Abstract:
A control circuit and method for a PWM voltage regulator combine a high frequency feedback technique with a constant on-time or constant off-time topology to improve the transient performance of the PWM voltage regulator. The PWM voltage regulator generates a constant on-time or constant off-time depending on a current for generating a PWM signal, and dynamically adjusts the current according to the droop-voltage at its output during a transient period. Therefore, the PWM voltage regulator boosts its transient response without any threshold for load step detection.

Description:
FIELD OF THE INVENTION 
     The present invention is related generally to a pulse width modulation (PWM) voltage regulator and, more particularly, to a control circuit and method for a PWM voltage regulator. 
     BACKGROUND OF THE INVENTION 
     Recently, central processing units (CPUs) have to bear highly dynamic load currents that usually change very quickly from a light load to a maximal-load. The CPU current transient may happen within 1 μs, smaller than a switching cycle of a typical PWM voltage regulator, whether it is controlled in a voltage mode or a current mode. For solving this problem, a PWM voltage regulator that serves to provide a voltage to a CPU is usually set with a threshold of output voltage variation so that when the variation of its output reaches the threshold, another non-closed loop adjusting mechanism can be triggered. For example, another PWM on-time can be triggered or its off-time can be immediately stopped or the duty of a PWM signal can be increased. However, such an approach has two major problems. First, the threshold of voltage variation is discrete, so transient response can only be improved when the droop-voltage at the output exceeds the threshold. Second, the threshold of voltage variation is fixed, and thus it can&#39;t meet a variety of applications. Additionally, in the event that the threshold setting relies on external components, additional pins will be required, which increases manufacturing costs and reduce the flexibility of circuit design. 
     Currently, voltage regulators for CPUs, for example U.S. Pat. No. 7,436,158, mostly use native adaptive voltage positioning (N-AVP) control. Conventional PWM structures usually use a ramp signal as the reference to be compared with the output voltage or the inductor current for generating PWM signals to control switching of voltage regulators. During transient where the load changes from a light to a heavy, the output voltage of a PWM voltage regulator drops suddenly, and this may lead to shutdown of the CPU. For improving control loop transient, there have been proposed many solutions. For example, U.S. Pat. Application publication No. 20070013356 uses a voltage-mode control loop to achieve quick transient response, while it suffers a timing issue caused by a synchronous clock and is unable to act instantly when transient occurs, U.S. Pat. Application Publication No. 20070109825 changes timing sources by detecting a load current. Although this art is helpful to solve the foregoing problem about clock timing, it is also unable to act instantly when transient occurs. U.S. Pat. No. 7,615,982 inserts a non-closed loop PWM pulse when the load current exceeds a preset threshold to improve transient response. While this art realizes instant transient response, its non-linear control can undesirably make the control loop of the voltage regulator unstable. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a control circuit and method for improving transient performance of a PWM voltage regulator. 
     Another objective of the present invention is to provide a control circuit and method for a PWM voltage regulator, which serve to dynamically adjust a PWM signal according to a voltage variation at the output of the PWM voltage regulator during transient. 
     A further objective of the present invention is to provide a control circuit and method for a PWM voltage regulator, which serve to determine whether transient occurs according to an output voltage ripple frequency of the PWM voltage regulator. 
     According to the present invention, a control circuit and method for a PWM voltage regulator combines high-frequency feedback technology with a structure of constant on-time or constant off-time to improve transient performance of the PWM voltage regulator. 
     According to the present invention, a PWM voltage regulator generates constant on-time or constant off-time according to a current to generate a PWM signal, and dynamically adjusts the current according to its output voltage variation during transient. 
     According to the present invention, a PWM voltage regulator detects its output voltage ripple frequency and uses a high-frequency feedback loop to adjust the current. 
