Abstract:
A programmable parameter or feature is provided for a power converter through a multi-function connection on the converter controller. The parameter or feature selection is active for programming during a startup mode, and the connection is used for other control purposes during a steady state run mode. A reference signal is read on the multifunction connection during startup mode and a selection of a parameter value or feature is made based on a value of the reference signal. The reference signal is compared to preset, internal reference values to select a desired parameter value or feature. An internal preset value is chosen based on the selection and the programming circuitry is disconnected from the connection to permit alternate functionality for the connection. The programmable circuit permits selection from a variety of parameter values or features based on an external signal, without dedicating an external pin on the controller.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     N/A  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     N/A  
       BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The present invention relates generally to a system and method for selecting an internal parameter value or feature in a power converter, and relates more specifically to selecting an internal parameter value or feature by comparing a programmable reference signal on a multifunction pin to one or more threshold levels.  
         [0005]     2. Description of Related Art  
         [0006]     Power converters have become more flexible in providing functionality and features in recent years. One reason for advances in power converters is the design flexibility permitted by digital/analog hybrid controllers. These controllers can measure analog signals and perform digital logic and control functions to provide a wide range of features and functionality in realizing a power converter control.  
         [0007]     Among the various parameters that influence power converter control is overcurrent detection and response. A popular technique for detecting and responding to an overcurrent condition involves the use of a current limit threshold, where an overcurrent event causes a power controller to register a current related parameter going beyond a threshold. Due to the importance of providing flexibility in setting an overcurrent threshold level, a dedicated pin or connection for the power controller is typically used. A power converter designer typically specifies a passive component for connection to the dedicated pin to achieve an overcurrent threshold setting for the power converter. Typically, the passive component is a resistor that sets or selects a current threshold level that can be compared against a value representative of current flowing through a power stage of the power converter.  
         [0008]     If an external component and dedicated pin are not used to set, or program, the overcurrent threshold limit, an internal overcurrent threshold limit is used, which is typically not programmable. Accordingly, flexibility in the design of the power converter is limited. While it is possible to select from multiple controller models that offer various internal overcurrent thresholds, such a power converter design can be inefficient and costly in practice.  
         [0009]     It would be desirable to provide a means for permitting a user to select an internal parameter value, in this example an overcurrent threshold, using one controller design, without having to dedicate a controller connection for that purpose.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     In accordance with the present invention, there is provided a system and method for selecting one of several internal parameters, or programming one of several features in a power converter by utilizing one of its connectors as a multifunction connector. The system and method permits the elimination of a dedicated connector for the purpose of setting parameter values or selecting features. There are numerous types of parameters and features in a power converter that are advantageously set or programmed by a user. The list includes, and is not limited to, frequency, output voltage level, soft start time, overcurrent protection level, methods for output voltage sequencing and method of fault protection, for example. The present invention describes an exemplary embodiment in detail, where the system and method is directed to setting an overcurrent threshold level by selecting one from a plurality based on a programming reference signal.  
         [0011]     According to an aspect of the present invention, there is provided a system and method for permitting a user to select an internal parameter, such as overcurrent threshold limit, in a power controller. A multifunction pin or connection in a controller is used during one interval to set one of several internal overcurrent threshold limits, and is used for other purposes during other intervals or modes. The connection can be provided with a tri-state gate to permit the secondary function to be disconnected while an overcurrent threshold limit is programmed or set.  
         [0012]     In one embodiment, an error amplifier output pin is multiplexed with an overcurrent threshold limit function. During an initial startup, the error amplifier output is disconnected from its pin. A resistor is connected between the pin and a common reference. The resistor does not interfere with the error amplifier during normal operation, and serves the purpose of programming the overcurrent threshold limit during startup. An internal voltage source applied to the resistor permits a sensed current to be obtained. The sensed current is compared internally to one of several preset current levels. The result of the comparison is then decoded, latched, and used to select one of several internally preset overcurrent thresholds. The overcurrent threshold limit is modified, or programmed by adjusting the value of the external resistor.  
         [0013]     In accordance with another aspect of the present invention, a sensed current value is obtained from an external resistor during startup for a controller and decoded to select a preset overcurrent threshold limit. The current is decoded by comparison with a number of internal preset current values when a startup sequence in the controller is activated. If the sensed current is greater than a given preset current value, the preset value selects a given overcurrent threshold limit and latches that value for use in the controller during normal operating periods.  
