Abstract:
A controller integrated circuit (IC) for controlling a power converter uses its input voltage pin with a plurality of functions, including receiving an input voltage to the power converter, charging an external startup capacitor through charging circuitry coupled internally to the input voltage pin, and also receiving a test signal for programming a programmable resistance in an input voltage scale down circuitry coupled to the input voltage pin. Use of the input voltage pin with a plurality of functions reduces the number of pins required in the controller IC, thereby reducing the cost of manufacturing the controller IC.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119(e) to co-pending U.S. Provisional Patent Application No. 60/735,522 entitled “Digital Off-line Low Power Supply Controller,” filed on Nov. 10, 2005, which is incorporated by reference herein in its entirety, 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power converter and, more specifically, to a power converter controller IC (integrated circuit) that has an input voltage pin with multiple functions. 
     2. Description of the Related Art 
     With the explosive growth of the number of electronic devices recently, the demand for power converters used as adapters or changes for these electronic devices is also growing at a rapid rate. These power converters are typically controlled by power converter controller ICs. Especially, switching mode power converters are typically controlled by power converter controller ICs that control the on-times (T ON ) or off-times (T OFF ) of the switch in the power converters to regulate the output voltage and power of the power converters. 
     The power converter industry is under significant pressure to manufacture power converter controller ICs that are highly efficient but can also be manufactured at low cost. Because the manufacturing cost of ICs is highly dependent upon the die size, the number of pins, the packaging, and testing of the IC, it is desirable to reduce the number of pins of an IC. However, it is difficult to reduce the number of pins in conventional power converter controller ICs, because in conventional power converter controller ICs each pin of the IC is associated with a single, separate parameter or function and thus the IC required as many pins as the number of parameters or functions either input to or output from the controller IC. Thus, it is difficult to reduce the number of pins in the power converter controller IC without reducing the number of parameters either input to or output from the controller IC and thereby sacrificing the performance of the power converter. 
     Therefore, there is a need for a technique to reduce the number of pins used in a power converter controller IC and reduce manufacturing costs of the IC without reducing the number of parameters or sacrificing the performance of the power converter. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention include a power converter controller IC that uses the input voltage pin with a plurality of functions, including receiving an input voltage to the power converter, charging an external startup capacitor through charging circuitry coupled internally to the input voltage pin, and also for receiving a test signal used for programming a programmable resistor in input voltage scale down circuitry coupled to the input voltage pin. Use of the input voltage pin with a plurality of functions reduces the number of pins required in the controller IC, thereby reducing the cost of manufacturing the controller IC. 
     In one embodiment, the controller IC comprises charging circuitry coupled internally to the input voltage pin of the controller IC for charging a capacitor coupled externally to a supply voltage pin of the controller IC. The charging circuitry comprises a switch that is turned on and off in response to a power-on-reset (POR) signal, where the POR signal is in a first state to turn on the switch when the input voltage to the power converter is below a first threshold voltage and in a second state to turn off the switch when the input voltage to the power converter rises above the first threshold voltage but does not fall below a second threshold voltage. When the input voltage falls below the second threshold voltage, the POR signal transitions to the first state again to turn on the switch. The capacitor is charged while the switch is turned on, and the capacitor provides supply voltage to the controller IC during a startup mode of the power converter. 
     In another embodiment, the controller IC comprises scale down circuitry for scaling down the input voltage received at the input voltage pin to a voltage level compatible with the device characteristics of the controller IC. The scale down circuitry comprises a first transistor coupled to receive the input voltage, a second transistor connected in series to the first transistor, and a programmable resistor connected in series to the second transistor. The input voltage to the power converter is scaled down by the programmable resistor when both the first and second transistors are turned on. The second transistor is always on, while the first transistor is turned on and off in response to a power-on-reset (POR) signal. The POR signal is in a first state when the input voltage to the power converter rises above a first threshold voltage to turn on the first transistor and in a second state when the input voltage to the power converter falls below a second threshold voltage to turn off the first transistor. 
     The programmable resistor comprises a plurality of resistors connected in series and a plurality of switches each coupled to one of the resistors, where each of the switches is configured to short one of the resistors to which each of the switches is coupled when the switch is closed. The programmable resistor is programmed by a clock count signal determining how many of the switches are closed and how many of the resistors of the programmable switches are shorted by said closed switches. A test signal is input to the input voltage pin, where the test signal includes a first positive pulse indicating the start of a test mode, a plurality of negative pulses following the first positive pulse, and a second positive pulse indicating the end of the test mode. The count of the number of negative pulses is used to set the programmable resistor, in such a way that the count of the number of negative pulses in the test signal is inversely proportional to the set resistance of the programmable resistor. 