     A control circuit and method according to the present invention achieve at least the following effects. First, there is no need of a threshold for load step detection because its triggering is only related to transient speed, and its response is directly proportional to transient speed and steps. Second, transient response is improved without using additional pins. Third, circuit board design is provided with high flexibility of changing the capability of speeding up transient response. At last, the control circuit and method according to the present invention are adaptive to various applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objectives, 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: 
         FIG. 1  is a circuit diagram of a CCR COT PWM voltage regulator according to the present invention; 
         FIG. 2  is a circuit diagram of an embodiment for the power management IC and high-frequency feedback controller shown in  FIG. 1 ; 
         FIG. 3  is a circuit diagram of another embodiment for the high-frequency feedback controller shown in  FIG. 1 ; 
         FIG. 4  is a waveform diagram of the PWM voltage regulator shown in  FIG. 1 ; 
         FIG. 5  is a circuit diagram of another embodiment for the power management IC and high-frequency feedback controller shown in  FIG. 1 ; and 
         FIG. 6  is a circuit diagram of a control circuit for a constant frequency COT PWM voltage regulator according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Traditionally, a PWM voltage regulator usually has an additional compensation circuit for filtering out the high-frequency component of a feedback signal. The present invention acts in a diametrically opposite way to provide a specially high-frequency feedback loop to control a high-frequency feedback signal for effectively improving a transient response of a PWM voltage regulator. The high-frequency feedback loop is a linear control loop and only acts on the high-frequency component of the control loop. The high-frequency feedback loop can be implemented by simple passive components and configurations. 
     As shown in  FIG. 1 , an embodiment according to the present invention is applied to a constant current ripple (CCR) constant on time (COT) PWM voltage regulator using N-AVP control and having a PWM triggering mechanism similar to a valley current mode COT control loop. As is well known, the PWM voltage regulator includes a control circuit  10  and an output stage  12 , and the output stage  12  generates an output voltage Vout for a power input pin  142  of a CPU  14  according to a PWM signal Spwm from the control circuit  10 . The output stage  12  includes a driver  20  for switching switches SW 1  and SW 2  according to the PWM signal Spwm, to control an inductor current IL to charge a capacitor Co, thereby generating the output voltage Vout. All the above are known in the art. The control circuit  10  includes a resistor Rset, a power management IC  18  and a high-frequency feedback controller  16 . The resistor Rset is connected between a voltage input terminal Vin and a constant time setting pin  182  of the power management IC  18 , for supplying a current Iset to the constant time setting pin  182 . The power management IC  18  generates a constant time Tcon according to a current I 1  received from the constant time setting pin  182 , for defining on-time or off-time of the PWM signal Spwm. In the CCR COT PWM voltage regulator, the current I 1  is directly proportional to the inductor current IL to make the ripple of the output voltage Vout fixed. The high-frequency feedback controller  16  is connected between the constant time setting pin  182  and the power input pin  142 , for establishing a high-frequency feedback loop. The constant time setting pin  182  and the power input pin  142  are both pins originally provided on an IC or a chip, so there is no additional pin required in the embodiment. 
     Referring to  FIG. 1 , during normal operation, the ripple of the output voltage Vout is at a low frequency, and the high-frequency feedback controller  16  is open circuit, so the current I 1 =Iset that is flowing into the power management IC  18  via the constant time setting pin  182 . When the output voltage Vout supplied to the CPU  14  drops fast and significantly, the ripple frequency of the output voltage Vout exceeds a threshold preset in the high-frequency feedback controller  16 , making the high-frequency feedback controller  16  establish the high-frequency feedback loop that extracts a current Iqr through the constant time setting pin  182 , thereby adjusting the current I 1 =Iset−Iqr and in turn adjusting the constant time Tcon. The current Iqr is dependent on a droop-voltage of the output voltage Vout, so the high-frequency feedback controller  16  can automatically track the droop-voltage of the output voltage Vout and adjust the current Iqr accordingly, thereby dynamically adjusting the constant time Tcon. Since the high-frequency feedback controller  16  is deposited on the circuit board but not in the power management IC  18 , the accelerating ability of the high-frequency feedback controller  16  can be easily changed to meet requirements of different applications by properly arranging components in the high-frequency feedback controller  16 . Thus, to circuit designers, the present invention has more flexible in terms of circuit design. 
