Abstract:
Various embodiments of LED drivers and associated methods of are described below. In one embodiment, a method for controlling an LED driver includes receiving a reference voltage, receiving a feedback voltage from said LED driver, receiving said input voltage as a first feed forward voltage and said output voltage as a second feed forward voltage, generating a hysteretic width based on said first feed forward voltage and said second feed forward voltage, and generating a hysteretic band voltage using said hysteretic width and said reference voltage. The method also includes generating a first control signal for controlling said LED driver based on said hysteretic band voltage and said feedback voltage, inverting said first control signal to generate a second control signal for controlling said LED driver, and achieving a generally fixed frequency for said LED driver.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to light emitting diode (LED) drivers, and more particularly, to adaptive hysteretic control circuits and methods thereof for LED drivers with a buck converter. 
       BACKGROUND 
       [0002]    In some system designs, a fixed frequency is required. For example, a fixed frequency is needed to reduce electromagnetic interference (EMI) in some portable devices. One prior art solution is to use a clock circuit, a ramp compensation circuit and an amplifier to form a closed loop to obtain the fixed frequency. The drawback of this conventional solution includes, but is not limited to, a slow regulation speed. 
         [0003]    Another existing solution is to use an adaptive constant on time control with an input voltage feed forward. This solution can almost achieve a fixed frequency. However, it regulates an inductor valley current so that the accuracy of the LED current regulation is typically not good. In addition, a load step down transient may have a big overshoot due to the constant on time. 
         [0004]      FIG. 1  shows a circuit  10  illustrating a conventional hysteretic control circuit  100  for an LED driver with a buck converter. As shown in  FIG. 1 , an input voltage V in  is provided to a first terminal of a high side switch Q 1  whose second terminal is electrically coupled to a first terminal of a low side switch Q 2 . A second terminal of the low side switch Q 2  is electrically coupled to the ground. An inductor L is electrically coupled between a node SW formed by the second terminal of the high side switch Q 1  and the first terminal of the low side switch Q 2  and an output voltage port which provides a regulated output voltage V o  to an LED string. A capacitor C o  is electrically coupled between a first terminal and a second terminal of the LED string. A sensing resistor R sensed  is electrically coupled between the second terminal of the LED string and the ground. 
         [0005]    The hysteretic control circuit  100  comprises a fixed hysteretic width production circuit  101 , a comparator CMP, an inverter INV and a hysteretic band voltage generating circuit  102 . As shown in  FIG. 1 , the inductor current I L  is sensed by the sensing resistor R sensed  across which the voltage drop acts as a sensing voltage V s  to be compared by the comparator CMP with a hysteretic band voltage which comprises a high hysteretic band voltage V_h by adding half of a hysteretic width ΔV generated by the fixed hysteretic width production circuit  101  with a reference voltage V ref  and a low hysteretic band voltage V_l by subtracting half of the hysteretic width ΔV from the reference voltage V ref . When V s  is lower than V_l (V s &lt;V_l), the comparator CMP outputs a high level to turn on Q 1  and to turn off Q 2  with a low level which is generated by inverting the high level with the inverter INV. Accordingly, the inductor current I L  and the sensing voltage V s  start to increase. When V s  increases to such a point that it is higher than V_h (V s &gt;V_h), the comparator CMP outputs a low level to turn off Q 1  and meanwhile to turn on Q 2  through the inverter INV. Accordingly, the inductor current I L  and the sensing voltage V s  start to decrease. When V s  decreases to be lower than V_l (V s &lt;V_l) again, the comparator CMP outputs a high level to turn on Q 1  and to turn off Q 2  through the inverter INV. A new control cycle begins. 
         [0006]    The on time T 1  and the off time T 2  of the high side switch Q 1  are determined by the hysteretic width ΔV, the inductor L, the input voltage V in , the output voltage V o  and the sensing resistor R sensed , following the equations below: 
         [0000]      Δ l=ΔV/R   sensed   (1)
 
         [0000]        T   1 =( L×Δl )/( V   in   −V   o )  (2)
 
         [0000]        T   2 =( L×Δl )/ V   o   (3)
 
         [0000]    Thus, the switching period T s  of the switches can be written as: 
         [0000]        T   s   =T   1   +T   2 =( L×ΔV×V   in )/( R   sensed ×( V   in   −V   o )× V   o )  (4)
 