     The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the embodiments of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings. 
         FIG. 1  illustrates an AC-DC flyback power converter with primary-side sensing, according to one embodiment of the present invention. 
         FIG. 2  illustrates the circuitry connected to the V IN  pin of the power converter controller in the AC-DC flyback power converter of  FIG. 1 , according to one embodiment of the present invention. 
         FIG. 3  illustrates the POR (Power On Reset) signal used in the circuitry of  FIG. 2 , according to one embodiment of the present invention. 
         FIG. 4  illustrates the test signal used in triggering a test mode for trimming the voltage divider resistance coupled to the V IN  pin of the power converter controller in the AC-DC flyback power converter of  FIG. 1 , according to one embodiment of the present invention. 
         FIG. 5  illustrates the programmable resistor used in the voltage divider resistance coupled to the V IN  pin of the power converter controller in the AC-DC flyback power converter of  FIG. 1 , according to one embodiment of the present invention. 
         FIG. 6  is a flowchart illustrating a method of trimming the voltage divider resistance coupled to the V IN  pin of the power converter controller in the AC-DC flyback power converter of  FIG. 1 , according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The Figures (FIG.) and the following description relate to preferred embodiments of the present invention by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of the claimed invention. 
     Reference will now be made in detail to several embodiments of the present invention(s), examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
       FIG. 1  illustrates AC-DC flyback power converter with primary-side sensing, according to one embodiment of the present invention. Although the power converter of  FIG. 1  is an AC-DC flyback converter with primary side sensing of the feedback signals, it should be noted that the present invention is not limited to a flyback converter and that it can be applied to any type of power converter of any topology and any type of feedback sensing. The power converter includes, among other components, a bridge rectifier BR 1 , a transformer T 1 , a switch Q 4 , an output rectifier diode D 1 , output filter capacitor C 7 , and a power converter controller  100 . The controller  100  includes a digital controller  104  together with a number of other components (not shown herein). 
     Referring to  FIG. 1 , the rectifier BR 1  receives an input AC voltage and converts it into a full-wave rectified voltage for transfer to the output OUTPUT. The power converter controller  100  controls the opening and closing of the switch Q 4  using its output control signal  102  generated by the digital controller  104  in the form of pulses with on-times (T ON ) and off-times (T OFF ). The output control signal  102  may be a periodic pulse with a fixed period, or a pulse with its period varying as necessary. When the switch Q 4  is turned on because the pulse  102  is high during the on-time, energy is stored in the primary side windings of the transformer T 1  because the diode D 1  is reverse biased. When the switch Q 4  is turned off, the energy stored in the primary windings of the transformer T 1  is released to the secondary side of the transformer T 1  because the diode D 1  becomes forward biased. The diode D 1  rectifies the output voltage on the secondary windings of the transformer T 1  and the capacitor C 7  filters the output voltage signal on the secondary windings of the transformer T 1  for outputting as the output voltage OUTPUT. By controlling the period of time during which the switch Q 4  is on or off, i.e., the on-times (T ON ) and off-times (T OFF ), the power converter can control the amount of power delivered to the output. Note that explanation on other components of  FIG. 1  is sometimes omitted herein merely because they are not particularly relevant to explaining the claimed inventions herein. 
     As shown in  FIG. 1 , the power converter controller IC  100  has only 5 pins, V IN /Startup, a ground pin (PGND), a V SENSE  pin, a chip supply voltage pin Vcc, and an Output pin. The power converter controller  100  receives an input voltage (V IN  also referred to herein as V LINE ) which is scaled down from the output voltage of the rectifier BR 1 , via the V IN /Startup pin. In addition, the V IN /Startup pin is associated with additional functions in addition to receiving the input voltage as explained below with reference to  FIGS. 2 and 4 . The power converter controller  100  receives a divided-down version (V SENSE ) of the reflected secondary voltage on the auxiliary windings T 1 -AUX 1  of the transformer T 1  through the V SENSE  pin, receives the supply voltage (Vcc) via the Vcc pin, and is connected to ground via the PGND pin. Finally, the power converter controller  100  generates and outputs the pulse  102  for controlling the switch Q 4  via the OUTPUT pin. 
