Abstract:
An integrated circuit and a related method for determining an operation mode are disclosed. The exemplified integrated circuit includes a controller, a multi-function pin, and a mode determination circuit. The controller controls a power switch and is being set to operate in one of the operation modes including a first operation mode and a second operation mode. The multi-function pin is connected to an external resistor. The mode determination circuit detects a signal from the multi-function pin. The signal represents the resistance of the external resistor. If the resistance is within a first range, the controller is operated in the first operation mode. If the resistance is within a second range, the controller is operated in the second operation mode.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for determining operation modes and a related circuit, and more particularly, to a method for determining operation modes and a related circuit used in a power supply. 
         [0003]    2. Description of the Prior Art 
         [0004]    In order to decrease the package size, the pin count of the integrated circuit (IC) is also preferably smaller. The concept of a multi-function pin is thus developed. The power management IC of the power supply also needs the multi-function pin to reduce the packaging cost. 
         [0005]    For instance, a light emitting diode (LED) driver IC, HV9910B, manufactured and sold by a company, Supertex, located in California, U.S.A., has a multi-function pin RT.  FIGS. 1 and 2  show two light emitting diode (LED) driver systems, of which integrated circuit IC 1  is HV9910B, suggested in the datasheet of HV9910B. The detailed system circuit operation of  FIGS. 1 and 2  can be found in the datasheet of HV9910B. Briefly speaking, as power switch Q 1  is turned on, inductor L starts to store the energy and current I LED  flows from the power source V IN  through LEDs LED 1  to LED N , inductor L, power switch Q 1 , and resistor R CS . If current I LED  is as large as a preset value, then power switch Q 1  is turned off, inductor L starts to release the stored energy and current I LED  flows through a loop composed of LEDs LED 1  to LED N , inductor L, and diode D. As for the turned-off power switch Q 1 , it is again turned on depending on the operation mode.  FIG. 1  shows the system operated in the pulse width modulation (PWM) mode;  FIG. 2  shows the system operated in the constant off-time mode. 
         [0006]    The difference between  FIG. 1  and  FIG. 2  is only the way for connecting resistor R T . If integrated circuit IC 1  determines that one end of resistor R T  is connected to the ground (GND) through multi-function pin RT, as shown in  FIG. 1 , integrated circuit IC 1  would make the entire LED driver system operate in the PWM mode, such that switching frequency f PWM  would be about a fixed value. If integrated circuit IC 1  determines that the end of resistor R T  is connected to pin GATE to receive a high voltage (e.g. 12V) therefrom, as shown in  FIG. 2 , integrated circuit IC 1  would make the entire LED driver system operate in the constant off-time mode, such that off-time T OFF  is about a fixed value. The resistance of resistor R T  is also used to determine switching frequency f PWM  in the PWM mode or off-time T OFF  in the constant off-time mode. 
         [0007]    In other words, multi-function pin RT is used not only to determine the operation mode of integrated circuit IC 1  but also to determine the off-time of power switch Q 1 . 
         [0008]    However, such multi-function pin RT design may cause the negative effects to the internal circuit design of integrated circuit IC 1 . 
         [0009]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIGS. 1 and 2  are two conventional light emitting diode (LED) drivers. 
           [0011]      FIG. 3  is an LED driver system according to a preferred embodiment of the present invention. 
           [0012]      FIG. 4  is a schematic view of a part of a circuit in  FIG. 3 . 
           [0013]      FIG. 5  is a flow chart illustrating a method for operating the circuit in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    For easily understanding the objectives, features, and advantages of the present invention, a detailed description with preferred embodiments accompanying figures is described as follows. 
         [0015]      FIG. 3  is an LED driver system  200  according to an embodiment of the present invention. Different to integrated circuit IC 1  in  FIGS. 1 and 2 , integrated circuit IC 2  in  FIG. 3  is determined to operate in the PWM mode or the constant off-time mode according to the resistance of resistor R T , so as to control power switch Q 1  for converting the power and then driving LEDs LED 1  to LED N . Since the principle and the operation between power switch Q 1 , current detecting resistor R CS , inductor L, capacitors C IN  and C OUT , diode D, and LEDs LED 1  to LED N  in  FIG. 3  are identical or similar to the corresponding elements in  FIG. 1 , the detailed description thereof will be omitted. 
         [0016]    With reference to  FIG. 3 , if the resistance of resistor R T  is larger than 8.5 KΩ, integrated circuit IC 2  operates in the constant off-time mode. Thus, off-time T OFF , i.e. the period from power switch Q 1  turned off to it turned on again, is an approximate fixed time that is determined by the resistance of resistor R T . 
         [0017]    As shown in  FIG. 3 , if the resistance of resistor R T  is smaller than 8.5 KΩ, integrated circuit IC 2  operates in the PWM mode. Thus, switching frequency f PWM , i.e. the reciprocal of the time interval for one turn-on time and a consecutive turn-off time of power switch Q 1 , is an approximate fixed frequency that is determined by the resistance of resistor R T . 
