Abstract:
An ON-OFF timer circuit for use in a DC-DC converter to minimize or eliminate the risk of developing sub-harmonic oscillations that may cause the dc-dc system to be unstable is presented. The apparatus controls and limits the ‘On’ time duration and ‘Off’ time duration within one pulse cycle.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to an ON-OFF timer circuit for use in a dc-dc converter. More specifically, the present invention relates to a boost dc-dc converter type for use in driving LEDs. 
         [0002]    A conventional boost dc-dc converter is shown in  FIG. 1A  which is used for driving LEDs, and the signals observed in the boost dc-dc converter are shown in  FIGS. 1B and 1C . As shown in  FIG. 1B , dc-dc control  2  outputs the switching control signal to a driver circuit  1  before driving the switch  3  for the dc-dc operation. In this mode of operation, there is a possibility of the occurrence of sub-harmonic oscillations, which in turn may cause the dc-dc converter system to become unstable. These sub-harmonic oscillations occur if the following conditions are met:
       Operating in continuous conduction mode;   Duty cycle is 50% or higher.
 
According to a technical report on “Ramp Compensation for Current-Mode Converters” by Christophe Basso of ON Semiconductor, as published in the Power Electronics Technology magazine in July 2004, a resultant perturbation due to the sub-harmonic oscillations may be represented by the formula:
       
 
         [0000]      Δ I   L ( n )=Δ I   L (0)×( D/ 1 −D ) n  
 
         [0000]    where D=duty cycle;
 
n=number of switching cycles;
 
I L =inductor current;
 
ΔI L =inductor current step as a result of the said perturbation.
 
