Abstract:
In a noise sensitivity improved switching system and method thereof, comprised sensing the output voltage of the switching system to generate a feedback signal, respectively amplifying the feedback signal by two gains to generate two signals in phase or out of phase, filtering one of the two amplified signals, and summing or comparing the filtered signal and the other one, thereby reducing the noise interference to the switching system.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to a switching system and more particularly, to an apparatus and method for noise sensitivity improvement to a switching system.  
       BACKGROUND OF THE INVENTION  
       [0002]     The conventional fixed-frequency pulse width modulation (PWM) controller has poor transient response. Recently, people devote to develop variable-frequency PWM controller system to overcome this issue. For the ultra-fast transient response characteristic, however, the output capacitor of a variable-frequency PWM controller system is smaller than that of a fixed-frequency PWM controller system, and another problem, noise sensitivity, is introduced eventually. The popular variable-frequency PWM controller senses the ripple of the output voltage of the switching system to be a ramp to compare with a reference so as for its system control. For example, as shown in  FIG. 1 , a switching system  10  comprises an error amplifier  102 , a constant on-time circuit  104 , two drivers  106  and  108 , an output stage  110 , and a voltage sense circuit  112 , among which the output stage  110  includes a high-side switch SW 1  connected between a supply voltage VDD and a phase node  114 , a low-side switch SW 2  connected between the phase node  114  and ground GND, an inductor L connected between the phase node  114  and the output Vout, and an output capacitor C having an equivalent series resistance (ESR) R ESR  connected between the output Vout and ground GND. The voltage sense circuit  112  senses the output voltage Vout to generate a feedback signal FB, the error amplifier  102  receives the feedback signal FB to compare with a reference V REF  to generate a signal PM, and the constant on-time circuit  104  controlled by the signal PM generates an on-time signal T on  for the drivers  106  and  108  to drive the switches SW 1  and SW 2 , such that an inductor current I L  is generated to charge the capacitor C through the inductor L to provide the output voltage Vout to a load R L . As the feedback signal FB is lower than the reference V REF , it will trigger a constant on time to the driving signal UG for the high-side switch SW 1  by the constant on-time circuit  104 . Following the falling edge of the on time duty, a minimum off time is triggered to keep the system stable. Responsive to a load transient, the output voltage Vout will drop a value equal to the product of R ESR  and I L  instantly, and this voltage drop will apply to the error amplifier  102 , such that the driving signal UG turns on in the shortest response time. The topology has fast transient response, but induces noise sensitivity problem.  
         [0003]      FIG. 2  shows a timing diagram of various signals in the circuit  10  of  FIG. 1  during a load transient, in which waveform  202  represents the inductor current I L , waveform  204  represents the feedback signal FB, waveform  206  represents the reference V REF , waveform  208  represents the signal PM, waveform  210  represents the on-time signal T on , waveform  212  represents the minimum off-time signal T offmin , and waveform  214  represents the driving signal UG generated by the driver  106 . At time t, a load transient is occurred, and thereby the inductor current I L  begins to increase. As a result, the output voltage Vout drops rapidly, and eventually, the feedback signal FB also drops rapidly. By the error amplifier  102 , the duty of the signal PM is enlarged, and by which the constant on-time circuit  104  increases the frequency of the on-time signal T on , the frequency of the driving signal UG is increased accordingly, such that the high-side switch SW 1  is switched more frequently to recover the output voltage Vout to its original level as soon as possible. However, in reality, the feedback signal FB involves high-frequency noise, as shown in  FIG. 3  by waveform  204  indicated by noisy portions  218  and  220  for example. Moreover, to achieve fast transient response, the variable-frequency PWM system  10  uses the output capacitor C much smaller than that in a fixed-frequency PWM system, and thus the system becomes more sensitive to noise. When the noise near the valley of the waveform  204  is so large to pull the feedback signal FB lower than the reference V REF , error operation may occurred. For example, as shown in the portion  220 , the high-frequency noise near a valley of the waveform  204  is large enough for the feedback signal FB lower than the reference V REF  incorrectly, resulting in error operation or malfunction of the system  10 , as shown in the driving signal UG corresponding to the portion  220 .  
         [0004]     Therefore, it is desired an apparatus and method for noise sensitivity improvement and fast transient response for a switching system.  
       SUMMARY OF THE INVENTION  
       [0005]     One object of the present invention is to provide an apparatus and method for noise sensitivity improvement to a switching system.  
         [0006]     Another object of the present invention is to provide an ultra-fast response and low noise-sensitive switching system.  
         [0007]     A noise sensitivity improved switching system according to the present invention comprises generating a feedback signal by sensing the output voltage of the switching system, respectively amplifying the feedback signal by two gains to generate two signals in phase or out of phase, filtering one of the two amplified signals by a filter, and combining the filtered signal and the other one to reduce the noise interference to the switching system.  