         [0018]    Therefore, multi-function pin RT is used to determine not only the operation mode of integrated circuit IC 2  but also the switching time of power switch Q 1 . 
         [0019]    Compared to multi-function pin RT of integrated circuit IC 2  in  FIG. 3 , multi-function pin RT of integrated circuit IC 1  in  FIGS. 1 and 2  will have two problems as follows. 
         [0020]    1. A high voltage circuit is needed for the input/output (I/O) circuit of multi-function pin RT of integrated circuit IC 1 . Because pin GATE is used to turn on/off power switch Q 1 , the output voltage from pin GATE is usually a high voltage, for example, 12 volts. Since multi-function pin RT of integrated circuit IC 1  in  FIG. 2  is connected to pin GATE, the I/O circuit of pin RT has to use high voltage circuit for tolerating the high voltage (12V) signal from pin GATE. 
         [0021]    2. The I/O circuit of multi-function pin RT of integrated circuit IC 1  may be interfered by the high frequency switching signal. Pin GATE of integrated circuit IC 1  outputs a high frequency signal to turn on/off power switch Q 1 . When integrated circuit IC 1  operates in the constant off-time mode in  FIG. 2 , such high frequency switching signal enters multi-function pin RT through resistor R T , and further affects a constant off-time control circuit connected to multi-function pin RT. Hence integrated circuit IC 1  needs extra protection to prevent the I/O circuit of multi-function pin RT from being interfered by the high frequency switching signal outputted from pin GATE. 
         [0022]    Multi-function pin RT of integrated circuit IC 2  in  FIG. 3  does not have the problems as mentioned above. A low voltage, for example, 5 volts, circuit may be used for the I/O circuit of multi-function pin RT of integrated circuit IC 2 . This is because multi-function pin RT of integrated circuit IC 2  is always coupled to the ground (GND) and does not receive the high voltage (12V) signal, whether integrated circuit IC 2  operates in the constant off-time mode or the PWM mode. It is well-known that low voltage (e.g. 5V) circuit generally costs a smaller chip area than high voltage (e.g. 12V) circuit, and thus it is more likely to reduce the cost if the low voltage circuit is put to use. Similarly, as shown in  FIG. 3 , since multi-function pin RT of integrated circuit IC 2  and pin GATE are separated whether integrated circuit IC 2  operates in the constant off-time mode or the PWM mode, multi-function pin RT of integrated circuit IC 2  is not interfered by the high frequency signal outputted from pin GATE. This is why an extra cost for the protection of the high frequency interference is not needed for integrated circuit IC 2 . 
         [0023]      FIG. 4  is a schematic view of a part of the circuit in  FIG. 3 . With reference to  FIG. 4 , integrated circuit IC 2  comprises controller  202 , mode determination circuit  204 , and voltage setting circuit  206 . Controller  202  controls switching of power switch Q 1  in  FIG. 3  through pin GATE. Mode determination circuit  204  detects the current flowing through multi-function pin RT and thus equally detects the resistance of resistor R T  as well. According to the detected resistance, Mode determination circuit  204  sends out the mode signal S MODE  to set controller  202  operating in one of the operation modes. For example, if the resistance of resistor R T  is larger than 8.5 KΩ, mode determination circuit  204  sets controller  202  to operate in the constant off-time mode and sends out time signal S TIME  according to the detected resistance of resistor R T  to determine off-time T OFF . When the resistance of resistor R T  is smaller than 8.5 KΩ, mode determination circuit  204  sets controller  202  to operate in the PWM mode and sends out time signal S TIME  according to the detected resistance of resistor R T  to determine switching frequency f PWM . 
         [0024]    Voltage setting circuit  206  sets voltage V RT  of multi-function pin RT according to mode signal S MODE . As shown in  FIG. 4 , voltage setting circuit  206  comprises a multiplexer  2062  that has three input terminals receiving the fixed voltages V REFH , V REF0 , and V REFL , respectively. In this embodiment, V REFH , V REF0 , and V REFL  are 1V, 0.6 V, and 0.2V, respectively. According to mode signal S MODE , multiplexer  2062  selects one of fixed voltages V REFH , V REF0 , and V REFL  as a reference voltage V REF  to output to comparator  2064 . The circuit connection between comparator  2064  and switch Q C  can maintain voltage V RT  of multi-function pin RT about equal to the reference voltage V REF . Besides, current I RT  passing through resistor R T  will also flow through switch Q C  and be detected by mode determination circuit  204 . 