         [0005]    From the formula, we can see that if the duty cycle D is less than 50%, the perturbation will die out after several cycles. However, for duty cycles more than 50%, the perturbation continues to grow with every cycle. An exemplary waveform showing this phenomenon is as shown in  FIG. 1C . In  FIG. 1C , a typical inductor current under normal operating condition is given as I L0 . Upon introduction of the said perturbation (ΔI L0 ), the resultant oscillating waveform is given by I Lsh . 
         [0006]    Conventional means to prevent the sub-harmonic oscillations would be to use the Slope Compensation Technique, as described in U.S. Pat. No. 4,837,495 (Current Mode Converter With Controlled Slope Compensation) by Zansky; as well as by using the Hysteretic Control, as described in U.S. Pat. No. 6,628,106 (Control Method and Circuit to Provide Voltage and Current Regulation for Multiphase DC/DC Converters) by Batarseh et al. 
         [0007]    These two techniques, however, involve complex systems. The present invention aims to provide a solution to resolve the instability problems described above. Based on the present invention, a technique is introduced for controlling the ON and OFF widths of the output pulse-width modulated waves. 
       SUMMARY OF THE INVENTION 
       [0008]    The purpose of this invention is to provide a technique to minimize or eliminate the risk of developing sub-harmonic oscillations that may cause the dc-dc system to be unstable. 
         [0009]    The technique basically comprises controlling the ‘On’ (pulse duration, τ) and ‘Off’ (T−τ) durations (Refer to  FIG. 1 ), where T is the period of one pulse cycle. 
         [0010]    According to the present invention, an ON-OFF timer circuit for use in a A DC-DC converter comprising: an Off-Timer to keep the Off duration constant; an On-Timer to limit the On duration so as not to exceed a pre-determined time limit; and a flip-flop operable to for generate a first level signal and a second level signal from its output for controlling said Off duration and On duration. 
         [0011]    According to the present invention, the Off-Timer comprises: a first current source; a first switch operable to receive said first and second level signals from said flip-flop; a first charging capacitor connected to said first current source through said first switch, said first charging capacitor operable to charge current according to said first current source during when said flip-flop generates said first level signal so as to produce an ascending DC voltage across said first charging capacitor; and a first comparator operable to compare said ascending DC voltage with a predetermined first threshold level, and operable to generate a latch signal when said ascending DC voltage reaches said first threshold level, said latch signal applied to said flip-flop to generate said second level signal. 
         [0012]    According to the present invention, the On-Timer comprises: a second current source; a second switch operable to receive said first and second level signals from said flip-flop; a second charging capacitor connected to said second current source through said second switch, said second charging capacitor operable to discharge current according to said second current source during when said flip-flop generates said second level signal so as to produce a descending DC voltage across said second charging capacitor; and a second comparator operable to compare said descending DC voltage with a predetermined second threshold level, and operable to generate a reset signal when said ascending DC voltage drops to said second threshold level, said reset signal applied to said flip-flop to generate said first level signal. According to the present invention, said first charging capacitor is in common with said second charging capacitor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1A  is a circuit diagram showing a typical set-up for a boost type dc-dc converter according to prior art. 
           [0014]      FIG. 1B  is a waveform diagram showing a typical rectangular waveform according to prior art. 
           [0015]      FIG. 1C  is an exemplary waveform diagram showing the effect of sub-harmonic oscillation according to prior art. 
           [0016]      FIG. 2  is a block diagram showing a first embodiment of the present invention. 
           [0017]      FIG. 3A  is a block diagram showing an exemplary implementation of first embodiment of the present invention, highlighting off timer block. 
           [0018]      FIG. 3B  is a block diagram showing an exemplary implementation of first embodiment of the present invention, highlighting on timer block. 
           [0019]      FIG. 3C  is a circuit diagram showing a PMOS switch, as exemplarily used for switch SW 1 . 
           [0020]      FIG. 3D  is a circuit diagram showing an NMOS switch, as exemplarily used for switches SW 2  and  131 . 
           [0021]      FIG. 3E  is a diagram showing a collection of waveforms of selected nodes, illustrating the operation of the first embodiment. 
           [0022]      FIG. 4  is a circuit diagram showing an exemplary circuit to counteract effect of variable output voltage. 
           [0023]      FIG. 5A  is a circuit diagram showing an exemplary implementation of the second embodiment of the present invention, with the off-timer highlighted. 
           [0024]      FIG. 5B  is an exemplary implementation of the second embodiment of the present invention, with the on-timer highlighted. 
           [0025]      FIG. 5C  is a diagram showing a collection of waveforms of selected nodes, illustrating the operation of the second embodiment. 
           [0026]      FIG. 6A  is a circuit diagram showing an exemplary implementation of the third embodiment of the present invention, with the off-timer highlighted. 
           [0027]      FIG. 6B  is a circuit diagram showing an exemplary implementation of the third embodiment of the present invention, with the on-timer highlighted. 
           [0028]      FIG. 7  is a diagram showing waveforms demonstrating the situation when capacitor C 1  is not fully discharged when timing duration for On-Timer  102  is ended, and the full discharge by switch  131  prior to start of Off Timer  103 . 
           [0029]      FIG. 8  is a circuit diagram showing an exemplary implementation of the logic control of the third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0030]    Referring to  FIG. 2 , a first embodiment of an On-Off Timer according to the present invention is shown. 
         [0031]    As shown in  FIG. 2 , the On-Off Timer  100  outputs to a driver block  99 , which drives an exemplary output stage comprising an NMOS output power transistor  98  and sense resistor  97  coupled to the emitter terminal of the NMOS output power transistor  98 . On-Off Timer  100  may be implemented within the DC-DC control  2  of  FIG. 1A . 
         [0032]    The On-Off Timer  100  comprises an Off-Timer  101 , an On-Timer  102  and a flip-flop  103  which generates a first level signal and a second level signal from its output. The first level signal and the second level signal can be a LOW level signal and a HIGH level signal, or vice versa. As the name suggests, flip-flop  103  may be of any known types of flip-flops, such as the D flip-flop, SR flip-flip or the JK flip-flop. DC voltage levels VREG and Vin are inputted to provide bias voltage and input voltage respectively. VREG is a constant voltage that may be generated from within the IC chip or external source. 
         [0033]    The operation of the On-Off Timer  100  is described as follows. 
         [0034]    The Off-Timer  101  serves to keep the ‘Off’ duration constant, where the ‘Off’ duration is the duration when the pulse is at a logic LOW level (see  FIG. 1B ), as shown by (T−τ), where T is the period of one pulse cycle; τ is the ‘On’ duration, when the pulse is at a logic HIGH level. For this case, we are referring to the pulse output to the driver block  99 . Off-Timer  101  communicates with flip-flop  103  via control lines  103 A and  105 A. It is via these control lines that proper synchronization is accomplished between counting of the ‘Off’ duration and actual turning on or off of the driver block  99 . 
         [0035]    The On-Timer  102  is the essential block to prevent instability. On Timer  102  serves to limit the ‘On’ duration, that is, to not exceed a pre-determined time limit. This way, for cases of abnormality, where ‘On’ duration persists for a duration exceeding T, On-Timer  102  ensures that ‘On’ duration goes to logic LOW after the pre-determined time limit. 
         [0036]      FIG. 3A  shows an exemplary implementation of the Off-Timer  101  of the first embodiment of the present invention. 
         [0037]    To explain the operation of this exemplary implementation, we shall explain in two parts, namely the Off-Timer  101  and the On-Timer  102 . We will explain the Off-Timer  101  first. 
         [0038]    As highlighted in  FIG. 3A , the Off-Timer  101  comprises a comparator  105 , a charging capacitor C 1 , a switch SW 1 , a constant current source  106  and a voltage source for producing a reference voltage VTH 1 . 
         [0039]    Switch SW 1  may be in the form of any known solid-state switches. For the purpose of explanation, an exemplary PMOS switch is used as switch SW 1 , as shown in  FIG. 3C . 
         [0040]    The operation of the Off-Timer  101  is as follows: 
         [0041]    The timing duration for the Off-Timer  101  starts when output Q (node  103 A) of D flip-flop  103  goes to logic LOW. The timing duration for the Off-Timer  101  is realized by charging up capacitor C 1  using constant current Ichg supplied from constant current source  106 . Thus, an ascending DC voltage is produced across capacitor C 1 . The instantaneous voltage across capacitor C 1 , VC 1 , is constantly compared with a reference voltage VTH 1  using comparator  105 . Once VC 1  reaches VTH 1 , this marks the end of the timing duration for the Off-Timer  101 . The corresponding waveforms of output Q (node  103 A) of D flip-flop  103 , output  105 A of comparator  105  and instantaneous voltage across capacitor C 1 , VC 1 , illustrating the timing duration of the Off-Timer is as shown in  FIG. 3E , and denoted by “OFF Time”. 
         [0042]    From the relationship between the charging current to a capacitor and rate of change of voltage across the capacitor, we have: 
         [0000]        I=CdV/dT   (1)
 