       FIG. 2  illustrates the circuitry connected to the V IN  pin of the power converter controller in the AC-DC flyback power converter of  FIG. 1 , according to one embodiment of the present invention.  FIG. 2  shows only part of the power converter controller  100  circuitry coupled to the V IN /Startup pin and the Vcc pin, and other parts of the power converter controller  100  that are not specifically relevant for explaining the present invention are omitted in  FIG. 2  as denoted by the dotted line on the right edge of the power converter controller IC  100  in  FIG. 2 . The power converter controller  100  includes Vcc charging circuitry and V IN  scale down circuitry, both of which are connected to the V IN /Startup pin. 
     Referring to  FIGS. 1 and 2 , the Vcc charging circuitry is comprised of the p-type MOSFET (Metal Oxide Semiconductor Field Effect Transistor) switch Q 20  (including its parasitic diode D 20 ). When the switch Q 20  is turned on, the Vcc charging circuitry serves as a bleeding charge current path internal to the controller IC  100 , from the input line voltage V IN  via the V IN /Startup pin to the capacitor C 7 , to charge the capacitor C 7 . The capacitor C 7  is connected to the Vcc pin externally to the controller IC  100 , in order to provide the supply voltage Vcc to the power converter controller  100 . For example, if 15 μA is provided through the V IN /Startup pin, part (e.g., 7 μA) of that current may be drawn by the startup circuitry  206  that includes circuitry to power up and start the chip  100 , and the remaining part (e.g., 8 μA) of that current may be used to charge the Vcc capacitor C 7 . The Vcc supply voltage (Vcc) across the capacitor C 7  is used to start the power converter controller IC  100  before the power converter operates in normal operation mode and electrical energy for running the power converter controller IC  100  is received through the auxiliary winding T 1 -AUX 1  of the transformer T 1 . 
     Note that the Vcc charging circuitry is implemented on-chip on the power converter controller IC  100 , and there is no circuitry external to the power converter controller  100  that connects the input line voltage V IN  to the capacitor C 7 . In essence, the V IN /Startup pin is used as a charging path internal to the controller IC  100  for charging the supply voltage (Vcc) capacitor C 7  connected externally to the controller IC through the power converter controller IC  100 . Therefore, the entire power converter circuitry may be implemented in a simple configuration with fewer external parts. 
     The switch Q 20  may be turned on and off according to a POR (Power On Reset) signal that is generally high while the input line voltage V IN  is high.  FIG. 3  illustrates the POR (Power On Reset) signal used in the circuitry of  FIG. 2 , according to one embodiment of the present invention. In one embodiment, the POR signal is generated by the startup circuitry  206 . As shown in  FIG. 3 , POR becomes high when the input line voltage V IN  rises above a predetermined voltage (e.g., 10-13 Volt) and becomes low when the input line voltage V IN  falls below another predetermined voltage (e.g., 5.5-6.5 Volt). The switch Q 20  is on while the POR signal is low to charge the capacitor C 7  and the switch Q 20  is off while the POR signal is high to shut off the charging path to the capacitor C 7 . 
     In one embodiment, the power converter controller IC  100  is fabricated on silicon using a low voltage CMOS (Complementary Metal-Oxide Semiconductor) process, which typically cannot withstand a voltage higher than 3.6 V in the devices. This is at odds with a power converter controller that typically should be able to receive and withstand 260 V of input line voltage V IN . This is why the V IN  scale down circuitry is needed. 
     Referring back to  FIG. 2 , the V IN  scale down circuitry is comprised of the two n-type MOSFETs Q 22 , Q 24  and a programmable resistor R 20 . As shown in  FIG. 2 , the transistor Q 22  is turned on and off according to the POR signal, and the transistor Q 24  is always turned on by applying a high voltage (e.g., 6 V) to its gate. 
     When the controller IC  100  is turned on, POR becomes high and the transistor Q 22  is turned on. The transistor Q 24  is always turned on. The transistor Q 24  has resistor-like characteristics in its MOSFET linear region, providing a certain voltage drop. Thus, the input line voltage V IN  (also referred to herein as V LINE ) is scaled down by a resistive divider comprised of the resistor R 40  (which is typically very large, for example 6 Mohm), the two transistors Q 22 , Q 24  in their linear regions, and the programmable resistor R 20  to generate the scaled down input voltage  236 . The scaled down input voltage  236  is input to an analog-to-digital converter (ADC)  210  to generate a digital representation  230  of the scaled down input voltage  236 , which is used by the digital controller  104  in a variety of ways to generate the pulse  102  and determine its on-times/off-times as well as set a variety of analog parameters in the power converter (e.g., internal oscillator frequency, reference voltage, etc.). 
