate and source connections would split I sense  between them (according to respective aspect ratios). However, copies of I sense  obtained from the drain of PMOS device  130  can be generated by passing it through NMOS mirrors.  
         [0042]     A further problem with the circuits depicted above is that the main switch  100  is switching on and off, and the measured current is valid only when it is on. When the main switch  100  is off, its drain voltage swings below ground. This causes massive swings on the difference amplifier, resulting in large recovery times.  
         [0043]      FIG. 5  shows an improvement to the circuit of  FIG. 3 . This shows essentially the same circuit as  FIG. 3  with the addition of dummy PMOS devices  135   a ,  135   b ,  140   a ,  140   b  connected as shown. The amplifier senses the main PMOS switch  100  and mirror PMOS switch  105  via switches  135   a  and  140   a , when the main PMOS switch  100  is ON. When the main PMOS switch  100  is OFF, the amplifier senses the supply via switches  135   b  and  140   b  to maintain the common mode point. Two copies  150   a  and  150   b  of the PMOS mirror switch are shown in this example, one ( 150   a ) in series with main PMOS switch  100  and dummy transistor  135   a , the other ( 150   b ) in series with dummy transistor  135   b.    
         [0044]      FIG. 6  shows an equivalent circuit to  FIG. 4  but for sensing the second (NMOS) switch  20  in the converter of  FIG. 1  instead of the first (PMOS) switch  10 . This shows NMOS switch  200  being mirrored using NMOS mirror switch  205  in the same way as the PMOS switch was mirrored in previous examples. The NMOS mirror switch  205  is therefore identical to the main NMOS switch  200  in all but size. Devices  215 ,  220 ,  230  (NMOS in this case) form the current amplifier equalising the voltages through each leg as in the previous examples. As a result it will be apparent to the skilled person that this circuit operates essentially the same way as the circuit depicted in  FIG. 4 . Over-compensation for the quiescent current is provided in the form of the two copy NMOS switches  250 ,  260 .  
         [0045]     Although most examples shown have been created for current sensing in the PMOS switch of switching converters, the concept is applicable to any circuit that requires the sensing of current in a transistor, whether it is PMOS or NMOS.  
         [0046]     The above examples are for illustration only and should not be taken as limiting. For instance, although the circuit technique is particular useful in switching applications such as Class D drives (switching) and switching chargers, it is also envisaged that such techniques can be applied to a wider range of applications that do not include switching (for example non-switching regulators).