         [0043]    Hence, the timing duration for the Off-Timer  101  may be derived as follows: 
         [0000]        T   OFF   =C   C1   ×VTH 1 /Ichg   (2)
 
         [0044]    (Where C C1 =Capacitance of Capacitor C 1 ) 
         [0045]    Once the end of the timing duration for the Off-Timer  101  is reached, a logic HIGH pulse signal at  105 A will be outputted by comparator  105  to clock input CK of D flip-flop  103 . This effectively latches the output Q of the D flip-flop  103  to logic HIGH, as its input D is connected to VREG. Output Q, when logic HIGH, pulls control line  103 A to logic HIGH. This in turn causes switch SW 1  to be open, thus effectively stopping the charging up of capacitor C 1 . 
         [0046]    Reset pin R of D flip-flop  103  is not used in the Off-Timer  101  operation, but will be used for the On-Timer  102  operation. 
         [0047]      FIG. 3B  shows an exemplary implementation of the On-Timer  102  of the first embodiment of the present invention. 
         [0048]    As highlighted in  FIG. 3B , the On-Timer  102  comprises a comparator  110 , a charging capacitor C 1 , a switch SW 2 , a constant current source  107  and a voltage source for produce a reference voltage VTH 2 . It is noted that the reference voltage VTH 1  is greater that the reference voltage VTH 2 . 
         [0049]    Switch SW 2  may be in the form of any known solid-state switches. For the purpose of explanation, an exemplary NMOS switch is used as switch SW 2 , as shown in  FIG. 3D . 
         [0050]    The operation of the On-Timer  102  is as follows: 
         [0051]    The timing duration for the On-Timer  102  starts when the output Q of D flip-flop  103  goes to logic HIGH. This corresponds to the end of the timing duration for the Off Timer. When node  103 A is logic HIGH, this will cause switch SW 2  to be closed. 
         [0052]    Similar to OFF timer, time limit is set by comparing VC 1  to reference voltage VTH 2 . However this time, capacitor is “discharged” from VTH 1  to VTH 2  under constant current (Idchg) from current source  107 . Thus, a descending DC voltage is produced across capacitor C 1 . When VC 1  has discharged to VTH 2 , comparator  110  produces a pulse which serves as a reset signal. The reset signal is applied to the reset terminal R of flip-flop  103  so that output Q goes to logic LOW. This means the end of ON time has been reached. From equation (1), ON time T ON  can be calculated as follows: 
         [0000]        T   ON   =C   C1 ×( VTH 1 −VTH 2)/ Ichg   (3)
 