     However, when the input line voltage V IN  increases beyond a certain level, the transistor Q 24  becomes saturated. When Q 24  is saturated, V GS  (gate-source voltage drop) of the transistor Q 24  is typically approximately 2 V, and thus the voltage at node  234  at the source of the transistor Q 24  is clamped to approximately 4 V. The 4 V clamped voltage at node  236  can be easily scaled down further by the resistive divider comprised of the programmable resistor R 20  to be under 3.6 V, which is the voltage limit that can be tolerated by semiconductor devices fabricated under the low voltage CMOS process. Thus, the devices of the power converter controller IC  100  are able to receive and process a high input line voltage V IN  even with devices fabricated using the low voltage CMOS process. 
       FIG. 4  illustrates the test signal used in triggering a test mode for trimming the voltage divider resistance coupled to the V IN  pin of the power converter controller in the AC-DC flyback power converter of  FIG. 1 , according to one embodiment of the present invention. The resistance of the programmable resistor R 20  in  FIG. 2  can be fixed by programming the fuses in a counter  208  associated with the programmable resistor R 20 . However, in order to program the programmable resistor R 20 , it is necessary to test the entire power converter with the power converter controller  100  to determine which programmable resistor R 20  value is appropriate for the various parameters (e.g., input voltage level, type of load, etc.) of the power converter. This is achieved by injecting a test signal  400  that has a positive pulse  402 , followed by negative pulses  404 ,  406 ,  408 , and again followed by a positive pulse  410 , into the V IN /Startup pin of the controller IC  100 . As will be explained with reference to  FIG. 6 , the negative pulses  404 ,  406 ,  408  are continuously generated until the appropriate value of the output voltage V SENSE  is achieved in the power converter in its operating environment, at which time the next positive pulse  410  is generated in the test signal  400 . Referring to  FIGS. 2 and 4  together, in one embodiment the test signal  400  is input to the V IN  pin and is received by the digital controller  102  to generate the test mode signal  420  and the counter clock signal  440 , both of which are input to the counter  208  with fuses. The voltage level of the test signal  400  is set high enough to be detected by the digital controller  104  but low enough so as not to damage the electronic components in the power converter controller  100 . 
     The test mode signal  420  indicates a test mode during which the values of T 1 -Tn for setting the programmable resistor R 20  are determined. The test mode signal  420  turns high at the rising edge of the first positive pulse  402  of the test signal  400 , and turns low at the rising edge of the second positive pulse  410  of the test signal  400 . The test mode signal  420  can be generated, for example, as the output signal of a flip flop (not shown) that is set and reset in response to rising edges of the test signal  400 . However, any logic circuitry that can generate the test mode signal  420  in accordance with the test signal  400  shown in  FIG. 4  may be used. 
     The counter clock signal  440  indicates the number of negative pulses  404 ,  406 ,  408  in the test signal  400  while the test mode signal  400  is positive (during test mode). The counter clock signal  440  turns high at the falling edge of the negative pulses  404 ,  406 ,  408  and turns low at the rising edges of the negative pulses  404 ,  406 ,  408  of the test signal  400 . The counter clock signal  440  is input to the counter  208 . The counter  208  counts the number of positive pulses in the counter clock signal  440  and converts the resulting count into a programming signal T 1 -Tn representing the count. The programming signal T 1 -Tn sets the value of the programmable resistor R 20 , as is explained below with reference to  FIG. 5 . Since the negative pulses  404 ,  406 ,  408  of the test signal are generated until the appropriate value of the output voltage V SENSE  is achieved in the power converter in its operating environment, the final count T 1 -Tn generated by the counter  208  represents the proper value using which the programmable resistor R 20  should be set for the power converter controller  100  to appropriately scale the input voltage V IN  to the power converter. Note that the counter  208  can also include a fuse, to permanent fix the count at the value set immediately prior to the end of the test mode  420 . The counter clock signal  440  can be generated, for example, as the output signal of a flip flop (not shown) that is set and reset in response to falling and rising edges of the negative pulses  404 ,  406 ,  408  of the test signal  400 . However, any logic circuitry that can generate the test mode signal  440  in accordance with the test signal  400  shown in  FIG. 4  may be used. 
