Abstract:
A step up converter with overcurrent protection is disclosed. The step up converter can precisely limit the output current of the upstream device. Current from the input terminal of the converter is detected and compared with a predetermined maximum current to get a comparison value which is delivered to a close-loop regulator. The overcurrent protection is achieved by the regulator outputting a control signal to fulfill the conduction or resistance increase of a resistive element of the protection circuit. Furthermore, detection of the temperature or the output voltage may trigger shut off of the protection circuit to implement a protection function.

Description:
RELATED APPLICATION(S) 
       [0001]    This application claims priority to Chinese patent application No. 200710044337.2 entitled “A STEP UP CONVERTER WITH OVERCURRENT PROTECTION”, filed on Jul. 28, 2007. 
       TECHNICAL FIELD 
       [0002]    This invention relates to a step up converter with overcurrent protection, and more particularly, to a step up converter which limits the output current of an upstream device. 
       BACKGROUND 
       [0003]    Many consumer electronic devices are now charged not through the use of an external dedicated charger, but rather through a USB charger. The use of a USB charger eliminates the need for an AC power outlet which is required by traditional chargers. Further, the USB charger can obtain the power directly from a computer or other device with a USB port. 
         [0004]    A USB port typically can deliver a voltage of 5V and a maximum current of 500 mA. If the load causes the current to rise above the maximum value, the USB port and the mainboard may be damaged. It is important to ensure normal operation of the USB charger and at the same time realize the maximum power output. Prior art attempts to limit the current from a USB port utilize a current limiting integrated circuit (IC) on the mainboard. Another prior art technique of overcurrent protection is achieved by integrating the current limiting circuit with the charger circuit. An example of this is shown in U.S. Pat. No. 6,664,765 entitled “Lithium-ion Battery Charger Power Limitation Method”. The &#39;765 patent compares the power, voltage and current and shifts among the three modes to achieve current limiting. This current limiting function is realized in the charger circuit and the current limiting circuit cannot be used in other devices with an internal charging control circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  illustrates a block diagram of a step up converter with overcurrent protection function in accordance with a disclosed embodiment. 
           [0006]      FIG. 2  illustrates a schematic input current detection circuit unit in accordance with a disclosed embodiment. 
           [0007]      FIG. 3  illustrates a schematic step-up circuit unit in accordance with a disclosed embodiment. 
           [0008]      FIG. 4  illustrates a schematic protection circuit unit in accordance with a disclosed embodiment wherein  FIG. 4(   a ) is a BJT and  FIG. 4(   b ) is a MOSFET. 
           [0009]      FIG. 5  illustrates a schematic auxiliary power supply unit in accordance with a disclosed embodiment. 
           [0010]      FIG. 6  illustrates a schematic close-loop regulator of the control circuit in accordance with a disclosed embodiment. 
           [0011]      FIG. 7  illustrates a schematic control circuit in accordance with a disclosed embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Disclose is a step up converter with overcurrent protection function for a USB port and which limits the output current delivered to a device that includes internal charging control circuitry. 
         [0013]    In one embodiment, a current limiting step up converter comprises: an input terminal, connected to the power supply device (such as USB port); an output terminal, delivering current to the load; an input current detection circuit, connected to said input terminal, sensing the input current of the input terminal and sending a current detection signal to a control circuit. The control circuit is connected to the input current detection circuit which generates a maximum current reference signal representing a predetermined maximum current by an internal maximum current reference generator. The current detection signal is compared with the maximum current reference signal to get a comparison signal which is sent to a close-loop regulator for outputing a control signal to adjust the resistance of the protection circuit. 
         [0014]    The step up converter may also comprise an auxiliary power source, supplying power to the protection circuit and the control circuit. When the current detection signal is less than the maximum current reference signal, the resistance of the protection circuit is decreased by the control circuit to achieve a desired power delivered to the load. When the current detection signal reaches the maximum current reference signal, the resistance of the protection circuit is dynamically adjusted by the control circuit to maintain the input current at the predetermined maximum current. 
         [0015]    The step-up circuit of the step up converter may be a boost converter, increasing the voltage of the USB port to a level required by the load and delivering the regulated voltage to the protection circuit. The current limiting step up converter further includes an auxiliary power supply, coupled to the switch node (SW) of the step-up circuit and input terminal, and to supply power to the protection circuit and the control circuit. The auxiliary power supply may be a charge pump. The input current detection circuit detects the input current from the input terminal by sensing the voltage a resistance device such as a simple resistor, the “on” resistance of a switching element or the DC resistance of the inductive device of the step-up circuit. The switching element of the protection circuit can be a BJT or a MOSFET. 
