Abstract:
A voltage converter circuit includes a voltage converter controller which generates a PWM signal to operate a power switch for voltage conversion. The voltage converter controller includes a sensing pin for sensing a current and the voltage converter controller receives a power supply. A parameter setting method for the voltage converter circuit includes: during a start-up stage, when the power supply increases above a predetermined reference level, the voltage converter controller outputting a current through the sensing pin; and setting at least one parameter of the voltage converter controller according to a voltage at the sensing pin.

Description:
CROSS REFERENCE 
     The present invention is a continuation-in-part application of U.S. Ser. No. 13/869,684 filed on Apr. 24, 2013. The present invention also claims priority to TW 103104199, filed on Feb. 10, 2014. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to a voltage converter circuit and a voltage converter controller, especially to a voltage converter circuit and a voltage converter controller which includes an integrated circuit and capable of setting a parameter thereof without extra pins. 
     Description of Related Art 
     U.S. Pat. No. 7,315,190 discloses a voltage converter controller  200 . This voltage converter controller  200  was implemented by an integrated circuit capable of reducing geometric size and cost. In order to set circuit parameters of the voltage converter controller  200 , a discrete resistor Roc is connected to an output pin P 4  of a power switch driver stage, and a default current source of the voltage converter controller  200  provides a current flowing through the resistor Roc to generate a voltage when the voltage converter controller  200  is in a start-up stage and does not enter a normal operating state yet. Then the voltage is sampled and held to set the circuit parameters of the voltage converter controller  200 . Thus an extra pin is not required for the voltage converter controller  200  to setup the circuit parameters. Furthermore, the resistance of the resistor Roc can be adjusted to change the circuit parameters. 
     This prior art U.S. Pat. No. 7,315,190 can only set the circuit parameters during the start-up stage before the circuit enters normal operation, but can not set the circuit parameters during normal operation. 
     SUMMARY OF THE INVENTION 
     In view of above drawback, this invention provides a voltage converter circuit and a voltage converter controller including an integrated circuit and capable of setting a parameter thereof without extra pins. The present invention can set the circuit parameters during the start-up stage before the circuit enters normal operation, and the present invention also can set the circuit parameters during normal operation. Therefore, a user can set the circuit parameters at any desired timings, either during the start-up stage or during normal operation, or both. 
     In one embodiment, a voltage converter controller is adapted to a voltage converter circuit which generates a pulse-width-modulation (PWM) signal to operate a power switch thereof so as to drive a current load. The PWM signal toggles between a first level and a second level. The voltage converter controller includes a sensing pin and a parameter sampling and setting unit. The sensing pin receives a first sensing signal when the PWM signal is at the first level, and the sensing pin receives a second sensing signal when the PWM signal is at the second level. The parameter sampling and setting unit has an input terminal coupling to the sensing pin. When the PWM signal is at the second level, the parameter sampling and setting unit generates a default current or a default voltage on the sensing pin to generate the second sensing signal and simultaneously samples the second sensing signal to generate a sampling signal. And when the PWM signal is at the first level, the parameter sampling and setting unit holds the sampling signal to set a parameter of the voltage converter controller. 
     In another embodiment, a voltage converter circuit includes a power switch, a sensing pin, and a parameter sampling and setting unit. A power switch is controlled by a PWM signal to drive a current load. The PWM signal toggles between a first level and a second level. The sensing pin receives a first sensing signal when the PWM signal is at the first level, and the sensing pin receives a second sensing signal when the PWM signal is at the second level. The parameter sampling and setting unit has an input terminal coupling to the sensing pin. When the PWM signal is at the second level, the parameter sampling and setting unit simultaneously generates a default current or a default voltage on the sensing pin to generate the second sensing signal and samples the second sensing signal to generate a sampling signal. And when the PWM signal is at the first level, the parameter sampling and setting unit holds the sampling signal to set a parameter of the voltage converter circuit. 
     In another aspect, the present invention provides a parameter setting method for a voltage converter circuit, the voltage converter circuit including a voltage converter controller which generates a PWM signal to operate a power switch for voltage conversion. The voltage converter controller includes a sensing pin for sensing a current and the voltage converter controller receives a power supply. The parameter setting method includes: during a start-up stage, when the power supply increases above a predetermined reference level, the voltage converter controller outputting a current through the sensing pin; and setting at least one parameter of the voltage converter controller according to a voltage at the sensing pin. 