       FIG. 5  illustrates the programmable resistor used in the voltage divider resistance coupled to the V IN  pin of the power converter controller in the AC-DC flyback power converter of  FIG. 1 , according to one embodiment of the present invention. Once the counter value T 1 -Tn is set by the counter  208 , T 1 -Tn can be used to set the value of the programmable resistor R 20 . The example in  FIG. 5  illustrates where the programmable resistor R 20  is set by a 2 bit counter  208  generating 4 values (T 1 , T 2 , T 3 , T 4 ), although the counter  208  may generate any number of counter values in other embodiments. 
     Referring to  FIG. 5 , programmable resistor R 20  includes a plurality of resistors R 50 , R 520 , R 54 , R 56  connected in series, an offset resistor R 58 , and a plurality of switches S 51 -S 58 . In one embodiment, the resistors R 50 , R 520 , R 54 , R 56  all have the same resistance R. Switches S 51  and S 55  are closed when the count from the counter  208  is T 1 , at which time the resistor R 50  is bypassed (shorted). Switches S 52  and S 56  are closed when the count from the counter  208  is T 2 , at which time the resistors R 50  and R 52  are bypassed (shorted). Switches S 53  and S 57  are closed when the count from the counter  208  is T 3 , at which time the resistors R 50 , R 52 , and R 54  are bypassed (shorted). Switches S 54  and S 58  are closed when the count from the counter  208  is T 4 , at which time the resistors R 50 , R 52 , R 54 , and R 56  are bypassed (shorted). The offset resistor R 58  provides the minimum resistance in the programmable resistor R 20 . Note that the on-resistance of the switches S 51 -S 58  is much smaller than the resistors R 40 , R 50 , R 52 , R 54 , R 56 , and R 58 , and is thus practically negligible. 
     Thus, for example, when the count from the counter is T 1 , the programmable resistor R 20  is “trimmed” with a remaining trimmed resistance R TRIM =R 52 +R 54 +R 56 +R 58 , and R 50  is bypassed (shorted). When the count from the counter is T 2 , the programmable resistor R 20  is “trimmed” with a remaining trimmed resistance R TRIM =R 54 +R 56 +R 58 , and R 50  and R 52  are bypassed (shorted). When the count from the counter is T 3 , the programmable resistor R 20  is “trimmed” with a remaining trimmed resistance R TRIM =R 56 +R 58 , and R 50 , R 52 , and R 54  are bypassed (shorted). When the count from the counter is T 4 , the programmable resistor R 20  is “trimmed” with a remaining trimmed resistance R TRIM  of R 58 , and R 50 , R 52 , R 54 , and R 56  are bypassed (shorted). Thus, R TRIM  is inversely proportional to the count (T 1 -T 4 ). The voltage at node  236  input to the ADC  210  is V LINE ×R TRIM /(R 40 +R 50 +R 52 +R 54 +R 56 +R 58 ). Since R 40  is typically a very large resistor (e.g., 6 Mohm) and much larger than the bypassed resistor(s) in the programmable resistor R 20 , the voltage  236  is determined by the ratio of the R TRIM  to the bypassed resistors in the programmable resistor R 20  with sufficient accuracy. 
       FIG. 6  is a flowchart illustrating a method of trimming the voltage divider resistance coupled to the V IN  pin of the power converter controller in the AC-DC flyback power converter of  FIG. 1 , according to one embodiment of the present invention. As explained above, the test signal  400  as shown in  FIG. 4  is provided 604 to the V IN  pin of the controller IC  100  with the negative pulses  404 ,  406 ,  408  continuing until the proper the value of V SENSE  is achieved. The number of pulses in the counter clock signal  400  generated based on the test signal  400  is counted  606  by the counter  208  to generate the counter value T 1 -Tn until the proper value of V SENSE  is achieved. The value of the trimmed resistance R TRIM  of the programmable resistor R 20  continues to be adjusted  608  while the count  606  changes, and is fixed  608  when the proper the value of V SENSE  is achieved, at which time the test signal  400  generates its second positive pulse  410  and the test mode signal  420  becomes inactive. 
     Upon reading this disclosure, those of ordinary skill in the art will appreciate still additional alternative structural and functional designs for an input voltage pin for a power converter controller IC through the disclosed principles of the present invention. Thus, while particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and components disclosed herein. Various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the invention as defined in the appended claims.