         [0016]    The control circuit may further comprise an output voltage detection circuit. When the output is shorted for a time, a protection circuit is triggered by the control circuit to protect the charger. 
         [0017]    The control circuit may further comprise a temperature detection circuit. When the temperature is higher than a predetermined value, a protection circuit is triggered to protect the charger. 
         [0018]    Now the invention will be described by the following detailed description in combination with the drawings. 
         [0019]    According to  FIG. 1 , a step up converter with overcurrent protection is illustrated. As shown, the input terminal, input current detection circuit  1 , step-up circuit  2 , protection circuit  3 , and the output terminal are all connected in serial. The input current detection circuit  1  is further coupled to a control circuit  5 . The control circuit  5  is coupled to the step-up circuit  2  and protection circuit  3 . The step-up circuit  2  is coupled to an auxiliary power supply circuit  4 . The auxiliary power supply is coupled to the protection circuit  3  and the control circuit  5 . 
         [0020]    When the input terminal is electrically coupled to a USB port, the input current detection circuit  1  begins to continuously detect the current provided by the USB port which is also the input current of the step up converter. The input current detection circuit sends a current detection signal to the control circuit  5 . Based upon the current detection signal, the control circuit  5  controls the status of protection circuit  3  to limit the current at the output terminal, thereby in turn limiting the current at the input terminal. The step-up circuit  2  is used to boost the voltage at the input terminal to the level required by the load and then outputs the boosted voltage to the load through the protection circuit  3 . The load coupled to the output terminal may be a USB charger or other device with charging control circuitry. Moreover, the step up converter further comprises an auxiliary power supply circuit  4  which supplies power to the control circuit  5  and the protection circuit  3 . 
         [0021]    Referring to  FIG. 2 , the input current detection circuit  1  may be a simple resistor R, the conduction resistance of a switching element, or the DC resistance of the inductive device in the step-up circuit  2 . The input current detection circuit  1  senses the input current by sensing the voltage drop across the resistive devices described above. 
         [0022]    Referring to  FIG. 3 , the step-up circuit  2  may be a boost converter, which comprises an inductor L 1 , switch S 1 , rectifier diode D 1 , input capacitor C 1  and output capacitor C 2 . By controlling the duty cycle of the switch S 1 , the boost converter boosts the voltage at node N 1  to the level required by the load at node N 2 . This voltage is filtered by the output capacitor C 2 . Node N 1  connects to the input current detection circuit  1 . The voltage at node N 1  is determined by the voltage at the input terminal (Vin) and the resistive value (R) of the input current detection circuit  1  as: VN 1 =Vin−I*R. R is selected to be small, which results in VN 1  almost being the same as Vin. 
         [0023]    When the input current detection circuit  1  uses the DC resistance of inductor L 1 , the voltage at node N 1  equals to Vin. In other words, the resistive element R may be in fact the inductor L 1  which serves dual purposes: (1) resistive element R and (2) inductor for the step-up boost circuit. This has the advantage of eliminating one element from the embodiment. 
         [0024]    The node between the inductor L 1  and the anode of the diode D 1  is a switch node (SW), which delivers a switching signal to the auxiliary power supply circuit  4 . While the step-up circuit  2  begins operation, the auxiliary power supply circuit  4  operates. The auxiliary power supply circuit  4  is a charge pump which uses the switching signal of step-up circuit  2  at the switch node (SW) and the input voltage at input terminal to for auxiliary power. 
         [0025]    Referring to  FIG. 5 , the auxiliary power supply circuit  4  may be a charge pump. The charge pump comprises diodes D 2 , D 3  and capacitors C 3 , C 4  wherein the anode of diode D 2  connects to IN terminal, C 3  has one end connected to the cathode of D 2  and anode of D 3  and the other end connected to the SW terminal, C 4  has one end connected to the cathode of D 3  and the other end connected to the ground GND. A Vbias terminal connects to the cathode of D 3  and delivers power to the control circuit  5  and protection circuit  3 . The IN terminal connects the input terminal (also the USB port terminal) which has the voltage of Vin, and the SW terminal connects the SW node of the step-up circuit  2 . When the step-up circuit  2  operates, SW terminal has a high or low level pattern. When the signal at SW terminal is low, C 3  is charged to Vin. When SW terminal is high (such as 6 volts), C 4  is charged to be a sum of the voltage of C 3  and the SW, that is Vc 4 =Vc 3 +Vsw=Vin+Vsw, and gets an auxiliary voltage of about 10 volts to supply the protection circuit  3  and the control circuit  5 . 
