Abstract:
A failure detector circuit for detecting status of a protected circuit, the failure detector circuit having an operating cycle, has an enabling signal generator, a comparator circuit, a delay circuit. The enabling signal generator enables the comparator for an enable time in each operating cycle. The comparator circuit compares an output of the protected circuit with a reference signal. The delay circuit receives an output signal of the comparator to decide whether a failure occurred within a give delay time.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of CN application No. 201110403342.4, filed on Dec. 7, 2011, and incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention generally relates to power supply, and particularly but not exclusively relates to a failure detector circuit and associated method. 
       BACKGROUND 
       [0003]    In applications of switch-mode power supply and battery power supply circuit, circuit protection, for example, over-voltage protection (OVP), over-current protection (OCP) or over-temperature protection (OTP) is important. In prior art, a control chip is applied to monitor the voltage, current, temperature or other parameters of the protected circuit and thereby protects it. Since the control chip needs to consume quiescent current, it directly affects the operating efficiency of the protected circuit. 
         [0004]    In prior art, the operating efficiency of circuit is generally improved through decreasing the quiescent current. However, the degree of decreasing quiescent current is limited. Therefore, the improvement of the operating efficiency is constrained. 
       SUMMARY 
       [0005]    One embodiment of the present invention discloses a failure detector circuit for detecting status of a protected circuit, the failure detector circuit comprising: an enabling signal generator, generating a periodic enabling signal which has an operating cycle, configured to indicate an enable time in every operating cycle; a comparator circuit, having an enabling terminal, two input terminals and a output terminal, wherein a enabling terminal receives the enabling signal, and wherein the two input terminals respectively receive an output signal from a protected circuit and a reference signal, and wherein the output terminal generates a comparative result signal according to the enabling signal, the output signal and the reference signal; and a delay circuit, receiving and detecting the comparative result signal for a given delay period, generating a delay signal according to the comparative result signal, wherein the given delay period is larger than the operating cycle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Non-limiting and non-exhaustive embodiments are described with reference to the following drawings. The drawings are not depicted to scale and only for illustration purpose. 
           [0007]      FIG. 1  illustrates a failure detector circuit  10  according to an embodiment of the present invention. 
           [0008]      FIG. 2  illustrates a schematic wave-form diagram of the failure detector circuit  10  according to the embodiment shown in  FIG. 1  when the protected circuit is in normal status. 
           [0009]      FIG. 3  illustrates a schematic wave-form diagram of the failure detector circuit  10  according to the embodiment shown in  FIG. 1  when the protected circuit is in failure. 
           [0010]      FIG. 4  schematically illustrates a circuitry of a delay circuit  40  according to an embodiment of the present invention. 
           [0011]      FIG. 5  schematically illustrates a circuitry of a delay circuit  50  according to another embodiment of the present invention. 
           [0012]      FIG. 6  schematically illustrates a method for detecting failure in circuit according to an embodiment of the present invention. 
       
    
    