         [0000]    In a particular application, the sensing resistor R sensed  is determined by a setting LED current, and the inductor L is determined by the inductor current ripple and the output voltage V o  is expected to be constant. So the switching period T s  is dependent of the input voltage V in  and the hysteretic width ΔV. 
         [0007]    For the conventional hysteretic control circuit for an LED driver with a buck converter shown in  FIG. 1 , the hysteretic width ΔV is a fixed value generated by the fixed hysteretic width production circuit  101 , so the switching period T s  is only dependent of the input voltage V in . When V in  changes, T s  changes accordingly. That is to say, the switching frequency F s =1/T s  changes in response to the input voltage V in .  FIG. 2  shows a waveform diagram illustrating examples of signals in the conventional hysteretic control circuit shown in  FIG. 1 . The signals from top to bottom are followed by the sensing voltage V s , the voltage V SW  at the node SW and the inductor current I L . Accordingly, improved hysteretic control circuits and methods thereof for LED drivers are needed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    In the Figures, identical or similar components are designated by the same or similar reference numerals throughout. 
           [0009]      FIG. 1  shows a conventional hysteretic control circuit for an LED driver with a buck converter. 
           [0010]      FIG. 2  shows a waveform diagram illustrating examples of signals in the conventional hysteretic control circuit shown in  FIG. 1 . 
           [0011]      FIG. 3  shows an adaptive hysteretic control circuit for an LED driver with a buck converter according to an embodiment of the present technology. 
           [0012]      FIG. 4  shows a hysteretic width production circuit for the adaptive hysteretic control circuit shown in  FIG. 3 . 
           [0013]      FIG. 5  shows an adaptive hysteretic control circuit for an LED driver with a buck converter according to another embodiment of the present technology. 
           [0014]      FIG. 6  shows a hysteretic width production circuit for the adaptive hysteretic control circuit shown in  FIG. 5 . 
           [0015]      FIG. 7  shows a waveform diagram illustrating the simulation results of the signals of the adaptive hysteretic control circuit shown in  FIG. 5 . 
           [0016]      FIG. 8  shows a waveform diagram illustrating a frequency corresponding to an input voltage according to the adaptive hysteretic control circuit shown in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Various embodiments of LED drivers and associated methods are described below. In the following detailed description of the present technology, numerous specific details are set forth in order to provide a thorough understanding of the present technology. However, it will be obvious to one of ordinary skill in the art that the present technology may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present technology. A person skilled in the relevant art will also understand that the technology may have additional embodiments, and that the technology may be practiced without several of the details of the embodiments described below with reference to  FIGS. 3-8 . 
         [0018]      FIG. 3  shows a circuit  30  illustrating an adaptive hysteretic control circuit  300  for an LED driver with a buck converter according to an embodiment of the present technology. The components of the circuit  30  are connected in a manner similar to that described above in connection with  FIG. 1  except that an adaptive hysteretic width production circuit  301  replaces the fixed hysteretic width production circuit  101 . 
         [0019]    As shown in  FIG. 3 , the adaptive hysteretic control circuit  300  comprises the adaptive hysteretic width production circuit  301  which receives an input voltage V in  as a first feed forward voltage and an output voltage V o  as a second feed forward voltage and provides an adaptive hysteretic width ΔV. The adaptive hysteretic width ΔV changes in response to V in  and V o  following the equation below: 
         [0000]      Δ V=K   1 ×( Vin   −V   o )× V   o   /V   in   (5)
 
         [0000]    wherein K 1  is a proportional factor which is a fixed value with a certain integrated circuit (IC) design. Thus, according to the equations (1) to (4) described above, the switching period T s  is: 
         [0000]        T   s   =K   1   ×L/R   sensed   (6)
 
         [0000]    The switching frequency F s  is: 
         [0000]        F   s   =R   sensed /( L×K   1 )  (7)
 