         [0014]     In accordance with another aspect of the present invention, a sensed current is provided to the controller on a temporary basis during a startup sequence, and is maintained over a short delay period while a preset overcurrent threshold limit is latched. In this embodiment, the overcurrent threshold limit setup may be permitted for a short time, and the delay and latching elements contribute to ensuring an overcurrent threshold limit is set.  
         [0015]     In accordance with another aspect of the present invention, a method for selecting an operating parameter for a power controller, such as an overcurrent threshold limit, for example, is provided without the use of a dedicated connection to a controller. During startup mode, the method provides for sensing a reference signal, such as a current applied to the multi-function connection, for example. The method may include selecting a preset value for the operating parameter, such as an overcurrent threshold limit, for example, based on the sensed reference signal. The preset value selected may be taken from a plurality of presets based on the reference signal, such as a sensed current. The sensed current may be provided in relation to a passive component or network of components coupled to the multi-function connection.  
         [0016]     In accordance with another aspect of the present invention, a plurality of overcurrent threshold limits are provided internally to a controller that are selectable through an appropriate choice of a passive component or network coupled to the controller. In at least one instance, one of the plurality of preset overcurrent threshold limits can be selected when no external passive component is provided to the multi-function connection, that is, the connection is left open. Accordingly, an overcurrent threshold limit is automatically selected even if no programming passive component is present. The overcurrent threshold may also be programmed through a network of passive components coupled to the multi-function connection.  
         [0017]     In accordance with another aspect of the invention, an impedance coupled to the multifunction connector provides a reference voltage signal upon application of a reference current. The derived reference voltage signal is used to select an internal parameter value or feature. A voltage selection circuit compares the reference voltage signal against one or more thresholds, and selects a parameter value or programmed feature based on a result of the comparison. A decoder circuit and/or latch may optionally be used to indicate the selection and maintain the indication or selection.  
         [0018]     Additionally, the concept of selecting one of several internal overcurrent thresholds, without having to dedicate a controller connection for that purpose, applies to other desirable functions or parameters, for example, operating frequency. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0019]     The invention will be more fully understood by reference to the following detailed description of the invention in conjunction with the drawings of which:  
         [0020]      FIG. 1  is an abstract circuit diagram of a conventional overcurrent threshold limit program circuit;  
         [0021]      FIG. 2  is a simplified circuit diagram of a programmable overcurrent threshold limit in accordance with the present invention;  
         [0022]      FIG. 3  is a circuit diagram of a power converter control circuit in accordance with the present invention;  
         [0023]      FIG. 4  is a block diagram illustrating internal operations of a controller in accordance with the present invention;  
         [0024]      FIG. 5  is a circuit diagram illustrating selection of a preset parameter value in accordance with the present invention;  
         [0025]      FIG. 6  is a circuit diagram of a preset parameter value selection circuit according to another embodiment of the present invention;  
         [0026]      FIG. 7  is a circuit diagram of a selection circuit on a multifunction connection in accordance with another embodiment of the present invention; and  
         [0027]      FIG. 8  is a circuit diagram of a selection circuit using voltage reference thresholds in accordance with another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     Referring now to  FIG. 2 , a simplified diagram for a parameter value selection circuit is illustrated generally as diagram  20 . The circuit of diagram  20  can be used to select an overcurrent threshold limit from among a number of preset values, for example. A resistor Rp is connected to an error amplifier circuit that includes an amplifier  22 . A switch  24  disconnects error amplifier  22  during certain stages of operation of the circuit in diagram  20 . For example, in a startup mode, switch  24  is open so that no error amplification is used when the switching stage is off. Because switch  24  disconnects amplifier  22  from external circuitry, resistor Rp can be used to program a current sensing circuit which decodes and selects one of several overcurrent threshold limits, for example. During normal running mode, switch  24  is closed so that amplifier  22  can provide a control feedback signal for the controller. In this situation, resistor Rp acts as a small load on the error amplifier circuit. Circuit  26  for selecting a parameter value is disconnected from connection COMP after startup mode is complete, and a parameter value selection has been established.  
         [0029]     Referring now to  FIG. 3 , an implementation of a power converter controller is illustrated as circuit  30 . A controller  32  provides synchronous control operation to drive a half bridge composed of two FETS. An output feedback signal from an output voltage VOUT is provided to connection FB for feedback control. Connection COMP provides a compensation for the error feedback connection FB, with an internal amplifier relating the two connections together. In an exemplary embodiment, controller  32  provides overcurrent or short circuit protection through the selection of one of several internal overcurrent thresholds determined by sensing the impedance of an external passive component, such as resistor Rp, connected from connection COMP to ground, for example. During startup, controller  32  forces a voltage on connection COMP, measures the current flowing through the impedance on connection COMP, compares the measured current to several internal current values, and asserts one comparison level to select and latch one of the several internal overcurrent threshold levels. In an exemplary embodiment, three internal overcurrent thresholds are provided. It should be apparent that controller  32  may easily be configured to force a current on connection COMP and compare the measure voltage against internal values to determine a parameter value selection. It should also be apparent that any number of threshold set points may be used, by simply decoding a larger number of values.  