         [0008]     From one aspect of the present invention, the switching system comprises a voltage sense circuit to sense the output voltage of the switching system to generate the feedback signal, an apparatus for noise sensitivity improvement to the switching system to receive the feedback signal and to generate a combined signal, a comparator to compare the combined signal with a reference to generate a comparator signal, and an output stage to generate the output voltage according to the comparator signal, among which the apparatus for noise sensitivity improvement to the switching system includes an amplifier having a first gain to amplify the feedback signal to generate a first signal, another amplifier having a second gain to amplify the feedback signal to generate a second signal in phase to the first signal, a filter to filter the second signal, and a summing circuit to combine the first signal and the filtered second signal to generate the combined signal to the comparator. As a result, the effective noise in the combined signal is level shifted to higher, thereby reducing its interference to the switching system.  
         [0009]     From another aspect of the present invention, the switching system comprises a voltage sense circuit to sense the output voltage of the switching system to generate the feedback signal, an apparatus for noise sensitivity improvement to the switching system to receive the feedback signal and to generate two signals that are compared by a comparator to generate a comparator signal, and an output stage to generate the output voltage according to the comparator signal, among which the apparatus for noise sensitivity improvement to the switching system includes an amplifier circuit to respectively amplify the differential of the feedback signal and a reference by a first gain and by a second gain to generate two amplified signals out of phase to each other to the comparator, a filter to filter one of the two amplified signals, and a voltage generator to provide a reference for level shifting of the two amplified signals.  
         [0010]     The apparatus and method according to the present invention is applicable to variable-frequency switching system for both fast transient response and noise sensitivity improvement. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0011]     These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which:  
         [0012]      FIG. 1  shows a conventional variable-frequency PWM controller system;  
         [0013]      FIG. 2  shows a timing diagram of various signals in the circuit of  FIG. 1  during a load transient;  
         [0014]      FIG. 3  shows a perspective diagram of noise interference to the switching system of  FIG. 1 ;  
         [0015]      FIG. 4  shows the first embodiment according to the present invention;  
         [0016]      FIG. 5  shows a timing diagram of various signals in the circuit of  FIG. 4 ;  
         [0017]      FIG. 6  shows a current sense circuit inserted additionally to the first embodiment according to the present invention;  
         [0018]      FIG. 7  shows the second embodiment according to the present invention;  
         [0019]      FIG. 8  shows a timing diagram of various signals in the circuit of  FIG. 7 ;  
         [0020]      FIG. 9  shows a current sense circuit inserted additionally to the second embodiment according to the present invention;  
         [0021]      FIG. 10  shows the third embodiment according to the present invention;  
         [0022]      FIG. 11  shows an embodiment for the two-output transconductive amplifier in  FIG. 10 ;  
         [0023]      FIG. 12  shows a scheme to sense the inductor current;  
         [0024]      FIG. 13A  shows another scheme to sense the inductor current;  
         [0025]      FIG. 13B  shows still another scheme to sense the inductor current;  
         [0026]      FIG. 14A  shows yet another scheme to sense the inductor current; and  
         [0027]      FIG. 14B  shows still yet another scheme to sense the inductor current. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]      FIG. 4  shows the first embodiment according to the present invention, in which a switching system  30  comprises an apparatus  302  for noise sensitivity improvement to the switching system  30 , in addition to a comparator  304 , a constant on-time circuit  306 , two drivers  308  and  310 , an output stage  312 , and a voltage sense circuit  314 . The output stage  312  includes a high-side switch SW 1  connected between a supply voltage VDD and a phase node  313 , a low-side switch SW 2  connected between the phase node  313  and ground GND, an inductor L connected between the phase node  313  and an output Vout, and a capacitor C having an equivalent series resistance R ESR  connected between the output Vout and ground GND. The voltage sense circuit  314  has two resistors R 4  and R 5  connected in series between the output Vout and ground GND, to sense the output voltage Vout of the switching system  30  to generate a feedback signal FB. The apparatus  302  generates a combined signal FBX according to the feedback signal FB. The comparator  304  compares the combined signal FBX with a reference VREF to generate a signal PM for the constant on-time circuit  306  to generate an on-time signal T on , to control the drivers  308  and  310  to generate two driving signals UG and LG to drive the switches SW 1  and SW 2  to modulate the output voltage Vout.  
         [0029]     In this embodiment, the apparatus  302  includes two amplifiers  3022  and  3024 , a low-pass filter (LPF)  3026 , and a summing circuit  3028 . The amplifier  3022  amplifies the feedback signal FB by a gain K to generate a signal FBF, while the amplifier  3024  amplifies the feedback signal FB by a gain N to generate a signal filtered by the low-pass filter  3026  to generate a signal FBS. Since the signal FBF is obtained directly by amplifying the feedback signal FB, it may be still noisy. In contrast, the low-pass filter  3026  filters out the high-frequency noise from the amplified feedback signal, and the signal FBS is thus clear. The summing circuit  3028  combines the signals FBF and FBS to generate the combined signal FBX to the comparator  304 .  