       [0013]    The use of the same reference label in different drawings indicates the same or like components. 
       DETAILED DESCRIPTION 
       [0014]    Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one of ordinary skill in the art that the present invention 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 invention. 
         [0015]      FIG. 1  illustrates a failure detector circuit  10  according to an embodiment of the present invention. As shown in  FIG. 1 , the failure detector  10  comprises an enabling signal generator  101 , a comparator  102  and a delay circuit  103 . 
         [0016]    Enabling signal generator  101  generates a periodical enabling signal EN, In one embodiment, the enabling signal EN is a square-wave signal which has an operating cycle T, and the enable time in each cycle is Te, Te&lt;T. Comparator  102  receives an output signal S from a protected circuit (not shown in  FIG. 1 ), and a reference signal R. An enabling terminal of the comparator  102  receives the enabling signal EN. Comparator  102  generates a comparative result signal COUT according to the output signal S, the reference signal R and the enabling signal EN. Delay circuit  103  receives the comparative signal COUT and monitors the states of the comparative result signal COUT in a given delay period Td, so that provides a delay signal DOUT to a failure protection circuit (not shown in  FIG. 1 ). The given delay period Td may be set according to the requirement of user, and is longer than the operating cycle T of the enabling signal EN (Td&gt;T). 
         [0017]    For further illustrating the failure detector circuit  10  shown in  FIG. 1  in detail,  FIG. 2  and  FIG. 3  illustrate schematic wave-form diagrams of the failure detector circuit  10  according to the embodiment shown in  FIG. 1 . As shown in  FIG. 2  and  FIG. 3 , from top to bottom, the schematic waveform curves in turn represent the enabling signal EN, the comparative result signal GOUT and the delay signal DOUT. The enabling signal EN is a square wave signal with the operating cycle T, and the enabled time in each operating cycle is Te. In the embodiment in  FIG. 2  and  FIG. 3 , the square wave signal is active high, which means during the enable time Te, EN=1. Otherwise the square wave signal is at low level, i.e. EN=0. 
         [0018]    In the embodiments shown in  FIG. 2  and  FIG. 3 , when the enabling signal EN is at low level, comparator  102  is out of operation, and the comparative result signal GOUT is originally set to high level. When the enabling signal EN turns to high, comparator  102  is in operation to compare the output signal S from the protected circuit with the reference signal R. In an exemplar, the comparative result signal GOUT is at low level when the protected circuit is operating normally, while the comparative result signal GOUT is at high level when the protected circuit is suffering a failure. Then delay circuit  103  detects the value of comparative result signal GOUT during the given delay period Td. If the comparative result signal GOUT keeps in high level during the given delay period Td, it indicates that the protected circuit is in failure, and the delay signal DOUT provided by delay circuit  103  is at high level to initiate the failure protection circuit. Once the comparative result signal GOUT emerges at low level during the given delay period Td, the failure detector circuit  10  decides that the protected circuit is operating normally. Delay circuit  103  generates a low level delay signal DOUT to disable the failure protection circuit. 
         [0019]      FIG. 2  illustrates a schematic wave-form diagram of the failure detector circuit  10  according to the embodiment shown in  FIG. 1  when the protected circuit is operating in normal status. At normal, when EN=1, comparator circuit  102  operates to compare the output signal S of the protected circuit with the reference signal S, configured to obtain a low level comparative result signal COUT, COUT=0. While EN=0, comparator circuit  102  is disabled, and the comparative result signal COUT is set to high level, COUT=1. During the given delay period Td, once the comparative result signal COUT emerges at low level when EN=1, the failure detector circuit  10  considers that the protected circuit is operating normally, DOUT=1, and the failure protection circuit is disabled. 
         [0020]      FIG. 3  illustrates a schematic wave-form diagram of the failure detector circuit  10  according to the embodiment shown in  FIG. 1  when the protected circuit is in failure. In failure status, when EN=1, comparator circuit  102  operates to compare the output signal S of the protected circuit with the reference signal S, configured to obtain a high level comparative result signal COUT, COUT=1. While EN=0, the comparative result signal COUT is set to high level, COUT=1. Therefore in failure status, the comparator result signal COUT always maintains at high level during the given delay period Td. The delay circuit  103  generates a high level delay signal, DOUT=1, and the failure detector circuit  10  initiates the failure protection circuit. 
         [0021]    The failure detector circuit in prior art generally keeps on during a whole operating cycle T, and consumes quiescent current ISS. For the embodiment shown in  FIG. 1 , the failure detector circuit  10  receives the enabling signal EN, and is on during the enabled time Te of the operating cycle T. Thus, the consumed quiescent current I′ss is 
         [0000]    
       
         
           
             
               
                 I 
                 ′ 
               
                
               ss 
             
             = 
             
               
                 Te 
                 T 
               
                
               Iss 
             
           
         
       
     