         [0030]     Referring now to  FIG. 4 , an internal block diagram of a buck controller with various selectable parameter values is illustrated generally as diagram  40 . Connection COMP is shown in diagram  40  as having a switch  42  used to switch connection COMP between the error amplifier and a parameter value selector  44 . Selector  44  provides the circuitry for reading the current or voltage determined by the passive component(s) on connection COMP and selecting a parameter value from several available values, which is latched into the control illustrated in diagram  40 .  
         [0031]     Switch  42  in diagram  40  is active during a startup mode, to connect connection COMP to selector  44 . Once the startup mode passes, and the controller of diagram  40  enters a normal running mode, switch  42  is closed to complete the error loop between connections FB and COMP. The latched value of the parameter, such as, for example, an overcurrent threshold, is then used for comparison against output current values to determine when an overcurrent condition occurs. By providing a selectable overcurrent threshold, for example, diagram  40  provides a control that is usable with a number of different applications in a wide variety of power levels. As indicted in selector  44 , other parameters that can be given a selectable value include oscillator frequency, soft-start time, output voltage level, an output sequencing method and fault protection or response methods.  
         [0032]     Referring now to  FIG. 5 , an exemplary embodiment of the circuitry for parameter value selector  44  is illustrated generally as circuit  50 . Circuit  50  provides a technique for selecting a parameter value based on voltage provided to resistor Rp. The resulting current flowing through resistor Rp, transistors  51  and M 0  is compared against current flows represented as I 1  and I 2 , provided through transistors M 1  and M 2 , respectively. Currents I 1  and I 2  have different values, with current I 2  being greater than I 1 , for example. Any number of comparative currents may be used, where each of the successive currents is higher than the previous, that is, a current I 3  would be higher than current I 2  and so on.  
         [0033]     During programming, switch S 1  is closed to determine the current through resistor Rp. Current IRp is compared to the preset internal currents I 1  and I 2 . A decoder determines when a given current path is active, to provide a selection for the parameter value or feature. According to this embodiment, circuit  50  provides a current mirror for current IRp that activates one or more internal current paths depending upon the value of current IRp. That is, if current IRp is greater than or equal to current path I 1 , and less than current path I 2 , then an active current path I 1  is decoded in decoder  53 . If IRp is greater than or equal to current I 2 , then current path I 2  is decoded as an active current in decoder  53 . Any current path that is not active because the value of current IRp is not great enough to make the path active, is decoded as an inactive path in decoder  53 . Accordingly, if current IRp is lower than the lowest value for the current paths I 1  and I 2 , a selection is still made based on currents I 1  and I 2  both being inactive.  
         [0034]     Referring now to  FIG. 6 , another exemplary embodiment of the present invention is illustrated as circuit  60 . Circuit  60  is similar in concept and function to circuit  50 , and further includes a latch  62  and a delay element  64 . The additional circuitry in circuit  60  provides a particular timing sequence for latching the parameter value or feature information before resistor Rp is disconnected with switch S 1 . A latch signal  65  is provided at a clock input of latch  62  to secure the decoded parameter value or feature information. Latch signal  65  also propagates through delay element  64  to open switch S 1  after a given delay. When switch S 1  is open, connection COMP is disconnected from the parameter value or feature selection circuit. In accordance with the configuration of circuit  60 , the feature or parameter value selection, such as, for example, an overcurrent limit threshold value, is latched in latch  62  prior to the opening of switch S 1 , to ensure a good reading of the parameter value or feature selection information is obtained.  
         [0035]     Referring now to  FIG. 7 , another exemplary embodiment of the present invention is illustrated in detailed circuit diagram  70 . Circuit  70  illustrates a specific component layout that may be used to achieve the present invention. A current IRp is drawn through connection COMP and compared against currents I 1  and I 2 . If current IRp is less than I 1  or I 2 , signals  72 ,  73  both remain at a low logic level. D flip flops  75 ,  76  produce outputs that are decoded by logic gates  78  to produce the set point levels for the selected parameter value. In the example illustrated in diagram  70 , an overcurrent limit threshold is selected based on three available levels. For example, logic gate  78   a  decodes two low levels from D flops  75 ,  76  to select a medium level for an overcurrent threshold. This selection level corresponds to high impedance value at connection COMP, such as in the case of no programming component being connected at all.  
