Abstract:
A method for tuning a parameter of an electronic device includes: measuring a tuning variable x; and translating the measured tuning variable into a tuning range out of a plurality of tuning ranges representing a piece-wise tuning function. The piece-wise tuning function represents a tuning variable to parameter value mapping. Each tuning range i is associated with a nominal parameter value M i . The method further includes determining a parameter value M within said tuning range, and setting the parameter to the determined parameter value.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for tuning a parameter of an electronic device as a function of a measured variable. The present invention specifically relates to a method for tuning a variable of a parameter of a power converter as a function of a measured variable 
       BACKGROUND OF THE INVENTION 
       [0002]    In the development process of power modules, parameters of a DC-DC converter can be configured to meet application requirements and configuration data can be determined for a DC-DC converter serving a specific platform and the same configuration data can be used to configure all other converters serving the same platform. In this context, power modules manufactures are coming across the need to design power modules that are flexible to serve different platforms where operating parameters, such as input voltage, output voltage, nominal output current, transient performance etc. vary from platform to platform. To cut down development costs the POL DC-DC converter implemented in the power module is requested to have tunable parameters that can be easily accessed by the end-user without the need of fully reconfiguring the DC-DC converter. 
         [0003]    Indeed, an electronic device can be preconfigured and a defined set of operating parameters may be adjusted directly in the application field by measuring a variable, generated internally or externally to the device, and selecting the value of a parameter out of a discrete set of values. To do that, the value of the parameter has to be uniquely identified by the value of the measured variable. Such measurement though is affected by errors both caused by tolerances of components used to generate the variable to be measured and errors introduced by the circuit used for the measurement. 
         [0004]    The simplest variable to be measured is the measurement of voltage in a specific point of the electronic device. Such voltage may be generated by a trimmed current source flowing through a network of passive components such as resistors. Resistor tolerances as well as measurement errors limit the number of possible values of the parameter to be configured. Different methods can be implemented to reduce errors in the measurement. 
         [0005]    U.S. Pat. No. 4,349,777 discloses a method using resistors with very accurate values to generate an accurate voltage measurement. As well, the use of external resistors, although more inaccurate, for such purpose has the advantage of providing an easy selection of a parameter value to the end-user. 
         [0006]    U.S. Pat. No. 7,292,019B1 discloses a method to measure a voltage with the purpose of uniquely identifying the value of the parameter to be used. The method has the main disadvantages of allowing only a limited set of values to be associated to the parameter to configure and of not allowing tuning of the parameter around a preconfigured value. The method can be used to select the value of the set point of the DC-DC converter, as well as the timing for the ramp up of the DC-DC converter, i.e. soft turn on, and other converter parameters such as thresholds. Considering the example of configuring the set-point of the DC-DC converter, although the desired set point value can be selected via the method disclosed in U.S. Pat. No. 7,292,019B1, voltage drops due to resistive components located in between the voltage sensing point and the applied load may lead to a lower set point value at the load side. This rises the need to fine tune the set point value to achieve the target value. 
         [0007]    Together with the need of setting operating parameters of a DC-DC converter to serve different platforms, there is a need to be able to tailor the action of the compensator in case the external components defining the power stage of the DC-DC converter have changed. Indeed, a POL DC-DC converter may be designed to meet dynamic requirements of a specific platform that may be too week when compared to the requirement of other applications. The end-user may want to add additional bulk capacitance to overcome this problem, thus affecting stability of the regulation loop. Compensator parameters may need to be tuned accordingly to preserve stability and maintain the same dynamic behavior of the regulated system. 
       DISCLOSURE OF THE INVENTION 
       [0008]    The present invention relates to a method for tuning a parameter of an electronic device. The method comprises: measuring a tuning variable x; translating said measured tuning variable into a tuning range out of a plurality of tuning ranges representing a piece-wise tuning function f(x). The piece-wise tuning function represents a tuning variable to parameter value mapping. Each tuning range i is associated with a nominal parameter value M i . The method further comprises determining a parameter value M within said tuning range and setting the parameter to the determined parameter value. 