         [0025]      FIG. 5  is a flow chart illustrating a method of an embodiment for operating the circuit in  FIG. 4 . With reference to  FIGS. 4 and 5 , step  502  shows that when integrated circuit IC 2  in  FIG. 4  is just connected to the power supply, the power supply will be stable after a settle time, and after fixed voltages V REFH , V REF0 , and V REFL  are steadily generated, other steps will then be performed. In step  504 , mode determination circuit  204  makes multiplexer  2062  select to output fixed voltage V REF0  according to mode signal S MODE , so that voltage V RT  of multi-function pin RT is approximately equal to fixed voltage V REF0 . In step  506 , mode determination circuit  204  detects current I RT  flowing through multi-function pin RT. In step  508 , current I RT  is compared to determine whether it is smaller than a predetermined current value I RT0 . With reference to  FIG. 4 , fixed voltage V REF0  is 0.6V and the predetermined value of current I RT0  is 70 μA. Thus, in steps  506  and  508 , mode determination circuit  204  equivalently determines whether the resistance of resistor R T  is larger than 8.5 KΩ (˜0.6V/70 uA). If the resistance of resistor R T  is determined to be larger than 8.5 KΩ, mode determination circuit  204  determines integrated circuit IC 2  to operate in the off-time mode and then step  510  is performed; if the resistance of resistor R T  is determined to be smaller than 8.5 KΩ, mode determination circuit  204  determines integrated circuit IC 2  to operate in the PWM mode and then step  520  is performed. 
         [0026]    Similar to step  504 , in step  510 , mode determination circuit  204  changes voltage V RT  of multi-function pin RT to fixed voltage V REFH  through voltage setting circuit  206 . As shown in  FIG. 4 , fixed voltage V REFH  is 1V. In step  512 , mode determination circuit  204  detects current I RT  flowing through multi-function pin RT. In step  514 , mode determination circuit  204  indicates controller  202  to operate in the constant off-time mode. Moreover, mode determination circuit  204 , as shown in step  516 , controls off-time T OFF  in the constant off-time mode according to current I RT . For example, if current I RT  is about 93.747 μA, which means the resistance of resistor R T  is about 10.667 KΩ (˜1V/93.747 uA), off-time T OFF  is about 0.5 micro-seconds; if current I RT  is about 0.93747 μA, which means the resistance of resistor R T  is about 1.06667 MΩ (1V/93.747 uA), off-time T OFF  is about 50 micro-seconds. 
         [0027]    In step  520 , mode determination circuit  204  changes voltage V RT  of multi-function pin RT to fixed voltage V REFL  through voltage setting circuit  206 . With reference to  FIG. 4 , fixed voltage V REFL  is 0.2V. In step  522 , mode determination circuit  204  then detects current I RT  flowing through multi-function pin RT. In step  524 , mode determination circuit  204  indicates controller  202  to operate in the PWM mode. Similarly, mode determination circuit  204  controls switching frequency f PWM  in the PWM mode in accordance with the current I RT . For example, integrated circuit IC 2  may be designed that if the current I RT  is about 30 μA, which means the resistance of resistor R T  is about 6.667 KΩ (˜0.2V/30 uA), switching frequency f PWM  of integrated circuit IC 2  is about 30 KHz; if current I RT  is about 200 μA, which means the resistance of resistor R T  is about 1 KΩ (0.2V/200 uA), switching frequency f PWM  of integrated circuit IC 2  is about 200 KHz. 
         [0028]    With reference to the above embodiment, 8.5 KΩ is a watershed. Resistor R T  with the resistance larger than 8.5 KΩ may make integrated circuit IC 2  operate in the constant off-time mode, and this resistance may be as large as 1.0667 MΩ. Resistor R T  with the resistance smaller than 8.5 KΩ may make integrated circuit IC 2  operate in the PWM mode, and this resistance may be as small as 1 KΩ. To sum up, in this embodiment, the usable resistance range of resistor R T  is from 1 KΩ to 1.0667 MΩ and the largest resistance is about 1000 (i.e. 10 to the power of three) times to the smallest resistance. 
         [0029]    According to the above embodiments, if the operation mode changes, voltage V RT  of multi-function pin RT will change; thereby the problem of serious variation in current I RT  is solved. Alternatively, if voltage V RT  of multi-function pin RT stays constant and does not change with the operation modes, it would be difficult to design integrated circuit IC 2  because integrated circuit IC 2  needs to identify current I RT  with variation up to 10 3  times. From the above embodiment, if voltage V RT  of multi-function pin RT is switched with the operation modes, the variation in current I RT  would decrease to 10 2  times (from 0.93747 μA to 200 μA) and the current detecting ability requirement of integrated circuit IC 2  can be relatively relaxed so that it can be designed more easily. 
         [0030]    As described above, the above embodiment discloses that the I/O circuit of multi-function pin RT needs only a low voltage circuit and the I/O circuit of the multi-function pin is prevented from being interfered by the high-frequency signals, and the problem of possibly wide variation in current I RT  is solved as well. Nevertheless, the scope of the present invention does not limit to the LED driver system and the effects disclosed in the above embodiments. 
         [0031]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.