         [0026]    The input current changes according to the resistance of the load. When the resistance of the load decreases, the input current increases. When the resistance increases, the input current decreases. The protection circuit  3  and the load are connected in serial to form the load of the step-up circuit  2 . The load of the step-up circuit  2  is changed by controlling the resistance of the protection circuit  3 , so as to control the input current of the step up converter. This principle is used to limit the current on the USB port (input current). 
         [0027]    Referring to  FIG. 6 , the input current detection circuit  1  sends a current detection signal to the control circuit  5 . The control circuit  5  has an internal maximum current reference generator  6  which offers a predetermined maximum current reference signal representing the predetermined maximum current. Then the current detection signal is compared to the maximum current reference signal to get a comparison signal which is sent to the close-loop regulator  7  of the control circuit  5 . According to the comparison signal, the close-loop regulator  7  outputs a control signal to drive the protection circuit  3 . The protection circuit  3  comprises a switching element (a resistive element) which is connected with the load. The resistance of the resistive element is controlled by the control signal. The load in one embodiment can be a lithium-ion battery charger. 
         [0028]    Referring to  FIG. 4 , the resistive device of the protection circuit may be a BJT ( FIG. 4(   b )) or a MOSFET ( FIG. 4(   b )) in according to an embodiment of the present invention. The close-loop regulator outputs the control signal according to the comparison signal and adjusts the resistance of the switching element of protection circuit  3  to limit the current. In practice, when the input current is less than the predetermined maximum current, the control signal from the close-loop regulator  7  controls the switching element of the protection circuit to be totally turned on, which allows minimal resistance and allows the converter to supply a desired power to the load. 
         [0029]    As the resistance of the load decreases, the input current increases. When the input current increases to be the same as the maximum current, that is, the current detection signal reaches the maximum current reference signal, the resistance of protection circuit  3  is dynamically adjusted by the control circuit  5  to maintain the input current at the predetermined maximum current. 
         [0030]      FIG. 7  shows the schematic close-loop regulator  7  and the maximum current reference generator  6  of the control circuit  5 . The maximum current reference generator  6  comprises voltage divider resistors R 1  and R 2  connected in series. One end of R 2  connects to ground GND, and one end of R 1  receives a reference voltage Vref. The maximum current reference signal is obtained from the connection node of R 1  and R 2 . By changing the values of R 1  and R 2 , the predetermined maximum current may be set. The close-loop regulator  7  comprises a comparator  70 , a close-loop feedback network consisting of resistor R 4  and capacitor C. The non-inverting input of the comparator  70  receives the maximum current reference signal. The inverting input of the comparator  70  receives the current detection signal  17  through resistor R 3 . 
         [0031]    The output  37  of the comparator  70  connects with the gate of the switching element of the protection circuit  3 . Comparator  70  compares the current detection signal with the maximum current reference signal. When the current detection signal is less than the maximum current reference signal, the close-loop regulator  7  outputs a high voltage to turn on the switching element of the protection circuit  3 ; and when the current detection signal surpasses the maximum current reference signal, close-loop regulator  7  outputs a voltage which leads the switching element of the protection circuit working under the linear region and maintain the input current at the maximum current. 
         [0032]    The control circuit  5  may further comprise an output voltage detection circuit to detect the output voltage. When the output terminal is shorted for a certain time, the resistive element of the protection circuit  3  would become the actual load of the entire circuit. In a short circuit condition, all the energy is converted to heat and would damage the circuit. Thus, the close-loop regulator  7  delivers a control signal to cut off the resistive element of the protection circuit  3 . The control circuit  5  further may comprise yet another circuit for temperature detection to detect the system temperature. When the temperature is higher than a predetermined value, the resistive element may be cut off by the control circuit  5 . 
         [0033]    Many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For example, the disclosure herein is mainly in the context of a USB charger, but one skilled in the art should know that the invention can also be used with other applications with internal charging control circuitry. It should be understood, of course, the foregoing disclosure relates only to a specific embodiment (or embodiments) of the invention and that numerous modifications may be made therein without departing from the spirit and the scope of the invention as set forth in the appended claims. Various modifications are contemplated and they obviously will be resorted to by those skilled in the art without departing from the spirit and the scope of the invention as hereinafter defined by the appended claims as only a preferred embodiment(s) thereof has been disclosed.