     In every cycle of a voltage converter circuit during the start-up stage or during normal operation, when the sensing pin thereof is not adopted for a feedback control, the parameter sampling and holding unit receives on the sensing pin a signal generated by applying a default current or a default voltage on resistor elements coupling to the sensing pin, and a parameter of the voltage converter controller is determined accordingly. Thus no extra pins are required for setting the parameter of the voltage converter controller and the hardware resource is saved. 
     These and other objectives of this invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiments that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a voltage converter circuit of a first embodiment. 
         FIG. 2  is a schematic diagram of a voltage converter circuit of a second embodiment. 
         FIG. 3  is a schematic diagram of a voltage converter circuit of a third embodiment. 
         FIG. 4  is a schematic diagram of an embodiment of a parameter sampling and setting unit. 
         FIG. 5  is a schematic diagram of another embodiment of a parameter sampling and setting unit. 
         FIG. 6  is a schematic diagram of an embodiment of a current sampling and holding circuit. 
         FIG. 7  is a schematic diagram of an embodiment of a parameter sampling and setting unit adopted in a power converter controller of a fly-back switching power converter. 
         FIG. 8  is a waveform diagram showing parameter setting during normal operation wherein the embodiment of  FIG. 1  is taken as an example. 
         FIG. 9  is a waveform diagram showing parameter setting during the start-up stage wherein the embodiment of  FIG. 1  is taken as an example. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  is a schematic diagram of a voltage converter circuit  100  of a first embodiment. In  FIG. 1 , the major components instead of a completed schematic of a voltage converter circuit are shown which is sufficient to fully describe the innovation of the invention to those skilled in the art. The voltage converter circuit  100  is a fly-back switching power converter by which converts an input voltage source to a either higher or lower DC output voltage and drives a current load on an output terminal. The voltage converter circuit  100  includes a power switch control unit  110 , a power switch driver unit  120 , a parameter sampling and setting unit  130 , a first resistor  140 , a second resistor  150 , a transformer  160 , a diode  170 , a power switch  180  and a sensing pin  190 . The voltage converter circuit  100  includes a feedback loop (not shown) to determine a duty cycle of the conduction of a channel of the power switch  180 . The power switch control unit  110  generates a control signal to the power switch driver unit  120  which accordingly generates a driving voltage signal or a driving current signal to drive the power switch  180  to control the conduction or cut-off of the channel of the power switch  180 . Then a pulse-width-modulation (PWM) signal is generated on the secondary side of the transformer  160 , that is, the side coupled with the diode  170 . The PWM signal drives the current load through the diode  170 . 
     In more detail, when the channel of the power switch  180  is conducted, no current is generated on the secondary side of the transformer  160 , and the PWM signal on the secondary side is at a first level which corresponds to the voltage value of the input voltage source. And when the channel of the power switch  180  is turned off, a current is generated on the secondary side of the transformer  160 , and the PWM signal on the secondary side is at a second level which corresponds to the voltage value of the DC output voltage. Since the channel of the power switch  180  conducts and turns off back and forth periodically, the PWM signal also toggles between the first level and the second level. 
     As shown in  FIG. 1 , the power switch control unit  110 , the power switch driver unit  120  and the parameter sampling and setting unit  130  are disposed in a voltage converter controller  195  which can be but not limited to an integrated circuit implemented by a semiconductor process by which the geometric size and the cost of the voltage converter circuit  100  are reduced. The voltage converter controller  195  further includes a sensing pin  190  coupled to the second resistor  150  and an input terminal of the parameter sampling and setting unit  130 . In the prior art, the sensing pin  190  is adopted to detect a first sensing signal relating to the feedback control. For example, when the channel of the power switch  180  is conducted, the sensing pin  190  is adopted to sense a sensing current flowing through the power switch  180 , wherein the quantity of the sensing current corresponds to the current on the current load. When the channel of the power switch  180  is turned off, no meaningful signal is generated or detected on the sensing pin  190 . The present invention adopts a default current or a default voltage applying on a resistor component to generate on the sensing pin  190  a signal which is then received by the parameter sampling and setting unit  130  to set a parameter of the voltage converter controller  195 . 