         [0000]    As evident from the equation (7), the switching frequency F s  is independent of the input voltage V in  and is a fixed value only decided by the inductor L or the sensing resistor R sensed . 
         [0020]    Referring to  FIG. 4 , an adaptive hysteretic width production circuit  40  according to an embodiment of the present technology is illustrated and may be used in the circuit  30  as the adaptive hysteretic width production circuit  301 . As shown in  FIG. 4 , the adaptive hysteretic width production circuit  40  comprises a first transistor  401 , a second transistor  402 , a third transistor  403 , a forth transistor  404 , a first current source  407 , a second current source  408 , a third current source  409 , a first resistor R 1  and a first current mirror comprising a first PNP transistor  405  and a second PNP transistor  406 . A current input terminal of the first current source  407  is electrically coupled to collectors of the second transistor  402  and the third transistor  403  and emitters of the first PNP transistor  405  and the second PNP transistor  406 , a current output terminal of the first current source  407  is electrically coupled to a collector of the first transistor  401  and bases of the second transistor  402  and the third transistor  403 , a base of the first transistor  401  is electrically coupled to an emitter of the second transistor  402  and a current input terminal of the second current source  408 , an emitter of the third transistor  403  is electrically coupled to a base of the forth transistor  404  and a current input terminal of the third current source  409 , a collector of the forth transistor  404  is electrically coupled to a collector of the first PNP transistor  405 , a first terminal of the first resistor R 1  is electrically coupled to a collector of the second PNP transistor  406 , an emitter of the first transistor  401 , current output terminals of the second current source  408  and the third current source  409 , an emitter of the forth transistor  404  and a second terminal of the first resistor R 1  are electrically coupled commonly to the ground. 
         [0021]    As shown in  FIG. 4 , the output current of  407  is proportional to the difference between the input voltage V in  and the output voltage V o  with a proportional factor K 2 . The output current of  408  is proportional to V o  with the proportional factor K 2 . The output current of  409  is proportional to V in  with the proportional factor K 2 . The resistor R 1  equals to the proportional factor K 1  multiplied by the proportional factor K 2 . The hysteretic width ΔV is the voltage drop across the resistor R 1  which is referred to as the output voltage of the adaptive hysteretic width production circuit  40 . 
         [0022]    As shown in  FIG. 4 , the currents I 1  of the current source  407 , I 2  of the current source  408 , I 3  of the current source  409  and the output current I 4  of the current mirror are respectively approximate equal to the emitter currents of transistors  401 ,  402 ,  403  and  404 : 
         [0000]        I   1   =I   ES × eVbe1   /VT   (8)
 
         [0000]        I   2   =I   ES × eVbe2   /VT   (9)
 
         [0000]        I   3   =I   ES × eVbe3   /VT   (10)
 
         [0000]        I   4   =I   ES   ×e   Vbe4/VT   (11)
 
         [0000]    Where I ES  is an emitter inverse saturation current, VT is a temperature equivalent voltage and Vbe 1 , Vbe 2 , Vbe 3 , Vbe 4  are respectively base-emitter voltages of the transistors  401 ,  402 ,  403 , and  404 . 
       Thus, 
       [0023]        I   1   ×I   2   =I   ES   2   ×e   (Vbe1+Vbe2)/VT   (12)
 
         [0000]        I   3   ×I   4   =I   ES   2   ×e   (Vbe3+Vbe4)/VT   (13)
 
         [0000]    As can be seen from  FIG. 4 , there exist: 
         [0000]        V   be1 + Vbe2   =V   be3   +V   be4   (14)
 
       Thus, 
       [0024]        I   4   =I   1   ×I   2   /I   3   (15)
 
         [0000]    For the current sources  407 ,  408  and  409 , there respectively exists: 
         [0000]        I   1 =( V   in   −V   o )/ K   2   , I   2   =V   o   /K   2   , I   3   =V   in   /K   2   (16)
 
         [0000]    From the equations (15) and (16), the current I 4  can be expressed as: 
         [0000]        I   4 =( V   in   −V   o )× V   o /( K   2   ×V   in )  (17)
 
         [0000]      Because: 
         [0000]      Δ V=I   4   ×R   1 =(( V   in   −V   o )× V   o   ×R   1 )/( K   2   ×V   in )  (18)
 
         [0000]    Also because R 1 =K 1 ×K 2 , there exists: 
         [0000]      Δ V=I   4   ×R   1   =K   1 ×( V   in   −V   o )× V   o   /V   in   (19)
 
         [0000]    As evident from the above equation (19), a hysteretic width ΔV can be achieved by choosing R 1 =K 1 ×K 2  to form a fixed switching frequency F s  independent of the input voltage V in . 
         [0025]      FIG. 5  shows a circuit  50  illustrating an adaptive hysteretic control circuit  500  for an LED driver with a buck converter according to another embodiment of the technology. The components of the circuit  50  are electrically coupled in a manner similar to that described above in connection with  FIG. 1  and  FIG. 3  except for the addition of a current source  510  and a frequency setting resistor  511 . In the circuit  50 , the hysteretic width ΔV is: 
         [0000]      Δ V=V   fre ×(( V   in   −V   o )× V   o   /V   in )= I   s   ×R   fre ×(( V   in   −V   o )× V   o   /V   in )  (20)
 
         [0000]    where I s  is the output current of the current source  610  and R fre  is the resistance value of the frequency setting resistor  611 . Therefore, according to equations (4) and (20), the switching period T s  can be written as: 
         [0000]        T   s   =T   1   +T   2   =L×I   s   ×R   fre   /R   sensed   (21)
 