         [0009]    Each tuning range i is defined around its respective nominal parameter value M i  and is bounded by an upper bound and a lower bound. The size of the tuning range can be varied in dependence of a tuning range resolution ΔM which can be set and programmed to an internal memory of the electronic device. The parameter values of M are identified around each nominal parameter value M i . Within each tuning range the tuning function f(x) is strictly monotone and thus bijective. Hence, each value of the tuning variable x corresponds to one and only one value of the parameter M to tune. Thus, the value of parameter M can be adjusted, within the tuning range, by acting on the tuning variable x. The tuning variable can be any measurable variable external to the electronic device whose measured value is used by the electronic device to tune the parameter M. 
         [0010]    One aspect of the present invention relates to measuring the tuning variable which may comprise quantizing a measured tuning variable, e.g. an input voltage V in  such as voltage V pin  impinged upon a pin, using an analog-to-digital converter (ADC) having an ADC resolution Q ADC ; wherein a dynamic output range of the ADC is subdivided into a plurality of sub-ranges, each sub-range I [V TL#i−1  V TL#i ] being associated with a respective nominal parameter value M i  and being bounded by its upper bound V TL#i  and by its lower bound V TL#i−1 . 
         [0011]    One aspect of the present invention relates to the tuning variable to parameter mapping of the tuning function. The mapping comprises a mapping of nominal value of the tuning variable V Mi  to the nominal parameter value M i . Thus, the nominal value of the tuning variable V Mi  can be set arbitrarily within the tuning range to produce asymmetric tuning ranges. 
         [0012]    The mapping of the tuning function further comprises a mapping of a subrange [V TL#i−1  V TL#i ] to a tuning range which may comprise a mapping of an ADC resolution Q ADC  to the tuning range resolution ΔM. In this way, the tuning resolution ΔM in connection with nominal parameter value M i  indirectly defines the boundaries of the tuning range. 
         [0013]    One aspect of the present invention relates to defining the tuning range. Defining the tuning range comprises setting an arbitrary value of the tuning variable within a subrange corresponding to the tuning range to a nominal value of the tuning variable V Mi  such that it maps to the nominal parameter value M i  using the tuning function, M i =f(V Mi ), and setting the tuning resolution ΔM. 
         [0014]    A quantized measured tuning variable V meas  falling into the subrange [V TL#i−1  V TL#i ] of the ADC identifies the nominal parameter value M i  associated with this subrange. Thus, V meas  selects M i  the nominal value for M. For a given resolution the parameter M to tune thus can assume values in the tuning range 
         [0015]    M∈[f(V TL#i−1 )*ΔM, f(V TL#i−1 )*ΔM]=[M i −f(V Mi −V TL#i−1 )*ΔM, M i +f (V TL#i −V Mi )*ΔM] where V Mi  is the value Vmeas such M i =f(V Mi )=f(V TL#i −1+a*(V TL#i −V TL#i−1 )), a∈[0,1], wherein f is the tuning function. 
         [0016]    In case a=0.5, V Mi  is the middle point of the interval. 
         [0017]    One aspect of the present invention relates to fine tuning. Determining the parameter value within said tuning range comprises identifying the nominal parameter value M i  associated to the subrange [V TL#i−1  V TL#i ] into which the quantized measured tuning variable V meas  falls. This is a pre-stage to the further fine tuning which comprises mapping a difference V meas -V MI  between the quantized measured tuning variable V meas  and a nominal tuning variable V Mi  corresponding to the nominal parameter value M i  in terms of the ADC resolution to a difference between the parameter value and the nominal parameter value in terms of a tuning range resolution f(V meas -V Mi ) using the tuning function f(x); and setting the parameter value M to the sum of the nominal parameter value and the difference between the parameter value and the nominal parameter value in terms of the tuning range resolution times the tuning range resolution: 
         [0000]        M=M   i   +f ( V   meas - V   Mi )*Δ M.  
 
         [0018]    The parameter M may be a parameter of a power module and any of the following: output voltage set-point, threshold over-current shut-down, capacitance, inductance, switching frequency of a switching signal for switching a switched power stage. 
         [0019]    Specifically, the determined parameter value M may be used to adjust a coefficient of a PID compensator controlling the electronic device such as a power module. 