         [0030]      FIG. 5  shows a timing diagram of various signals in the circuit  30  of  FIG. 4 , in which waveform  402  represents the feedback signal FB, waveform  404  represents the amplified signal FBF, waveform  406  represents the filtered amplified signal FBS, waveform  408  represents the combined signal FBX, waveform  410  represents the reference V REF , and waveform  412  represents the high-side driving signal UG.  
         [0031]     Referring to  FIG. 4  and  FIG. 5 , in the apparatus  302 , the amplifier  3022  amplifies the feedback signal FB together with the noise thereon by K times to generate the signal FBF, as shown by the waveform  404 . On the other hand, the amplifier  3024  amplifies the feedback signal FB together with the noise thereon by N times, and the filter  3026  filters the amplified signal to remove the high-frequency noise thereon to generate the signal FBS, as shown by the waveform  406 . The summing circuit  3028  combines the noisy signal FBF and the clear signal FBS to generate the combined signal FBX for the comparator  304  to compare with the reference V REF , as shown by the waveforms  408  and  410 . Since the signal FBS is filtered by the low-pass filter  3026 , when the two signals FBF and FBS are combined together, the noise on the combined signal FBX will be that on the signal FBF but level shifted to higher only, without amplitude amplification. Therefore, the noise near the valley of the combined signal FBX is shifted away from the reference V REF , as shown by portion  4082 , and error operation is therefore avoided. As a result, the noise sensitivity of the switching system  30  is improved.  
         [0032]     In the first embodiment, the signal FBS is a low frequency signal by filtered by the low pass filter  3026 , and the other signal FBF is still a high frequency signal since it is amplified by the amplifier  3022  only. Therefore, the gain K of the amplifier  3022  is preferably larger than the gain N of the amplifier  3024 , to maintain the switching system  30  having fast transient response.  
         [0033]     Using transconductance to implement error amplifier has another advantage of droop import. A droop import circuit  316  may be inserted additionally to the circuit  30  of  FIG. 4 , and  FIG. 6  shows such an embodiment, in which the droop import circuit  316  includes a transconductive amplifier  3162  shunt to a sense resistor R S  connected between the inductor L and the output Vout to sense the inductor current I L  by the voltage drop across the sense resistor R S  to generate a current sense signal, and a sample and hold (S/H) circuit  3164  to sample and hold the current sense signal to generate a droop signal Droop to combine with the signals FBF and FBS by the summing circuit  3028 . The S/H circuit  3164  filters the output ripple, and thus the output DC current level is injected to the inverting input of the comparator  304 . The injected level will degrade the feedback signal FB and droop will be generated. In other words, the droop signal Droop is set to adjust the load regulation droop voltage. When the load R L  changes, the droop signal Droop will adjust the level of the combined signal FBX to prevent the output voltage Vout from over-ripple that may cause the load R L  malfunction or damaged.  
         [0034]      FIG. 7  shows the second embodiment according to the present invention, in which, similar to the switching system  30  of  FIG. 4 , a switching system  50  comprises a constant on-time circuit  306 , two drivers  308  and  310 , an output stage  312 , and a voltage sense circuit  314 . However, the apparatus  502  for noise sensitivity improvement hereof comprises an amplifier circuit  504 , a voltage generator  506 , an RC filter  508 , a comparator  510  and a resistor R 1 . The RC filter  508  includes a resistor R 2  and a capacitor C 1  connected in parallel, and the amplifier circuit  504  includes two substantially equivalent transconductive amplifiers  5042  and  5044 . The transconductive amplifiers  5042  and  5044  transfer the differential of the feedback signal FB and the reference V REF1  to output currents PM 1  and PM 2  and both the output currents PM 1  and PM 2  are inverted to each other due to their inverted inputs. The voltage generator  506  includes a voltage follower to provide a reference V REF2 ′ from a reference V REF2  connected through the resistors R 1  and R 2  to the outputs PM 1  and PM 2  of the amplifier circuit  504  that are connected to the inputs of the comparator  510  to generate the signal PM for the constant on-time circuit  306 . R 1  and R 2  determine the transconductances of the transconductive amplifiers  5042  and  5044 , and R 2  and C 1  determine the pole of the signal PM 2 . The pole may be set between the PWM controller operation frequency (about 200-600 kHz) and the noise frequency (about 10 MHz) to filter out the noise on the feedback signal FB. As a result, the signal PM 2  will be clear by filtered by the RC filter  508 , and in the signal PM  1  the feedback signal FB is directly amplified together with the noise thereon. Preferably, R 1 ≧R 2 , i.e., PM 1 /FB≧PM 2 /FB, to ensure fast transient response. As a load transient occurs, the signal PM  1  will response fast, while the signal PM 2  could not follow such fast transient due to the bandwidth limitation. FB≧R 2 , the signal PM 1  will be lower than the signal PM 2  in the load transient condition and the high-side driving signal UG will be turned on quickly. Therefore, the apparatus  502  will combine fast response and excellent noise immunity characteristics for the switching system  50 .  
