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1. Field of the Invention
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.
2. Description of Related Art
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The invention will be more fully understood by reference to the following detailed description of the invention in conjunction with the drawings of which:
Referring now to
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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.
Referring now to
During programming, switch S1 is closed to determine the current through resistor Rp. Current IRp is compared to the preset internal currents I1 and I2. 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 I1, and less than current path I2, then an active current path I1 is decoded in decoder 53. If IRp is greater than or equal to current I2, then current path I2 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 I1 and I2, a selection is still made based on currents I1 and I2 both being inactive.
Referring now to
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If current IRp is equal to or greater than current I1, and less than current I2, 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 78b to select a high range of preset current limits. If current IRp is greater than both current I1 and I2, both signal 72 and 73 become logic high levels. The resulting overcurrent threshold limit selection in logic gates 78 results in gate 78c 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.
Referring to
Comparators 82-85 provide a comparison result based a respective comparison of voltage references Vref1-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 Vref1-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.
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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.
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.
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.
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.
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.