         [0020]    To summarize, the present invention provides a solution to solve the problem of fine tuning electronic device parameters by means of measuring a variable internal and external to the electronic device performing tuning around a build-in tuning function around a set of pre-configured nominal values. Resolution of the tuning method, e.g. resolution of a tuning range, can be programmed by a user in an internal memory of the electronic device. The range of tuning is determined as a function of the resolution of the tuning method and the build-in tuning function associated to the parameter to be tuned. 
         [0021]    The present method can be used to tune any parameters of an electronic device such as a DC-DC converter such as, but not limited to, output voltage set-point, fault detection thresholds, i.e. over voltage/over current thresholds, parameters related to sequencing such as ramp up timings, ramp down timings and indirectly or directly compensator parameters, i.e. PID coefficients. 
         [0022]    Nominal values of the parameter to be tuned form a set of discrete points of tuning function and tuning is performed around an identified nominal value following the mathematical law describing the tuning curve. The parameter to be tuned is tuned by means of measurement of an internal or external tuning variable containing information related to the tuning. 
         [0023]    The nominal value of the tuning variable V Mi  can be set arbitrarily such that it maps to the nominal parameter value using the configuration curve. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    Reference will be made to the accompanying drawings, wherein 
           [0025]      FIG. 1  shows a graph of the tuning function; 
           [0026]      FIG. 2  shows a tuning range of the tuning function with a nominal parameter value centered in the tuning range; 
           [0027]      FIG. 3  shows a tuning range of the tuning function with a nominal parameter value not centered in the tuning range; 
           [0028]      FIG. 4  shows a tuning range of the tuning function with a nominal parameter value set to the upper bound of the tuning range; 
           [0029]      FIG. 5  shows a tuning range of the tuning function with a nominal parameter value set to the lower bound of the tuning range; 
           [0030]      FIG. 6  shows a diagram showing pin-strapping by using a resistor; 
           [0031]      FIG. 7  shows a diagram showing pin-strapping by using a potenziometer; 
           [0032]      FIG. 8  shows a diagram showing pin-strapping by using a resistive network; 
           [0033]      FIG. 9  shows a diagram showing pin-strapping by using a resistor; 
           [0034]      FIG. 10  shows a diagram showing a dynamic range of an ADC being sub-divided into subranges; 
           [0035]      FIG. 11  shows a diagram showing the tuning mechanism; 
           [0036]      FIG. 12  shows a block diagram of buck converter; 
           [0037]      FIG. 13  shows a block diagram of the tuning mechanism with respect to tuning of compensator parameters; and 
           [0038]      FIG. 14  shows a frequency response of
       a controller plant (top);   a whole DC-DC converter system in open loop (middle);   a compensator with parameters tuned according to the tuning mechanism to compensate for power stage poles variation (bottom).       
 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0042]    The tuning mechanism will be explained with reference to a buck converter as shown in  FIG. 12 . The buck converter comprises a switched power stage, wherein an output voltage is generated according to a switching signal and an input voltage. The switching signal is generated in a controller that adjusts the output voltage to a reference voltage. The switched power stage  121  comprises a dual switch consisting of a high-side switch  122  and a low-side switch  123 , an inductor  124  and a capacitor  125 . During a charge phase, the high-side switch  122  is turned on and the low-side switch  123  is turned off by the switching signal to charge the capacitor  125 . During a discharge phase the high-side switch  122  is turned off and the low-side switch  123  is turned on to match the average inductor current to the load current. The controller comprises a compensator to compensate for the poles introduced by the inductance and capacitance of the output stage. 
         [0043]    A block diagram of the tuning mechanism is shown in  FIG. 13  wherein a pre-determined compensator  131  is tuned according to a tuning  133  to yield a tuned compensator  134 . The tuning  133  modifies the compensator parameters according to parameter value M determined by the tuning mechanism  132 . 
         [0044]    The tuning mechanism is based on pin-strapping and a tuning function f as shown in  FIG. 1 . The tuning function M=f(x) defines a set of possible values of the parameter M as a function of the tuning variable x. M i  (i=1, . . . n) is a set of nominal parameter values of the curve M=f(x). Each nominal parameter value M i  is associated with a respective tuning range bounded by a lower bound [and an upper bound]. Thus, the tuning function f(x) is a piece wise function comprising a plurality of tuning ranges and represents a tuning variable to parameter mapping. The tuning function can be linear as shown in  FIG. 2  or non-linear but monotonic as shown in  FIG. 3-5 . 