     As shown in  FIG. 1 , when the channel of the power switch  180  is turned off, that is, when the PWM signal is at the second level, the parameter sampling and setting unit  130  generates on the sensing pin  190  a default current or a default voltage applying on the serial connection of the first resistor  140  and the second resistor  150  and generates a second sensing signal on the sensing pin  190 . For example, the default current flows through the serial connection of the first resistor  140  and the second resistor  150  and generates the second sensing signal in a voltage type, or the default voltage biases on the serial connection of the first resistor  140  and the second resistor  150  and generates the second sensing signal in a current type. At the same time the parameter sampling and setting unit  130  samples the second sensing signal through the sensing pin  190  to generate a sampling signal. And when the channel of the power switch  180  is conducted, that is, the PWM signal is at the first level, the default current or default voltage is turned off, and the parameter sampling and setting unit  130  holds the sampling signal to set the parameter of the voltage converter controller  195 . At the same time the sensing current on the channel of the power switch  180  flows through the first resistor  140  and generates a voltage as the first sensing signal. Then the sensing pin  190  couples to the first sensing signal through the second resistor  150 , and the sensing signal is adopted by the voltage converter controller  195  for the feedback control. 
       FIG. 2  is a schematic diagram of a voltage converter circuit  200  of a second embodiment. The voltage converter circuit  200  is a boost switching power converter by which converts an input voltage source to a higher DC output voltage and drives a current load on an output terminal. The voltage converter circuit  200  includes a power switch control unit  210 , a power switch driver unit  220 , a parameter sampling and setting unit  230 , a first resistor  240 , a second resistor  250 , an inductor  260 , a diode  270 , a power switch  280  and a sensing pin  290 . The power switch control unit  210 , the power switch driver unit  220  and the parameter sampling and setting unit  230  are included in a voltage converter controller  295 . The functions of the power switch control unit  210 , the power switch driver unit  220  and the parameter sampling and setting unit  230  can be referred to the corresponding elements of the voltage converter controller  195  of the first embodiment. The voltage converter circuit  200  includes a feedback loop (not shown) to determine a duty cycle of the conduction of a channel of the power switch  280  and then a PWM signal is generated on the connecting node of the inductor  260  and the diode  270 . The PWM signal drives the current load through the diode  270 . 
     In the prior art, when the channel of the power switch  280  is turned off, no meaningful signal is generated or detected on the sensing pin  290 . Nonetheless in every period of the PWM signal when the channel of the power switch  280  is turned off, the second embodiment of the present invention adopts a default current or a default voltage applying on a resistor component to generate on the sensing pin  290  a signal which is then received by the parameter sampling and setting unit  230  to set a parameter of the voltage converter controller  295 . 
     As shown in  FIG. 2 , when the channel of the power switch  280  is turned off, the parameter sampling and setting unit  230  generates on the sensing pin  290  a default current or a default voltage applying on the serial connection of the first resistor  240  and the second resistor  250  and generates a second sensing signal on the sensing pin  290 . For example, the default current flows through the serial connection of the first resistor  240  and the second resistor  250  and generates the second sensing signal in a voltage type, or the default voltage biases on the serial connection of the first resistor  240  and the second resistor  250  and generates the second sensing signal in a current type. At the same time the parameter sampling and setting unit  230  samples the second sensing signal through the sensing pin  290  to generate a sampling signal. And when the channel of the power switch  280  is conducted, the default current or default voltage is turned off, and the parameter sampling and setting unit  230  holds the sampling signal to set the parameter of the voltage converter controller  295 . At the same time the sensing current on the channel of the power switch  280  flows through the first resistor  240  and generates a voltage as the first sensing signal. Then the sensing pin  290  couples to the first sensing signal through the second resistor  250 , and the sensing signal is adopted by the voltage converter controller  295  for the feedback control. 
       FIG. 3  is a schematic diagram of a voltage converter circuit  300  of a third embodiment. The voltage converter circuit  300  is a Buck switching power converter by which converts an input voltage source to a lower DC output voltage and drives a current load on an output terminal. The voltage converter circuit  300  includes a power switch control unit  310 , a power switch driver unit  320 , a power switch driver unit  325 , a parameter sampling and setting unit  330 , a first resistor  340 , a second resistor  350 , an inductor  370 , a power switch  360 , a power switch  380  and a sensing pin  390 . The power switch control unit  310 , the power switch driver unit  320 , the power switch driver unit  325  and the parameter sampling and setting unit  330  are included in a voltage converter controller  395 . The functions of the power switch control unit  310 , the power switch driver units  320  and  325  and the parameter sampling and setting unit  330  can be referred to the corresponding elements of the voltage converter controller  195  of the first embodiment. The voltage converter circuit  300  includes a feedback loop (not shown) to determine a duty cycle of the conduction of a channel of the power switches  380  and  380  and then a PWM signal is generated on the connecting node of the power switches  360  and  380 . The PWM signal drives the current load through the inductor  370 . 
