Abstract:
A method for sensing the supply current of a switched DC-to-DC converter is discussed. The method sensing a first voltage that is proportional to the supply current, wherein the first voltage has first noise; outputting a second voltage that is based on the first voltage, and wherein the second voltage has second noise that is smaller than the first noise; and comparing the second voltage to a reference voltage to provide an indication of the supply current. According to the systems and methods disclosed herein, accurate current sensing is provided.

Description:
[0001]    This application is a Continuation of U.S. application Ser. No. 11/330,883, filed Jan. 11, 2006, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to electronic circuits, and more particularly to a system for sensing the supply current, for example in a switched DC-to-DC converter. 
       BACKGROUND OF THE INVENTION 
       [0003]    DC-to-DC converters are well known. A DC-to-DC converter is a device that receives a DC input voltage and produces a DC output voltage. The output voltage is typically produced at a different voltage level from the input voltage. DC-to-DC converters can produce high voltage usable for low power applications including supplying power to mobile devices such as cellular phones and laptop computers. 
         [0004]    Sensing supply current is one task that a DC-to-DC converter performs during a voltage conversion process. Supply current sensing in a DC-to-DC converter is generally implemented by sensing the voltage generated by the supply current flowing through a sense resistor. Sometimes the voltage across a switch is directly sensed where the “on” resistance of the switch is used as a sense resistor. Because the sense voltage is based on the supply current, the behavior of the supply current (e.g., glitches) will be reflected in the behavior of the sense voltage. This is problematic, because the resulting sense voltage may be very noisy (i.e., having a high ripple). As a result, current sensing of the conventional DC-to-DC converter is performed with poor accuracy. 
         [0005]    Accordingly, what is needed is an improved system for sensing the supply current of a switched DC-to-DC converter. The present invention addresses such a need. 
       SUMMARY OF THE INVENTION 
       [0006]    A system for sensing the supply current of a switched DC-to-DC converter is disclosed. The system includes a first circuit that senses a first voltage that is proportional to the supply current, wherein the first voltage has first ripple; a second circuit coupled to the first circuit, wherein the second circuit outputs a second voltage that is based on the first voltage, and wherein the second voltage has a second ripple that is smaller than the first ripple; and a third circuit coupled to the second circuit, wherein the third circuit compares the second voltage to a reference voltage to provide an indication of the supply current. 
         [0007]    According to the system disclosed herein, the system provides accurate current sensing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a block diagram of a conventional buck DC-to-DC converter where current sensing is performed using a sense resistor in series with a switch. 
           [0009]      FIG. 2  is a block diagram of the conventional buck DC-to-DC converter similar to the DC-to-DC converter of  FIG. 1 , except that the current sensing is performed using an “on” resistance of a switch. 
           [0010]      FIG. 3  shows the behavior of the current sensing performed by the DC-to-DC converter of  FIG. 1 . 
           [0011]      FIG. 4  is a schematic diagram of a DC-to-DC converter in accordance with one embodiment of the present invention. 
           [0012]      FIG. 5  is a schematic diagram of a DC-to-DC converter in accordance with another embodiment of the present invention. 
           [0013]      FIG. 6  shows the behavior of voltages V sense1  and V sense1  for the DC-to-DC converter shown in  FIG. 4  when the DC-to-DC load is changed from 100 mA to 1 A, in accordance with the present invention. 
           [0014]      FIG. 7  is a schematic diagram of a DC-to-DC converter in accordance with another embodiment of the present invention. 
           [0015]      FIG. 8  is a schematic diagram of a DC-to-DC converter  800  in accordance with another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    The present invention relates to electronic circuits, and more particularly to a system for sensing the supply current of a switched DC-to-DC converter. The following description is presented to enable one of ordinary skill in the art to make and use the invention, and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. 