         [0022]    For an instance, if the operating cycle T=1 ms, and the enable time Te=10 us (the enable time is 1% of operating cycle), the consumed quiescent current of the embodiments in present invention may be just 1% of the consumed quiescent current of prior art. And the bias current of the failure detector circuit  10  still maintains to ISS to guarantee the normal operation of the failure detector circuit  10 . Accordingly, the failure detector circuit according to the embodiments of the present invention may consume much less quiescent current so that improves the circuit efficiency. 
         [0023]    One with ordinary skill in the art should understand the failure detector circuit according to the embodiments may be applied to detect the failure of over-voltage, over-current, over-temperature or other failure status on the protected circuit. Correspondingly, the output signal S of the protected circuit and the reference signal R, which are received by comparator circuit  102 , are signals that monitor voltage, current, temperature or other parameters in the protected circuit. 
         [0024]      FIG. 4  schematically illustrates a circuitry of a delay circuit  40  according to an embodiment of the present invention. As shown in  FIG. 4 , the delay circuit  40  comprises an N-bit counter (N is a positive integer), wherein a clock terminal CLK of the counter receives a clock signal with a cycle Tc. The value of N satisfies Td=2N×Tc configured to detect the status of comparative result signal COUT during the given delay period Td. 
         [0025]    An enabling terminal D of the counter receives the comparative result signal COUT. A set terminal R 1  of the counter is coupled to an output terminal of an inverter INV. An input terminal of the inverter INV receives the comparative result signal COUT. An output terminal OUT of the counter generates an output signal Qn. Delay circuit further comprises a latch, wherein a first set terminal S of the latch is coupled the output terminal of the counter to receive the output signal Qn, and wherein a second set terminal R 2  is also coupled to the output terminal of the inverter INV. The latch generates the delay signal DOUT. 
         [0026]    When the protected circuit is in normal, the comparative result signal COUT is at low level, COUT=0 when EN=1. Hence the counter is disabled, and the signal COUT resets the latch through the inverter INV. The latch generates a low level delay signal, DOUT=0 and the failure protection circuit is disabled. When the protected circuit is in failure, the comparative result signal COUT is at high level, COUT=1. Hence the comparative result signal enables the counter to start counting. The counter generates a pulse signal that has a cycle N×Tc, configured to set the latch. The latch generates a high level delay signal DOUT, DOUT=1 thus initiating the failure protection circuit. 
         [0027]      FIG. 5  schematically illustrates a circuitry of a delay circuit  50  according to another embodiment of the present invention. Seen in  FIG. 5 , delay circuit  50  comprises a current source Is, a first switch S 2 , a second switch S 2 , a capacitor C, an inverter INV 1  and a comparator CMP. 
         [0028]    A first terminal of the first switch  51  is coupled to an output terminal of the current source Is, and a control terminal of the first switch  51  receives the comparative result signal COUT. A first terminal of the capacitor C is coupled to the second terminal of the first switch  51 , and a second terminal of the capacitor C is connected to a reference ground. A first terminal of the second switch S 2  is coupled to the common terminal for the first switch and the capacitor C, and a second terminal of the second switch S 2  is coupled to the reference ground. The comparative result signal COUT is provided to a control terminal of the second switch through the inverter INV 1 . A non-inverting terminal of the comparator CMP is coupled to the common terminal for the first switch  51  and the capacitor C, while an inverting terminal of the comparator CMP receives the reference voltage VREF. Wherein, the reference voltage VREF is a positive voltage, and the comparator CMP generates the delay signal DOUT. 
         [0029]    By properly setting the capacitance of C 1  and the output current of the current source Is, the following equation could be satisfied: 
         [0000]    
       
         
           
             
               T 
               d 
             
             = 
             
               
                 
                   C 
                   C 
                 
                 · 
                 
                   V 
                   REF 
                 
               
               
                 I 
                 IS 
               
             
           
         
       
     
         [0030]    Wherein: Td is the given delay period; I IS  is the output current of the current source Is; C C  is the capacitance of the capacitor C. Once the protected circuit is in normal, COUT=1, the switch  51  is off and the switch S 2  is on. The capacitor C is discharged. The comparator generates a low level delay signal DOUT. While the protected circuit is in failure, COUT=1, the switch  51  is on and the switch S 2  is off. The capacitor C 1  is charged by the current source Is. At the end of the given delay period Td, the voltage across the capacitor C equals to the reference voltage VREF. Thus the comparator generates a high level delay signal DOUT configured to initiate the failure protection circuit. 
         [0031]    In the above embodiment, when EN is set disabled, the comparative result signal COUT is originally set to high level. However, one with ordinary skill in relevant art should understand that in other embodiments, when EN is set disabled, the comparative result signal COUT may originally be set to low level. 
         [0032]    The failure detector circuit according to the embodiments of the present invention may be utilized in the application of rechargeable battery, switch mode power supply and etc, and may be integrated into protection circuit or independent from protection circuit. 
         [0033]      FIG. 6  schematically illustrates a method for detecting failure in circuit according to an embodiment of the present invention. As shown in  FIG. 6 , a step  61  comprises monitoring a voltage level of a comparative result signal COUT 1 . If COUT 1  is at high level, it is latched. After an operating cycle T (a first cycle) passes, a step  62  is entered. Step  52  comprises detecting an output signal S of a protected circuit, and then comparing the output signal S and a reference signal R configured to obtain a comparative result signal COUT 2  during a enable time Te of a second cycle. If the comparative result signal COUT 2  is still at high level, then the comparative signal COUT 2  is latched. When the second cycle ends, the step  62  is repeated. After an nth cycle ends, a step  65  which is the same as step  62  is completed. If during these n cycles, that is, during the given delay period Td=n×T, the detected comparative result signals COUT 1 , COUT 2 , . . . , COUTn are all high level signals, a step  66  which comprises initiating a failure protection circuit is entered. Wherein, n is an integer larger than 1. While if at least one of comparative result signals COUT, COUT 2 , . . . , COUTn is low level signal, corresponding step  63 ,  64  or  67  would be entered to reset the given delay period Td and recheck the status of the protected circuit. 
         [0034]    The above description and discussion about specific embodiments of the present invention is for purposes of illustration. However, one with ordinary skill in the relevant art should know that the invention is not limited by the specific examples disclosed herein. Variations and modifications can be made on the apparatus, methods and technical design described above. Accordingly, the invention should be viewed as limited solely by the scope and spirit of the appended claims.