     In the prior art, when the channel of the power switch  380  is turned off, no meaningful signal is generated or detected on the sensing pin  390 . Nonetheless in every period of the PWM signal when the channel of the power switch  380  is turned off, the third embodiment of the present invention adopts a default current or a default voltage applying on a resistor component to generate on the sensing pin  390  a signal which is then received by the parameter sampling and setting unit  330  to set a parameter of the voltage converter controller  395 . 
     As shown in  FIG. 3 , when the channel of the power switch  380  is turned off, the parameter sampling and setting unit  330  generates on the sensing pin  390  a default current or a default voltage applying on the serial connection of the first resistor  340  and the second resistor  350  and generates a second sensing signal on the sensing pin  390 . For example, the default current flows through the serial connection of the first resistor  340  and the second resistor  350  and generates the second sensing signal in a voltage type, or the default voltage biases on the serial connection of the first resistor  340  and the second resistor  350  and generates the second sensing signal in a current type. At the same time the parameter sampling and setting unit  330  samples the second sensing signal through the sensing pin  390  to generate a sampling signal. And when the channel of the power switch  380  is conducted, the default current or default voltage is turned off, and the parameter sampling and setting unit  330  holds the sampling signal to set the parameter of the voltage converter controller  395 . At the same time the sensing current on the channel of the power switch  380  flows through the first resistor  340  and generates a voltage as the first sensing signal. Then the sensing pin  390  couples to the first sensing signal through the second resistor  350 , and the sensing signal is adopted by the voltage converter controller  395  for the feedback control. 
     In the aforementioned three embodiments, the quantity of the second sensing signal of the voltage converter controller  195 / 295 / 395  is determined by the default current, the default voltage, the resistance of the first resistor  140 / 240 / 340  and the second resistor  150 / 250 / 350 . For example the value of the default current or the default voltage can be fixed in the design, and the parameter of the voltage converter controller  195 / 295 / 395  determined by the second sensing signal can be adjusted by changing the resistance of the first resistor  140 / 240 / 340  or the second resistor  150 / 250 / 350 . The parameter can be for example an output driving current of the power switch driver unit  120 / 220 / 320 / 325 , or a current threshold of an over-current protection unit (not shown) in the voltage converter controller  195 / 295 / 395  wherein when the current on the current load exceeds the current threshold, the voltage converter controller  195 / 295 / 395  turns off the channel of the power switch  180 / 280 / 360 / 380 . 
     In one embodiment, the sampling and holding process on the second sensing signal by the parameter sampling and setting unit  130 / 230 / 330  is performed in every period of the PWM signal. Thus, the parameter is periodically updated, so the leakage problem does not produce any significant influence. As a result, the sensing signal can be processed in its analog form without being converted to a digital signal. That is, since it is not necessary to convert the sensing signal into a digital form, an analog-to-digital converter which is relatively large in size, and a memory circuit can be omitted, and the corresponding area and power consumption are saved. Referring to  FIG. 8 , taking the embodiment of  FIG. 1  as an example, the current sensing and feedback function can be performed during the conduction period of the power switch  180  (when the gate signal of the power switch  180  is at the first level), and the parameter setting function can be performed during the non-conduction period of the power switch  180  (when the gate signal of the power switch  180  is at the second level). 
     In another embodiment, the parameter sampling and setting unit  130 / 230 / 330  performs the parameter setting function during a start-up stage before the voltage converter circuit enters normal operation. If necessary, the parameter setting can be stored by any means, for example converted to and stored in a digital form; however, it is not always necessary to store the parameter setting, and in many occasions the parameter only needs to be set once, without storage. Referring to  FIG. 9 , taking the embodiment of  FIG. 1  as an example, the parameter setting function can be performed when the power supply received by the voltage converter controller  195  is higher than a predetermined reference level. To set the parameter, in one embodiment, the parameter sampling and setting unit  130  outputs a current through the sensing pin  190 , which flows through the second resistor  150  and the first resistor  140  to ground. The parameter sampling and setting unit  130  senses a voltage at the sensing pin  190 . Thus, for example, the parameter can be set by the resistance of the second resistor  150 . 