       FIG. 2  shows embodiments of the power management IC  18  and the high-frequency feedback controller  16  of  FIG. 1 . The power management IC  18  includes a constant-time generator  30  for generating the constant time Tcon according to the current I 1 , and a PWM comparator  32  for triggering a signal St for the constant-time generator  30  to trigger the PWM signal Spwm when a feedback voltage VFB is lower than a reference voltage Vref 1 . As well known, the feedback voltage VFB is the output feedback signal of the PWM voltage regulator and usually directly proportional to the output voltage Vout. The constant-time generator  30  includes a current control current source (CCCS)  34  for generating a current I 2  that charges the capacitor C 1  according to the current I 1 , a switch SW 3  connected in parallel to the capacitor C 1 , a comparator  36  for comparing a voltage Vc 1  at the capacitor C 1  to a reference voltage Vref 2  to generate the PWM signal Spwm. In one embodiment, when the controller  38 , in response to the triggering signal St, generates a short pulse Ssp to rest the capacitor C 1  to a grounding voltage, the PWM signal Spwm turns to a high level. When the short pulse Ssp ends so as to turn off the switch SW 3 , the voltage Vc 1  at the capacitor C 1  rises. When the voltage Vc 1  reaches the reference voltage Vref 2 , the PWM signal Spwm turns to a low level. The current I 2  determines the rising speed of the voltage Vc 1 , thereby determining the length of the constant on-time Tcon of the PWM signal Spwm. 
     Referring to  FIG. 2 , the high-frequency feedback controller  16  includes a high-pass RC filter composed of a quick response capacitor Cqr and a quick response resistor Rqr. The quick response capacitor Cqr and the quick response resistor Rqr are connected in series. The quick response capacitor Cqr is coupled to the power input pin  142 , and the quick response resistor Rqr is coupled to the constant time setting pin  182 . During normal operation, the ripple of the output voltage Vout is at a low frequency, so the quick response capacitor Cqr regards as open circuit. When transient occurs and the output voltage Vout drops suddenly (for high-frequency transient), the feedback voltage VFB drops to become lower than the reference voltage Vref 1  immediately, thereby triggering the signal St to trigger the PWM signal Spwm and achieve a real time response. Meanwhile, since the ripple frequency of the output voltage Vout rises into a high frequency range, the quick response capacitor Cqr regards as a short circuit, so the quick response capacitor Cqr and the quick response resistor Rqr establish a shunt current path for extracting the current Iqr from the constant time setting pin  182  to decrease the current I 1  and in turn the current I 2 . Due to the decrease of the current I 2 , the voltage Vc 1  rises slowly, so the constant on-time Tcon of the PWM signal Spwm is increased, thereby preventing the too low output voltage Vout leads to shutdown of the CPU  14 . By properly setting the RC value of the quick response capacitor Cqr and the quick response resistor Rqr in the high-frequency feedback controller  16 , the voltage regulator can be effectively improved in transient response. In other embodiments, the quick response capacitor Cqr and the quick response resistor Rqr in the high-pass filter may be replaced by active components. In addition to the high-pass filter, the high-frequency feedback controller  16  may be implemented by using other high-frequency signal filtering circuits. 
     Since the control circuit  10  does not determine the occurrence of transient according to the variation of the output voltage Vout, there is no need to set a voltage variation threshold. Instead, the constant time Tcon of the PWM signal Spwm can be linearly adjusted according to the variation of the output voltage Vout, so the control loop is more stable. When the input voltage Vin changes, the load changes, or the voltage identification VID changes, and the output voltage Vout has transient, the control circuit  10  can provide better transient performance. 
     In the embodiment shown in  FIG. 2 , the quick response resistor Rqr makes the shunt current path maintained for a period after occurrence of transient. However, the current Iqr is limited by the quick response resistor Rqr, so the length of the constant time Tcon of the PWM signal Spwm is limited, causing the increased performance limited.  FIG. 3  shows another embodiment of the high-frequency feedback controller  16 . In addition to the first shunt current path formed by the quick response capacitor Cqr and the quick response resistor Rqr of  FIG. 2 , the high-frequency feedback controller  16  of  FIG. 3  has a bypass capacitor Cbp that is connected in parallel to the quick response capacitor Cqr and the quick response resistor Rqr to establish a second shunt current path. Since the second shunt current path contains no resistor, after occurrence of transient, the bypass capacitor Cbp immediately draws a large current to generate a longer constant time Tcon. For the same reason that there is no resistor in the second shunt current path, the second shunt current path can only be maintained for a short period. After the second shunt current path becomes open circuit, the first shunt current path will be further maintained for a period, so the wide range load transient response can be significantly improved. 