         [0035]      FIG. 8  shows a timing diagram of various signals in the circuit of  FIG. 7 , in which waveform  600  represents the load current I LOAD , waveform  602  represents the signal PM 1 , waveform  604  represents the signal PM 2 , line  606  indicates the level of the reference V REF2 ′, waveform  608  represents the inductor current I L , and waveform  610  represents the driving signal UG of the high-side switch SW 1 .  
         [0036]     Referring to  FIG. 7  and  FIG. 8 , when the signal PM 1  is lower than the signal PM 2 , the output signal PM of the comparator  510  will trigger the constant on-time circuit  306  to generate the on-time signal T on . Although the noise  6022  near the valley of the signal PM 1  is lower than the reference V REF2 ′, it will not interfere the operations of the switching system  50  since the signal PM 1  is still higher than the signal PM 2 . At time t, a load transient is occurred in the switching system  50 , and at this time, the signal PM 1  drops rapidly in fast response to the dropped output voltage Vout and the signal PM 2  begins to raise, such that the frequency of the driving signal UG for the high-side switch SW 1  increases.  
         [0037]     Similarly, a droop import circuit  316  may be inserted additionally to the circuit  50  of  FIG. 7 , and  FIG. 9  shows such an embodiment, in which the droop import circuit  316  includes a transconductive amplifier  3162  shunt to a sense resistor R S  connected between the inductor L and the output Vout to sense the inductor current I L  by the voltage drop across the sense resistor R s  to generate a current sense signal, and a sample and hold (S/H) circuit  3164  to sample and hold the current sense signal to generate a droop signal Droop to combine with the signal PM 1  by a summing circuit  512 . The S/H circuit  3164  filters the output ripple, and thus the output DC current level is injected to the inverting input of the comparator  510 . The droop signal Droop is set to adjust the load regulation droop voltage. When the load R L  changes, the droop signal Droop will adjust the level of the signal PM  1  to prevent the output voltage Vout from over-ripple that may cause the load R L  malfunction or damaged. In other embodiments, the droop signal Droop may be combined with the signal PM 2 , instead of with the signal PM 1 .  
         [0038]     Applying current mirror skill could combine two transconductive amplifiers to a single transconductive amplifier with two outputs inverted to each other. Particularly, the circuits shown in  FIG. 7  and  FIG. 9  may be modified by replacing the amplifier circuit  504  with a two-output transconductive amplifier  5046 , as shown in  FIG.10 , which has two inputs connected-with the feedback signal FB and the reference V REF1  and generates the signals PM 1  and PM 2  accordingly at its two outputs.  FIG. 11  shows the output stage perspective for the two-output transconductive amplifier  5046 , which has two current sources IS 1  and IS 2  connected between the supply voltage VDD and ground GND, with the node therebetween to derive the signal PM 1 , and another two current sources IS 3  and IS 4  connected between the supply voltage VDD and ground GND, with the node therebetween to derive the signal PM 2 . The current sources IS 1  and IS 4  are set to generate substantially equivalent current I 1 , and the other two current sources IS 2  and IS 3  are set to generate substantially equivalent current I 2 , thereby the output signals PM 1  and PM 2  inverted to each other. In other words, it could use only one transconductive amplifier to amplify the ripple of the feedback signal FB and filter the noise thereon at the same time. By use of a single transconductive amplifier  5046  as shown in  FIG. 10 , instead of two transconductive amplifiers  5042  and  5044  as shown in  FIG. 9 , to amplify the differential of the feedback signal FB and the reference V REF1 , the problem resulted from offset between the two transconductive amplifiers  5042  and  5044  is avoided.  
         [0039]      FIG. 12  to  FIG. 14  show various schemes to sense the inductor current I L . In  FIG. 12 , a capacitor C 2  is shunt to the inductor L as a sense device. In  FIG. 13A , the sense resistor R S  is connected between the low-side switch SW 2  and ground GND. In  FIG. 13B , the equivalent resistor of the low-side switch SW 2  is serving as the sense resistor, and thus the voltage drop across the low-side switch SW 2  is directly sensed. In  FIG. 14A , the sense resistor R s  is connected between the supply voltage VDD and the high-side switch SW 1 . In  FIG. 14B , the equivalent resistor of the high-side switch SW 1  is serving as the sense resistor, and thus the voltage drop across the high-side switch SW 1  is directly sensed.  
         [0040]     While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.