     Furthermore, because the voltage converter controller  195 / 295 / 395  is an integrated circuit which is required to provide pins for electrical connection to another circuit, it is preferred that the number of pins is as smallest as possible, considering the geometric size and cost. The sensing pin  190 / 290 / 390  of the voltage converter controller  195 / 295 / 395  of the present invention achieves this purpose. Because the sensing pin is a multi-functional pin, the parameter setting function can be achieved without increasing the number of the pins. A parameter of the voltage converter controller can be set by an external component with great flexibility, without affecting the normal operation of the circuit. The present invention can be applied to various types of switching power converters, having a broad application range. 
     It is noted that the voltage converter controllers  195 ,  295  and  395  in the aforementioned embodiments are described herein for illustration purpose but not to limit the scope of the present invention. For example the voltage converter controller  195 ,  295  and  395  can be integrated circuits implemented by a semiconductor process, or effective circuits made by other arts. The voltage converter controller  195 ,  295  and  395  can also further include power switches or other components. People skilled in the art may implement the voltage converter controller of the present invention based on the requirements of the applications, the consideration of cost on design and the state-of-the-art knowledge in the art. 
       FIG. 4  is a schematic diagram of an embodiment of a parameter sampling and setting unit  400 . The parameter sampling and setting unit  400  can be adopted as the parameter sampling and setting unit  130 / 230 / 330  of the voltage converter controller  195 / 295 / 395 . Parameter sampling and setting unit  400  includes a setting current source  410 , a setting switch  420 , an input buffer stage  430 , a voltage sampling and holding circuit  440 , a parameter input terminal  490 , a first parameter output terminal  445  and a control terminal  480 . 
     As shown in  FIG. 4 , the parameter input terminal  490  is an input terminal of the parameter sampling and setting unit  400  and couples to the sensing pin  190 / 290 / 390  of the voltage converter controller  195 / 295 / 395 . The setting current source  410  is adopted to generate a default current. A channel of the setting switch  420  couples between the setting current source  410  and the parameter input terminal  490 . A control terminal of the setting switch  420  couples to the control terminal  480 . The signal on the control terminal  480  corresponds to the control signal of the power switch control unit  110 / 210 / 310  of the voltage converter controller  195 / 295 / 395 . When the aforementioned PWM signal is at the first level, the channel of the setting switch  420  is turned off. And when the PWM signal is at the second level, the channel of the setting switch  420  is conducted, and the default current of the setting current source  410  flows into the parameter input terminal  490 , that is, the sensing pin  190 / 290 / 390 , and also into the first resistor  140 / 240 / 340  and the second resistor  150 / 250 / 350  to generate the second sensing signal. The circuit design related to the function and the operation in this paragraph should be common knowledge to whom skilled in the art, and will not be described further herein. 
     As shown in  FIG. 4 , the input buffer stage  430  responds a voltage signal on the parameter input terminal  490  to the voltage sampling and holding circuit  440 . A gain value can be designed in the input buffer stage  430  to derive a better signal quality for the input of the voltage sampling and holding circuit  440 . Note that the input buffer stage  430  is not a must to the parameter sampling and setting unit  400 . The description herein is for the illustration of a best practice. The one who skilled in the art can choose to or not to implement the input buffer stage  430  in the parameter sampling and setting unit  400  based on the tradeoff between hardware cost and signal quality. Correspondingly, the input terminal of the voltage sampling and holding circuit  440  may connect directly to the parameter input terminal  490 . 
     As shown in  FIG. 4 , the voltage sampling and holding circuit  440  includes an input terminal  441 , an output terminal  442  and a control terminal  443 . The input terminal  441  couples to the output terminal of the input buffer stage  430 . The output terminal  442  couples to the first parameter output terminal  445 . The control terminal  443  couples to the control terminal  480 . When the pulse-width-modulation is at the second level, the voltage sampling and holding circuit  440  is adopted to sample the signal on the parameter input terminal  490  as the second sensing signal and generates a sampling signal. And when the pulse-width-modulation is at the first level, the voltage sampling and holding circuit  440  is adopted to set the parameter of the voltage converter controller  195 / 295 / 395 , for example to set a current threshold of an over-current protection unit. In case that when the current of the current load is detected to be larger than the current threshold, the voltage converter controller  195 / 295 / 395  turns off the channel of the power switch  180 / 280 / 360 / 380 . 