       FIG. 4  is a waveform diagram of the PWM voltage regulator of  FIG. 1 . Waveforms  44  and  46  represent the PWM signal Spwm and the output voltage Vout when the high-frequency feedback controller  16  of  FIG. 2  is used, respectively. Waveforms  48  and  50  represent the PWM signal Spwm and the output voltage Vout when the high-frequency feedback controller  16  of  FIG. 3  is used, respectively. For comparison, waveforms  40  and  42  represent the PWM signal Spwm and the output voltage Vout when the high-frequency feedback controller  16  is not used, respectively. When transient occurs, as at time t 1 , the constant time Tcon of the PWM signal Spwm without the high-frequency feedback controller  16  remains unchanged, as shown by the waveform  40 , so it is impossible to instantly provide enough energy to stabilize the output voltage Vout, causing the output voltage Vout to become lower than the minimum voltage Vsd required by the CPU  14 , as shown by the waveform  42 . In the PWM voltage regulator using the high-frequency feedback controller  16  of  FIG. 2 , when transient occurs, the constant on-time Tcon of the PWM signal Spwm is instantly increased, so as to prevent the output voltage Vout from being too low and help the output voltage Vout to become stable again sooner, as shown by the waveforms  44  and  46 . In the PWM voltage regulator using the high-frequency feedback controller  16  of  FIG. 3 , when transient occurs, the constant on-time Tcon of the PWM control signal Spwm is further increased, so the better performance is achieved, as shown by the waveforms  48  and  50 . 
     In the embodiments shown by  FIG. 1  and  FIG. 2 , the high-frequency feedback controller  16  changes the constant time Tcon by adjusting the current I 1 . However, in other embodiments, the high-frequency feedback controller  16  may be coupled to other nodes in the constant-time generator  30 . For instance, in an embodiment shown in  FIG. 5 , the high-frequency feedback controller  16  is connected to the output terminal of the current control current source  34  through the pin  184  of the power management IC  18 , and adjusts the current I 2  to adjust the constant time Tcon when transient occurs. The pin  184  may be an additional pin. 
     Although the above embodiments are designed based on a CCR COT PWM voltage regulator for illustrating the principles of the present invention, it would be appreciated that other types of PWM voltage regulators, for example, constant on-time PWM voltage regulators and constant off-time PWM voltage regulators, may also use the high-frequency feedback controller  16  to adjust the constant time Tcon of the PWM signal Spwm. 
       FIG. 6  is a circuit diagram of a control circuit for a constant frequency COT PWM voltage regulator according to the present invention, which is the same as that shown in  FIG. 2 , except that the input terminal of the CCCS  34  that receives the voltage VID in  FIG. 2  is grounded in this embodiment, and the reference voltage Vref 2  is replaced by Vout&#39; that is related to the DC component of the output voltage Vout, for example, extracted from the output voltage Vout by low-pass filtering. When transient happens to the output voltage Vout, the quick response capacitor Cqr and the quick response resistor Rqr establish a shunt current path for extracting a current Iqr from the constant time setting pin  182  of the power management IC  18  to decrease the current I 1 . The current I 2  will vary with the current I 1  and so adjust the constant on-time Tcon of the PWM control signal Spwm, thereby improving the transient response of the voltage regulator. The high-frequency feedback controller  16  shown in  FIG. 6  may also be added with a bypass capacitor Cbp parallel connected to the serially connected quick response capacitor Cqr and quick response resistor Rqr to establish a second shunt current path, as that shown in  FIG. 3 . The high-frequency feedback controller  16  shown in  FIG. 6  may be alternatively connected to the output terminal of the CCCS  34 , as that shown in  FIG. 5 , to adjust the current I 2  so as to adjust the constant time Tcon when transient occurs. 
     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.