         [0000]    Accordingly, the switching frequency F s  can be written as: 
         [0000]        F   s   =R   sensed /( L×I   s   ×R   fre )  (22)
 
         [0000]    As evident from equation (22), in this circuit  50 , with a given current I s , a fixed inductor L and a fixed resistor R sensed , the switching frequency F s  is fixed when the input voltage V in  changes. Furthermore, the switching frequency F s  can be programmed by changing the resistance value R fre  of the frequency setting resistor  611 . 
         [0026]    Referring to  FIG. 6 , an adaptive hysteretic width production circuit  60  according to an embodiment of the present technology is illustrated and may be used in the circuit  50  as the adaptive hysteretic width production circuit  501 . The adaptive hysteretic width production circuit  60  comprises the adaptive hysteretic width production circuit  40  (referring as the first hysteretic width production circuit  61  thereafter) as shown in  FIG. 4  and further comprises a second adaptive hysteretic width production circuit  62 . The second adaptive hysteretic width production circuit  62  has a similar configuration with the first hysteretic width production circuit  61 . Compared to  61 , the output current I 5  of  607  is proportional to the output voltage V o1  of the first hysteretic width production circuit  61  with a proportional factor K 3 . The output current I 6  of  608  is proportional to V fre  with the proportional factor K 3 . The output current I 7  of  609  is a constant value. The resistor R 2  equals to the square of the proportional factor K 3  multiply the output current I 7  of the current source  609  over the proportional factor K 1 . The hysteretic width ΔV is the voltage drop across the resistor R 2 . 
         [0027]    Referring to  FIG. 4  and  FIG. 6 , according to the similar derivation as expressed by equations (8) to (17), an output current I 8  of the current mirror in the second adaptive hysteretic width production circuit  62  can be expressed as: 
         [0000]        I   8   =I   5   ×I   6   /I   7 =( V   o1   /K   3 )×( V   fre   /K   3 )/ I   7 =( K   1 ×( V   in   −V   o )× V   o   ×I   s   ×R   fre   /V   in )/( K   3   2   ×I   7 )  (23)
 
         [0000]    The hysteretic width ΔV can be written as: 
         [0000]      Δ V=R   2   ×I   8 =( K   1 ×( V   in   −V   o )× V   o   ×I   s   ×R   fre   /V   in )×( R   2 /( K   3   2   ×I   7 ))  (24)
 
         [0000]      Because 
         [0000]        R   2   =K   3   2   ×I   7   /K   1   (25)
 
         [0000]    Thus, the hysteretic width ΔV can be written as: 
         [0000]      Δ V=I   s   ×R   fre ×( V   in   −V   o )× V   o   /V   in   (26)
 
         [0000]    As evident from equation (26), a hysteretic width ΔV can be achieved by choosing R 2 =K 3   2 ×I 3 /K 1  to form a fixed switching frequency F s  independent of the input voltage V in  and furthermore, the switching frequency F s  can be programmed by changing the resistance value R fre  of the frequency setting resistor  611  even with a fixed inductor L and a fixed resistor R sensed . 
         [0028]      FIG. 7  shows a waveform diagram illustrating the simulation results of the signals of the adaptive hysteretic control circuit shown in  FIG. 5 . Top trace is the input voltage V in , below the input voltage V in  is the inductor current I L , the third trace is the sensing voltage V s  and the forth trace is the output voltage V o . The LED current is depicted by the bottom trace. As shown in  FIG. 7 , the hysteretic width changes when the input voltage changes. 
         [0029]      FIG. 8  shows the switching frequency F s  when the input voltage V in  changes from 20V to 65V. The traces of the 5 LED series and the 10 LED series are respectively measured by using an inductor with an inductance of 47 uH and an inductor with an inductance of 33 uH. The traces of the 5 LED series and the 10 LED series are both measured when the LED current is 350 mA. As shown in  FIG. 8 , the switching frequency is basically kept in a constant value as the input voltage changes. 
         [0030]    From the foregoing, it will be appreciated that specific embodiments of the technology have been described herein for purposes of illustration, but that various modifications may be made without deviating from the disclosure. In addition, many of the elements of one embodiment may be combined with other embodiments in addition to or in lieu of the elements of the other embodiments. For example, one of ordinary skill in the art will understand that in  FIG. 3  and  FIG. 5 , the output capacitor C o  is used to absorb the alternative current (AC) element of the inductor current I L . However, the output capacitor C o  can be removed in other embodiments. Also, the switches Q 1  and Q 2  can be any suitable types of switches, such as MOSFET, IGBT, and BJT. The low side switch Q 2  can also be replaced by a diode. Accordingly, the disclosure is not limited except as by the appended claims.