         [0017]    A system in accordance with the present invention for sensing the supply current of a switched DC-to-DC converter is disclosed. The system includes a first circuit that senses a sense voltage, which is proportional to the supply current. The sense voltage has ripple that may be relatively high. The system also includes a second circuit that outputs a second voltage that is based on the sense voltage, where the second voltage has a ripple that is substantially smaller than the ripple of the sense voltage. The system also includes a third circuit that compares the second voltage to a reference voltage to provide an indication of the supply current. Because the second voltage has a smaller ripple than that of the sense voltage, the circuit can sense the supply current with better accuracy. To more particularly describe the features of the present invention, refer now to the following description in conjunction with the accompanying figures. 
         [0018]    Although the present invention disclosed herein is described in the context of switch DC-to-DC converters, the present invention may apply to types of converters, and still remain within the spirit and scope of the present invention. 
         [0019]      FIG. 1  is a block diagram of a conventional buck DC-to-DC converter  50  where current sensing is performed using a sense resistor  52  in series with a switch  54 . The DC-to-DC converter  50  also includes a comparator  56 , an OR gate  58 , a delay block  60 , a switch  62 , an inductor  64 , and a capacitor  66 .  FIG. 2  is a block diagram of the conventional buck DC-to-DC converter  70  similar to the DC-to-DC converter  50  of  FIG. 1 , except that the current sensing is performed using an “on” resistance of the switch  54 . With both DC-to-DC converters  50  and  70 , a sense voltage V sense  is compared to a pre-defined reference voltage V r . The reference voltage V r  may represent, for example, the maximum current allowed to flow through the switch  54 . 
         [0020]      FIG. 3  shows the behavior of the current sensing performed by the DC-to-DC converter  50  of  FIG. 1 . Referring to both  FIGS. 1 and 3  together, the two switches  54  and  62  alternatively turn on and off such that the signals dh and dl behave like a two-phase clock signal, the duty cycle of which defines the value of the DC-to-DC converter output voltage V out . The switch  54  is controlled by the signal dh. Accordingly, when the signal dh is low, the switch  54  turns on, and the supply current I dcdc  flows from the voltage supply V dd  to the output voltage V out . The sense resistor  52  is connected between the voltage supply V dd  and the switch  54 . When the switch  54  is on, the supply current I dcdc  flows through the resistor  52  and generates a sense voltage V sense , which is proportional to and thus represents a measure of the supply current I dcdc . When the switch  54  is off, no current flows through the resistor  52  (i.e., I dcdc =0) and the sense voltage V sense  is equal to the supply voltage V dd . By comparing the sense voltage V sense  with a proper reference voltage V r , the output is only valid when the switch  54  is on. That is, the output of the DC-to-DC converter  50  must be gated with the same signal used to turn on the switch  54 ; i.e., the output of the DC-to-DC converter  50  must be ORed with the signal dh. A problem with the DC-to-DC converter  50  is that glitches can occur in the current due to direct path current (i.e., non-ideal switching of the switches  54  and  62 ) as well as in the sense voltage V sense . Consequently, the output of the comparator can change state, even if the current threshold determined by the voltage V r  is not reached. This is shown in  FIG. 3  where glitches on the supply current I dcdc  generate glitches on the sense voltage V sense , which makes the output of the DC-to-DC converter  50  change state. To avoid this behavior, the OR gate  58  is controlled by signal dh 1  instead of signal dh. Signal dh 1  (generated through delay block  60 ) has a delayed falling edge with respect to dh, and thus, as shown in  FIG. 3 , the comparator changes its state only if the sense voltage V sense  becomes lower than the threshold V r . Glitches have no effect on the comparator output. 
         [0021]    The DC-to-DC converter  70  of  FIG. 2 , where the current sensing is performed by sensing the voltage drop across switch  54 , has the same problems as the DC-to-DC converter  50  of  FIG. 1 . The problem with the DC-to-DC converters  50  and  70  is that the sense voltage elements have a very high ripple, as shown in  FIG. 3 . Consequently, this makes it difficult to perform accurate current sensing. Assuming, for example, that the current threshold at 1 A and the sense resistor  52  0.1Ω, the ripple voltage is about 100 mV. Both circuits of  FIGS. 1 and 2  have this problem. 