         [0000]    (Where C C1 =capacitance of capacitor C 1 ) 
         [0053]    Now, the relationship between the timing duration for Off Timer, T OFF  and the timing duration for On-Timer  102  T ON  is as follows: 
         [0000]      Duty Cycle, D=T   ON ( T   ON   +T   OFF )  (4)
 
         [0054]    Hence, from equation (4), we can see that it is possible to limit the duty cycle D by limiting T ON . 
         [0055]    The corresponding waveforms of voltage at output Q (node  103 A) of D flip-flop  103 , output  105 A of comparator  105  and instantaneous voltage across capacitor C 1 , VC 1 , illustrating the timing duration of the On-Timer is as shown in  FIG. 3E , and denoted by “ON Time”. 
         [0056]    In practical applications, input voltage, Vin, is usually variable, depending on the battery used. It is known that in general, the duty cycle needed to produce a desired output voltage increases as input voltage decreases. This thus implies that to produce the same output voltage, the maximum duty cycle would need to increase as input voltage decreases. Hence, to counteract the effect of variable input voltage, the duty limit of the present invention needs to change with the applied input voltage Vin. An exemplary circuit to implement this counteracting effect is as shown in  FIG. 4 . 
         [0057]      FIG. 4  is an exemplary circuit to implement constant current source  107  which comprises resistor Rin, NPN transistor  1070  and NPN transistor  1071 , whose emitter area is ‘M’ times the emitter area size of NPN transistor  1070 . The value of ‘M’ is a constant value, determined based on user&#39;s preference. Typically, ‘M’ is designed in a way to optimize both the discharge current and in-chip capacitor value. Using Kirchoff&#39;s Voltage Law, we may derive the relationship between Idchg and Vin as follows: 
         [0000]        Idchg =(Vin− VBE )/( M*RIN )  (5)
 