         [0036]     If current IRp is equal to or greater than current I 1 , and less than current I 2 , signal  72  becomes a logic high level, while signal  73  remains a logic low level. The resulting selection at logic gates  78  turns on high level gate  78   b  to select a high range of preset current limits. If current IRp is greater than both current I 1  and I 2 , both signal  72  and  73  become logic high levels. The resulting overcurrent threshold limit selection in logic gates  78  results in gate  78   c  being turned on to select a low overcurrent threshold limit. The logic level  72 ,  73  selected during startup with the impedance coupled to connection COMP are latched in D flip flops  75 ,  76  with a clock signal that is provided as an enable signal  71 . Signal  71  is active during a startup mode to latch the logic levels  72 ,  73  into D flip flops  75 ,  76 . After startup mode, signal  71  remains low and the latched values in D flip flops  75 ,  76  continue to select the programmed overcurrent threshold limit determined through logic gates  78 .  
         [0037]     Referring to  FIG. 8 , another exemplary implementation of the present invention is illustrated as circuit  80 . Circuit  80  operates on a voltage reference basis to determine which parameter value or feature should be selected based on the value of resistor Rp. A current Iref, coupled to switch S 1  is forced on resistor Rp when switch S 1  is closed during startup. The resulting voltage across resistor Rp is supplied to comparators  82 ,  83  and  85 , as well as any other comparators represented by dashed lines in circuit  80 . It should be noted that a selection according to this technique may be made using a single comparator, and additional comparators permit additional selection decisions.  
         [0038]     Comparators  82 - 85  provide a comparison result based a respective comparison of voltage references Vref 1 -VrefN to the input voltage across resistor Rp. A decoder  86  manipulates the results to provide an indication of the selected parameter or feature. In accordance with one exemplary embodiment, voltage references Vref 1 -VrefN have different values, so that their respective comparator outputs indicate a digitized value of the voltage across resistor Rp. That is, comparators  82 - 85  act as an analog-to-digital converter (ADC) for selection of a desired parameter value. Decoder  86  assesses the active lines of comparators  82 - 85  and provides a selection based on the assessment. Other exemplary embodiments include a direct selection of a parameter through activation of a given comparator output or set of outputs, for example.  
         [0039]     Although not shown in  FIG. 8 , a latch may also be used store a selection indication or to store a selected parameter value or feature. The operation of the latch could be similar to that of the D-flip flops shown in  FIG. 7 , for example.  
         [0040]     The circuit for selecting the overcurrent threshold limit operates as follows. An error amplifier output is initially tri-stated to disconnect it from its other functionality. An external impedance connected between the tri-stated connection and ground connection receives an arbitrary supply voltage from the connection to develop a current that can be measured in the controller. The current through the impedance is measured or sensed and compared to one or more internal levels. As the comparison to the internal levels is made, a decision is made for selection of one of the internal preset levels based on the current supply to the external component. Once the decision is made, the error amplifier output is reconnected using the tri-state gate.  
         [0041]     The above series of operations can be provided through an internal sequencer to obtain good overcurrent threshold data in a very short time period. For example, threshold data may be acquired in approximately one millisecond, which includes sensing the current supplied to the external impedance and deciding on which preset overcurrent threshold limit to select. Once the internal sequence is completed, the controller can move on to a normal start sequence and normal operation, for example.  
         [0042]     In accordance with the present invention, a connection to a power controller is used to drive an external impedance to obtain a programmable selection for an overcurrent threshold limit. The choice of controller connection can vary, and is made according to an exemplary embodiment to incorporate an amplifier or driver that is operable to provide a current to an external component or network. Accordingly, the tri-stated functionality need not be an error amplifier, but can be any function that has a free operating range available for use for other functionality in a multi-purpose connection.  
         [0043]     The present invention is not limited to overcurrent threshold limit detection and setting, but can be used with any type of control function where a programmable set point is desired without the use of a dedicated external connection. One example of an internal programmable function is the operating frequency of the controller. Another example would be selection of several soft start timing ranges.  
         [0044]     Finally, it will be appreciated that modifications to and variations of the above-described apparatus and method may be made without departing from the inventive concepts disclosed herein. Accordingly, the invention should not be viewed as limited except by the scope and spirit of the appended claims.