         [0022]    In accordance with the present invention, a DC-to-DC converter senses DC-to-DC current by sensing the voltage drop across a sense resistor or across a switch by providing to the comparator a clean sense voltage V sense  (i.e., a sense voltage with a very small ripple). 
         [0023]      FIG. 4  is a schematic diagram of a DC-to-DC converter  400  in accordance with one embodiment of the present invention. The DC-to-DC converter  400  includes a switched capacitor circuit  402  that has switches  404  and  406  and capacitors  408  and  410 . The DC-to-DC converter  400  also includes switches  412  and  414 , a sense resistor  416 , an inductor  418 , a capacitor  420 , a delay block  422 , and a comparator  424 . In one embodiment, the switches  412  and  414  are PMOS and NMOS transistors, respectively. 
         [0024]    In operation, the supply current I dcdc  of the DC-to-DC converter  70  is sensed through the sense resistor  416 . The switches  412  and  414  and the sense resistor  416  sense the sense voltage V sense , which is proportional to and thus representative of the supply current I dcdc . The sense voltage V sense  is input to the switched capacitor circuit  402 , which generates voltage V sense1  that is used as input for the comparator  424 . The voltage V sense1  has a very small ripple. A signal dh 1  controls the switches  404  and  406 . The signal dh 1  is used instead of dh in order to avoid glitches on the sense voltage V sense . 
         [0025]    When the switch  412  turns on, the switch  404  closes and the switch  404  opens. The capacitor  408  is then charged such that the capacitor  408  stores a charge that has a voltage that matches the sense voltage V sense . The capacitor  410  remains floating, making voltage V sense1  constant. The input resistance of the comparator  424  is assumed to be infinite. When the switch  412  turns off, the switch  404  opens and the switch  406  closes. When the switch  406  closes, the capacitors  408  and  410  share their charges. In other words, the capacitor  408  charges the capacitor  410  such that the capacitor  410  stores a voltage that matches the sense voltage V sense , thereby biasing the voltage V sense1  such that the voltage V sense1  is based on the sense voltage V sense  and thus reflects or represents the supply current I dcdc . The voltage V sense1  then represents the current I dcdc  flowing from the supply. Accordingly, when the load current changes, the voltages V sense  and V sense1  change as well. The transmit time depends on the ratio between the values of the capacitors  408  and  410 . 
         [0026]    In accordance with the present invention, due to the charge sharing aspect of the capacitors  408  and  410 , high values of the capacitor  410  relative to the capacitor  408  allow only a very small ripple on the voltage V sense1  but increase the transient time with respect to a load variation. The size of the switches and capacitors are optimized to minimize charge injection effects and to minimize the ripple on the voltage V sense1 . As a result, the fixed load condition is very small, thus allowing accurate current sensing. 
         [0027]    The comparator  424  compares the voltage V sense1  to the reference voltage V r , representing a threshold value of the supply current. The threshold value may be, for example, the maximum supply current allowed to flow through the switch  412 . Accordingly, comparing the voltage V sense1  to the reference voltage V r  provides an indication of the supply current I dcdc  (e.g., whether the supply current I dcdc  is above or below a threshold current represented by the reference voltage V r ). 
         [0028]      FIG. 5  is a schematic diagram of a DC-to-DC converter  500  in accordance with another embodiment of the present invention. The DC-to-DC converter  500  is similar to the DC-to-DC converter  400  of  FIG. 4 , except that in the DC-to-DC converter  500 , the DC-to-DC current I dcdc  is sensed through an “on” resistance of the switch  412 . In operation, the DC-to-DC converter  500  functions similarly to the DC-to-DC converter  400 . 