         [0058]    where VBE=base-emitter voltage of NPN transistor  1071 . 
       Second Embodiment 
       [0059]    Referring to  FIG. 5A  and  FIG. 5B , a second embodiment of an On-Off Timer according to the present invention is shown. The Off-timer and On-timer for the second embodiment are as highlighted in  FIG. 5A  and  FIG. 5B  respectively. 
         [0060]    Here, it is designed to accept an ENABLE signal, for extra control. The ENABLE signal may be generated from a CPU or initiated by the user. An AND gate  120  is incorporated into the circuit, with its input being the ENABLE signal and the output of comparator  110 . With the ENABLE signal, the output Q of D flip-flop  103  can be made low independent of the voltage at VC 1 . 
         [0061]    During initial condition, VC 1 =0; the ENABLE signal is logic LOW, causing the D flip-flop  103  to reset, that is, output Q goes to logic LOW. This further results in NMOS output power transistor  98  being turned off. When ENABLE signal goes logic HIGH, output Q of D flip-flop  103  remains logic LOW, corresponding to the start of the timing duration of the Off-Timer  101 . As described earlier, current source  106  charges up capacitor C 1  until VC 1  reaches VTH 1 . When this occurs, output Q of D flip-flop  103  goes logic HIGH, corresponding to the start of the timing duration for On-Timer  102 . This will thus cause switch SW 1  to be opened and switch SW 2  to be closed, thus causing current source  107  to discharge capacitor C 1 . Once fully discharged, the On-Off cycle continues. 
         [0062]    The corresponding waveforms of the ENABLE signal, voltage at output Q (node  103 A) of D flip-flop  103 , output  105 A of comparator  105  and instantaneous voltage across capacitor C 1 , VC 1 , illustrating the operation of the second embodiment is as shown in  FIG. 5C . 
       Third Embodiment 
       [0063]    Referring to  FIG. 6A  and  FIG. 6B , a third embodiment of an On-Off-Timer  101  according to the present invention is shown. The Off-timer and On-timer for the third embodiment are as highlighted in  FIG. 6A  and  FIG. 6B  respectively. 
         [0064]    In the present embodiment, additional circuit elements have been added. These additional elements are: Logic Control  130 , switch  131 , comparator  132 , and NOT gate  133 . Switch  131  may be in the form of any known solid-state switches. For the purpose of explanation, an exemplary NMOS switch is used as switch  131 , as shown in  FIG. 3D . 
         [0065]    Comparator  132  may be external or internal within the circuit. For the purpose of explanation, an exemplary implementation of comparator  132  external to the On-Off Timer  100  is used. 
         [0066]    The purpose of the comparator  132  is to detect when the voltage V S  is equal to a reference DC voltage V S     —     REF , where this would imply that the output current has reached the maximum threshold output current allowable (represented by reference voltage V S     —     REF ), beyond which would cause damage to the NMOS output power transistor  98 . Voltage V s , which is the voltage across the sense resistor  97 , essentially monitors the current through the NMOS output power transistor  98 . 
         [0067]    When such a situation happens, comparator  132  would emit a logic level so as to cause the timing duration of the On-Timer  102  to end. For an exemplary implementation of this step, comparator  132  outputs a logic HIGH signal to the inverter gate  133  before inputting as a third input for AND gate  120 , which would then cause the D flip-flop  103  to be reset. This results in output Q of the D flip-flop  103  to go to logic LOW—signaling the start of the timing duration of the Off-Timer  101 . NMOS output power transistor  98  would also turn off as a result. 
         [0068]    However, when this sudden reset occurs, the capacitor C 1  may not have been fully discharged yet. To start the timing duration of the Off-Timer  101  with a partially charged capacitor C 1  would mean that the timing duration of the Off-Timer  101  would be inaccurate, as the instantaneous voltage across capacitor C 1 , VC 1 , will not always start at the same potential. 
         [0069]    To overcome this problem, Logic Control  130  will close switch  131 , so as to discharge capacitor C 1  fully when such a situation occurs. By doing this, the instantaneous voltage across capacitor C 1 , VC 1 , will always start at 0V. The duration for which switch  131  will be closed is just sufficient for the capacitor to be fully discharged. 
         [0070]    The corresponding waveforms of the voltage at output Q (node  103 A) of D flip-flop  103 , output  105 A of comparator  105 , node VC 1  (instantaneous voltage across capacitor C 1 ), Voltage V s  (the voltage across the sense resistor  97 ) and node  130 A (output terminal of Logic Control  130 ) illustrating the operation of the third embodiment is as shown in  FIG. 7 . 
         [0071]    An exemplary implementation of Logic Control  130  is as shown in  FIG. 8 . Logic Control  130  comprises inverters  134  and  135 , delay block  136 , an AND gate  137  and an OR gate  138 . The operation of the Logic Control  130  is as follows: 
         [0072]    Initially, the output Q of D flip-flop  103  is at logic HIGH. Hence, node  103 A is at logic HIGH. This thus follows that the output  134 A of inverter  134  is at a logic LOW. Inverter  135  will output a logic HIGH signal at node  135 A. After a pre-determined time delay due to Delay block  136 , the logic HIGH signal at mode  135 A will be transmitted to node  136 A. Based on the inputs at nodes  134 A and  136 A, AND gate  137  then outputs a logic LOW signal at node  137 A. This is followed by an OR gate outputting a logic LOW signal at node  130 A. 
         [0073]    At the instance when the output Q of D flip-flop drops to logic LOW at instance of node ( 1 ) in  FIG. 7 , consequently node  134 A goes to logic HIGH, and node  135 A goes to logic LOW. However, AND gate  137  sees a logic HIGH signal at node  136 A, due to the initial condition and also because the Delay block  136  causes a delay in transmission from node  135 A to node  136 A. As a result, AND gate  137  outputs a logic HIGH signal at node  137 A. This further result in the OR gate  138  outputting a logic HIGH signal to node  130 A. This thus results in the closing of switch  131 , thus discharging any remaining charges still stored in capacitor C 1 . This will result in the drop of the voltage VC 1  across capacitor C 1  to node ( 2 ) of  FIG. 7 . 
         [0074]    After completion of the delay caused by Delay block  136 , the logic LOW signal at node  135 A will be transmitted to node  136 A. AND gate  137  will thus output a logic LOW signal at node  137 A, followed by the OR gate  138  outputting a logic LOW signal at node  130 A. This will then result in the opening of switch  131 , marking the end of the discharge of capacitor C 1 . The predetermined delay time set for Delay block  136  is set to a time sufficient to discharge the said charges. 
         [0075]    Having described the above embodiment of the invention, various alternations, modifications or improvement could be made by those skilled in the art. Such alternations, modifications or improvement are intended to be within the spirit and scope of this invention. The above description is by ways of example only, and is not intended as limiting. The invention is only limited as defined in the following claims.