         [0045]    The nominal parameter value may be placed arbitrarily within its respective tuning range as shown in  FIG. 2 - FIG. 5 . In  FIG. 2  the nominal parameter value is centered in the tuning range. In  FIG. 3  the nominal parameter value is not centered in the tuning range. In  FIG. 4  the nominal parameter value is set to the upper boundary of the tuning range while in  FIG. 5  the nominal parameter value is set to the lower boundary of the tuning range. 
         [0046]    With pin-strapping, the tuning variable x is a voltage impinged upon a pin of the controller by a current flowing through an external resistive component placed to the pin as shown in  FIGS. 6 to 9 . 
         [0047]      FIG. 6  shows an embodiment wherein the voltage Vmeas impinged upon the pin  61  is generated by the flowing of a provided bias current through resistor  64  that is placed at the pin  61 . The bias current is generated by current source  62  inside the controller of the buck converter. ADC  63  quantizes the voltage V meas  which is the quantized tuning variable. 
         [0048]      FIG. 7  shows another embodiment where the voltage V meas  impinged upon the pin  71  is generated by the flowing of a provided bias current through a potentiometer  74  that is placed at the pin  74  and adjusted to achieve the desired value of the parameter M to be tuned. The bias current is generated by current source  72  inside the controller of the buck converter. ADC  73  quantizes the voltage V meas  which is the quantized tuning variable. 
         [0049]      FIG. 8  shows an alternative embodiment where the voltage at the pin  81  is generated by a network  84  of passive components R 1  and R 2  placed externally to the controller. The bias current is generated by current source  82  inside the controller of the buck converter. ADC  83  quantizes the voltage V meas  which is the quantized tuning variable. 
         [0050]      FIG. 9  still shows another embodiment where the voltage at the pin  91  is generated by a combination of a passive component  94  placed externally to the controller and a passive component  95  located internally to the controller. The bias current is generated by current source  92  inside the controller of the buck converter. ADC  93  quantizes the voltage V meas  which is the quantized tuning variable. 
         [0051]    Now returning to  FIG. 6 , in one embodiment a resistor  64  of the series E24 is used for pin-strapping. Taking into account resistor tolerances and additional errors deriving from the internal circuitry used to generate and measure the voltage V pin , the dynamic range of the N ADC  bits ADC  64  given by [0, 2 N   ADC ] is sub-divided into n subranges where each subrange is delimited by the ADC output codes V TL#i−1  and V TL#i . The number n of subranges is designed in a way that for a defined set of resistors with nominal values belonging to the E24 series, the nominal parameter values M i  can be designed univocally. 
         [0052]      FIG. 10  shows an example of how the dynamic range of the ADC is divided into sub-ranges where each sub-range is associated to a measured voltage V meas  at the pin with two trip levels V TL . The number of sub-ranges in this example is n=16. 
         [0053]    Assuming the resistor of value “Resistor#3” is placed at the pin of the circuit in  FIG. 6 , the nominal value M 2  is identified as the measured voltage V meas  is located in the subrange bounded by V TL#1  and V TL#2  that is associated with M 2 . 
         [0054]      FIG. 11  shows the fine tuning according to the following formula M=M 2 +f(V meas −V M2 )*ΔM for the following example: M2=4, a=0.5 (symmetrical tuning range), ΔM=0.1, ADC resolution QADC=1.5 mV, V TL#1 =127 QADC=190.5 mV, V TL#2 =225 mV, V Mi =(V TL#2 −V TL#1 )*a*QADC=207.7 mV, V meas 137*QADC. 
         [0055]    The tuning range is given by [M 2 −f(V TL#2 −V TL#1 )*ΔM, M 2 +f(V TL#2 −V TL#1 )*ΔM]=[2.85, 5.15]. 
         [0056]    The value of the parameter M is computed to be M=M 2 +f(V meas −V M2 )*ΔM. 