     Besides, the parameter sampling and setting unit  400  can further include an output buffer stage  450  and a voltage to current converter  460 . The output buffer stage  450  has an output terminal and an input terminal. The input terminal of the output buffer stage  450  couples to the first parameter output terminal  445 . The output buffer stage  450  generates a parameter-setting voltage signal  470  on the output terminal thereof according to a signal on the input terminal thereof to determine a parameter of the voltage converter circuit  195 / 295 / 395 , for example a current threshold of an over-current protection unit. A voltage gain can be designed for the output buffer stage  450  to properly adjust the parameter-setting voltage signal  470 . The voltage to current converter  460  has an input terminal and an output terminal. The input terminal of the voltage to current converter  460  couples to the first parameter output terminal  445 . The voltage to current converter  460  generates a parameter-setting current signal  485  on the output terminal thereof according to a signal on the input terminal thereof to determine a parameter of the voltage converter controller  195 / 295 / 395 , for example a output driving current of the power switch driver unit  120 / 220 / 320 / 3235 . 
       FIG. 5  is a schematic diagram of another embodiment of a parameter sampling and setting unit  500 . The parameter sampling and setting unit  500  can be adopted as the parameter sampling and setting unit  130 / 230 / 330  of the voltage converter controller  195 / 295 / 395 . Parameter sampling and setting unit  500  includes a setting voltage source  510 , a voltage loop amplifier  520 , a voltage loop transistor  530 , a current sampling and holding circuit  540 , a parameter input terminal  590 , a parameter output terminal  550  and a control terminal  560 . 
     As shown in  FIG. 5 , the parameter input terminal  590  is an input terminal of the parameter sampling and setting unit  500  and couples to the sensing pin  190 / 290 / 390  of the voltage converter controller  195 / 295 / 395 . The setting voltage source  510  is adopted to generate a default voltage. The voltage loop amplifier  520  has a pair of input terminals, an output terminal and a enabling terminal  521 , wherein the pair of input terminals thereof couples to the setting voltage source  510  and the parameter input terminal  590  respectively, and the enabling terminal  521  couples to the control terminal  560 . The signal on the control terminal  560  corresponds to the control signal of the power switch control unit  110 / 210 / 310  of the voltage converter controller  195 / 295 / 395 . When the aforementioned PWM signal is at the first level, the voltage loop amplifier  520  is turned off. And when the PWM signal is at the second level, the voltage loop amplifier  520  is turned on. The voltage loop transistor  530  is a transistor element with a control terminal and a channel with two terminals, wherein one terminal of the channel of the voltage loop transistor  530  couples to the parameter input terminal  590 , and the control terminal of the voltage loop transistor  530  couples to the output terminal of the voltage loop amplifier  520 . When the voltage loop amplifier  520  is turned on, a negative feedback loop is formed with the voltage loop transistor  530  and the virtual short-circuited feature of the input terminals of an amplifier renders the voltage of the parameter input terminal  590 , that is, the voltage of the sensing pin  190 / 290 / 390  essentially equals to the default voltage generated by the setting voltage source  510 . Then a second sensing signal  591  in current type is generated by biasing the first resistor  140 / 240 / 340  and the second resistor  150 / 250 / 350  with the default voltage. 
     As shown in  FIG. 5 , a current sampling and holding circuit  540  has an input terminal  541 , an output terminal  542  and a control terminal  543 . The input terminal  541  couples to the other terminal of the channel of the voltage loop transistor  530 . The output terminal  543  couples to the parameter output terminal  550 . The control terminal  543  couples to the control terminal  560 . When the PWM signal is at the second level, the current sampling and holding circuit  540  samples the second sensing signal  591  and generates the sampling signal. And when the PWM signal is at the first level, the current sampling and holding circuit  540  holds the sampling signal on the output terminal  542 . The output current signal  551  on the parameter output terminal  550  can be adopted to determine a parameter of the voltage converter controller  195 / 295 / 395 , for example a output driving current of the power switch driver unit  120 / 220 / 320 / 3235 . 
       FIG. 6  is a schematic diagram of an embodiment of a current sampling and holding circuit  540 . The current sampling and holding circuit  540  further includes a current input transistor  610 , a current output transistor  620 , a current sampling switch  630 , a current sampling capacitor  640 , and a supply voltage source  650 . 