         [0029]      FIG. 6  shows the behavior of voltages V sense  and V sense1  for the DC-to-DC converter  400  shown in  FIG. 4  when the DC-to-DC load is changed from 100 mA to 1 A, in accordance with the present invention. This has been obtained by using C1=0.25 pF and C2=1 pF. Note that the sense voltage V sense  has very high ripple (i.e., more than 100 mV) compared to the very small ripple of the voltage V sense1  (i.e., a few mV). Because this ripple is very small, current sensing can be performed with more accuracy. 
         [0030]      FIG. 7  is a schematic diagram of a DC-to-DC converter  700  in accordance with another embodiment of the present invention. The DC-to-DC converter  700  is similar to the DC-to-DC converter  400  of  FIG. 5 , except for the switched capacitor circuit  702 . The switched capacitor circuit  702  of the DC-to-DC converter  700  includes switches  704 ,  706 ,  708 , and  710 , and includes capacitors  712  and  714 . Also, one node of the capacitor  712  is connected to V dd  (instead of to ground), and one node of the capacitor  714  is connected to a reference bandgap voltage V bg  (instead of to ground). 
         [0031]    Referring briefly to  FIG. 4 , if the supply current I dcdc  to be sensed is very low, the voltage V sense1  will be slightly below the supply voltage V dd . If the value of the sense resistor  416  is 0.1Ω and the current I dcdc  to be sensed is 100 mA, the voltage V sense1  will be about 10 mV below the supply voltage V dd , and the comparator  424  can thus work with a very-high-input common-mode voltage. 
         [0032]    Referring again to  FIG. 7 , the DC-to-DC converter  700  allows a change of the common-mode voltage of the voltage V sense1 . When the switch  722  turns on, the capacitor  712  is charged to the voltage across the resistor  726 . When the switch  722  turns off, the capacitor  712  shares its charge with the capacitor  714 , which has one node connected to the bandgap voltage V bg . Assuming that the sense resistor  726 =0.1Ω, the bandgap voltage V bg =1.2V, and the supply current I dcdc  to be sensed is 100 mA, the resulting voltage V sense1  will be 10 mV below the bandgap voltage V bg  (i.e., 1.19V). This means that the comparator  734  operates with a good common-mode input voltage relative to the DC-to-DC converters  400  and  500  of  FIGS. 4 and 5 , respectively. 
         [0033]    In an alternative embodiment, a ratiometric common-mode voltage can be used instead of the bandgap voltage V bg . This would guaranty that the DC-to-DC comparator  734  operates with its best common mode input voltage, thus minimizing the input offset. 
         [0034]      FIG. 8  is a schematic diagram of a DC-to-DC converter  800  in accordance with another embodiment of the present invention. The DC-to-DC converter  800  is similar to the DC-to-DC converter  700  of  FIG. 7 , except that in the DC-to-DC converter  800 , the DC-to-DC current I dcdc  is sensed through an “on” resistance of the switch  822 . 
         [0035]    According to the system disclosed herein, the present invention provides numerous benefits. For example, it provides accurate current sensing with low current overhead. 
         [0036]    A system in accordance with the present invention for sensing the supply current of a switched DC-to-DC converter has been disclosed. The system includes a first circuit that senses a sense voltage, which is proportional to the supply current. The sense voltage has ripple that may be relatively high. The system also includes a second circuit that outputs a second voltage that is based on the sense voltage, where the second voltage has a ripple that is substantially smaller than the ripple of the sense voltage. The system also includes a third circuit that compares the second voltage to a reference voltage to provide an indication of the supply current. Because the second voltage has a smaller ripple than that of the sense voltage, the system can sense the supply current with better accuracy. 
         [0037]    The present invention has been described in accordance with the embodiments shown. One of ordinary skill in the art will readily recognize that there could be variations to the embodiments, and that any variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.