         [0057]    The difference V meas −V M2  in terms of the ADC resolution is 137-138.5 QADC. This difference is mapped in terms of the tuning resolution according to the function, then multiplied by the tuning resolution and finally added to the nominal parameter value M 2 . Note, in this example the tuning function is a linear function that linearly maps the ADC resolution to the tuning range resolution. 
         [0058]    By using the presented method, parameters of the compensator of a digital point of load (POL) DC-DC converter can be tuned to fit stability and transient requirements for different power stage scenarios. In this example, three different power stage scenarios are considered, where power stage scenarios differ for external component values, that is the inductance L of the inductor  124 , the capacitance C of the capacitor  125  and the switching frequency Fsw of the switched power stage of the power module as shown in  FIG. 12 . 
       Scenario 1: L=400 nH, C=400 uF (MLCC), Fsw=500 kHz 
     Scenario 2: L=400 nH, C=330 uF (OSCON), Fsw=500 kHz 
     Scenario 3: L=330 nH, C=400 uF (MLCC), Fsw=500 kHz 
       [0059]    Power stage scenarios 1, 2, 3 may represent three different power modules embedding the same POL DC-DC converter with LC output filter set at a minimum load capacitance C and equivalent series resistance (ESR) configuration. 
         [0060]    Three different compensators may be designed to stabilize the power stage scenarios 1, 2, 3 by the use of a Graphical User Interface (GUI) and compensator parameters can be stored in an internal memory of the POL DC-DC converter. 
         [0061]    In one embodiment the controller used is of the digital Proportional Integral Derivate (PID) controller type having parameters Ki, Kp, Kd being integral, proportional and derivative coefficients. For each of the power stage scenarios, a set of parameters Ki, Kp, Kd is memorized: 
         [0000]    Scenario 1: compensator C1: {Ki_C1, Kp_C1, Kd_C1}
 
Scenario 2: compensator C2: {Ki_C2, Kp_C2, Kd_C2}
 
Scenario 3: compensator C3: {Ki_C3, Kp_C3, Kd_C3}
 
         [0062]    Module makes may increase the amount of load capacitance placed at the output of the converter in order to meet application requirements such as transient performance. Varying parameters of the power stage, in this example the output capacitance (Ctot) alters the position of poles and zeros of the power stage leading to the need to tune zeros and poles of the pre-determined compensator Cj: {Ki_Cj, Kp_Cj, Kd_Cj}, j={1,2,3}. In this respect, compensator parameters Cj: {Ki_Cj, Kp_Cj, Kd_Cj} need to be tuned to maintain the system performance obtained for the minimum load capacitance case. Tuning of compensator parameters is done in operation by reading the value of the parameter M, set by module makers using the presented tuning method, and computing the new value of compensator parameters via a built-in compensator adjustment function. In this way, the compensation can be continuously tuned to suit various output capacitor values. Indeed, tuning of the parameters {Ki_Cj, Kp_Cj, Kd_Cj}, j={1,2,3} correspond to move in frequency position of zeros and poles of the compensator as shown in  FIG. 14  bottom. The new set of compensator parameters, as a result of tuning, is expressed by 
         [0000]        Ki _ Cj _tuned= fi ( Ki _ Cj,M ) 
         [0000]        Kp _ Cj _tuned= fp ( Kp _ Cj,M ) 
         [0000]        Kd _ Cj _tuned= fd ( Kd _ Cj,M ) 
         [0000]    where fi, fp and fd are the built-in compensator adjustment functions of the integral, proportional and derivative coefficients. 
         [0063]    The advantages of the present invention are the following: 
         [0064]    Parameters of a pre-configured DC-DC converter can be fine-tuned around a set of pre-programmed nominal values without the need to fully reconfigure the device. 
         [0065]    Fine-tuning of parameters can be done using inaccurate external components. 
         [0066]    The resolution of the tuning method is programmable and can be stored among configuration data of the DC-DC converter. 
         [0067]    Built-in adjustment function allows tuning of parameters according to linear and non-linear laws. 
         [0068]    Although pre-configured values of the variable to tune may be located on a non-linear curve, the proposed method allows linearization of the tuning function around the pre-configured nominal values of the variable to be tuned. 
         [0069]    In case the resolution of the tuning method ΔM is set to 0, the values that the parameter to tune assumes are the pre-configured nominal values.