     As shown in  FIG. 6 , a channel of the current input transistor  610  couples between the supply voltage source  650  and the input terminal  541 . A control terminal of the current input transistor  610  couples to one terminal of the channel of the current sampling switch  630 . A channel of the current output transistor  620  couples between the supply voltage source  650  and the output terminal  542 . A control terminal of the current output transistor  620  couples to the other terminal of the current sampling switch  630  and the current sampling capacitor  640 . A control terminal of the current sampling switch  630  couples to the control terminal  543 . When the signal on the control terminal  543  renders the channel of the current sampling switch  630  conducting, the current input transistor  610  and the current output transistor  620  forms a current mirror and an output current  670  on the current output transistor  620  corresponds to an input current  660  on the current input transistor  610 . That is, the current sampling and holding circuit  540  is sampling the second sensing signal  591  and generating the output current  670  as a sampling signal. Note that an amplifying factor of the output current  670  to the input current  660  relates to the geometric size of the current input transistor  610  and the current output transistor  620 . And when the signal on the control terminal  543  is changed and renders the channel of the current sampling switch  630  turning off, the voltage signal on the control terminal of the current output transistor  620  is hold by the current sampling capacitor  640 , and the output current  670  is also hold. Thus the current sampling and holding circuit  540  holds the sampling signal until the state of the current sampling switch  630  is changed in the next cycle of the PWM signal. Note that a voltage variation of the current sampling capacitor  640  incurred by a leakage current thereon is relatively small and can be ignored in this design. 
       FIG. 7  is a schematic diagram of an embodiment of a parameter sampling and setting unit  500  adopted in a power converter controller  795  of a fly-back switching power converter  700 . The functions of the fly-back switching power converter  700  can be referred to the related descriptions of the voltage converter circuit  100 . Nonetheless, a power switch  780  of the fly-back switching power converter  700  is a bipolar junction transistor, i.e., BJT in short. Thus it is necessary for the power switch driver unit  720  to output a driving current to conduct a channel of the power switch  780 . However there is a tradeoff on the design that if the driving current is too large there will be unnecessary waste on the power consumption, and if the driving current is too small there will be sacrifices on the operating speed and also the converting efficiency of the fly-back switching power converter  700 . The power converter controller  795  thus adopts the parameter sampling and setting unit  500  of the present invention. By adjusting resistances of a first resistor  740  and a second resistor  750 , the output current signal  551  is changed, and an output driving setting current  730  and also the output driving current of the power switch driver unit  720  are determined. As a result the setting of the power switch driver unit  720  can be optimized thereby according to various types of the power switch  780  in different applications with different requirements on power consumption, operating speed and converting efficiency. 
     Besides, the power converter controller  795  can further include an output driving default current  760 , a switch  731  and a switch  761 . A channel of the switch  731  couples between the output driving setting current  730  and the power switch driver unit  720 . A channel of the switch  761  couples between the output driving setting current  760  and the power switch driver unit  720 . The current of the output driving setting current  760  is a fixed value. By conducting or turning off the switch  731  and the switch  761 , the output driving current can be determined by optional combinations of the driving setting current  730  and the driving setting current  760 . 
     It is to be noted that the aforementioned embodiments are described herein for the illustration purpose but not to limit the scope of the present invention. People skilled in the art can implement the present invention according to the practical requirements on applications, cost considerations on design, and with improved components and elements introduced by the state-of-the-art technique. 
     This invention is advantageous because in every cycle of a voltage converter circuit during a start-up stage or during normal operation, when a sensing pin thereof is not adopted for a feedback control, a parameter sampling and holding unit receives on the sensing pin a signal generated by applying a default current or a default voltage on resistor elements coupling to the sensing pin, and a parameter of the voltage converter controller is determined accordingly. Thus no extra pins are required for setting the parameter of the voltage converter controller and the hardware resource is saved. The present invention is different from and advantageous over the prior art U.S. Pat. No. 7,315,190. In U.S. Pat. No. 7,315,190, the parameter setting is achieved by a control pin for controlling the power switch, whereas in the present invention, the parameter setting is achieved by a sensing pin. In addition to this difference, U.S. Pat. No. 7,315,190 can only set the parameters during a start-up stage, but the present invention can set the parameters either during a start-up stage or during normal operation, or both. Hence, the present invention provides a broader application than U.S. Pat. No. 7,315,190. 
     The aforementioned descriptions represent merely the preferred embodiment of this invention, without any intention to limit the scope of this invention thereto. Various equivalent changes, alterations, or modifications based on the claims of this invention are all consequently viewed as being embraced by the scope of this invention.