Mode control circuit for switching regulators and method therefor

[0001] The present invention relates to switching regulators, and in particular, to control schemes for switching regulators during high and low load current.

[0002] When higher efficiency, smaller size, lighter weight, and prolonged battery lifetime are required, switching regulators are used for power management as replacements for linear regulators. The most common control scheme for switching regulators is pulse-width modulation (PWM). The control uses a constant switching frequency but varies the duty cycle as the load current varies. This scheme achieves good regulation, low noise spectrum, and high efficiency at relatively heavy load. However, when the load current is low, PWM mode is inefficient due to the switching loss and the high quiescent current.

[0003] There are prior art methods to improve the efficiency of switching regulators at light load. In order to reduce switching loss, the switching frequency can be reduced, cycles can be skipped, or enter into low dropout (LDO) mode at light load. However, those solutions still may not reduce the quiescent current.

[0004] The present invention provides a control scheme for switching regulators which offers smooth transition between two modes of operation automatically depending upon the load condition. These two modes enable the switching regulator to achieve efficient operation through a wide load range.

[0005] An exemplary mode control circuit of the present invention comprises: a current sensing amplifier, a sample circuit, an averaging circuit, and a mode control signal generator.

[0006] An exemplary mode control method of the present invention comprises: receiving an input signal; amplifying said input signal to get an amplified signal; sending said amplified signal to a sample circuit to get a sample signal; delivering said sample signal to an averaging circuit to get an averaged sensed output signal; comparing said averaged sensed output signal with a first pre-determined threshold or an second pre-determined threshold to get a PWM enable signal; determining mode based on said PWM enable signal.

[0007] These and other advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments, which are illustrated in the various drawing Figures.

[0011] 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.

[0012] Now referring to FIG. 1, a schematic diagram of mode control circuit 100 in accordance with an embodiment of the invention is illustrated. Circuit 100 includes a current sensing amplifier U₀ which is electrically coupled to receive an input current I_IN, and operable to output a current sensing signal I_IN—sns. Alternatively, the circuit 100 may also receive an input voltage signal, but for illustration purpose, signals flowing through the circuit 100 are adopted in current signal formats, such as I_IN, I_IN—sns, I_sns—out, I_{out—sns—ave} which will be illustrated hereinafter, but they may also be in the voltage domain, such as V_IN, V_IN—sns, V_sns—out, and V_{out—sns—ave}. Circuit 100 further includes a sample circuit, an averaging circuit, a mode control signal generator 10, and an input disconnect switch M₁. The input disconnect switch M₁ is fully on during normal operation when used in non-synchronous application. Switch M₁ is periodically on and off in complementary fashion to a low-side switch M₂ when used in synchronous application. In one embodiment, mode control signal generator 10 comprises a first switch S₁, a second switch S₂, a first inverter U₁, a second inverter U₂, and a comparator U_C. Further, there is a low-side switch M₂ and an inductor L as shown in FIG. 1, which are part of the main circuit of a switching regulator.

[0013] The output terminal of the sense current amplifier U₀ is coupled to the input terminal of the sample circuit. The output terminal of the sample circuit is in turn coupled to the input terminal of the averaging circuit. The output of the averaging circuit is coupled to the input terminal of mode control signal generator 10. The output of mode control signal generator 10 acts as the output terminal of circuit 100. In one embodiment, the inverting input terminal of the comparator U_C acts as the input terminal of mode control signal generator 10. The output of the comparator U_C is coupled to the input of the first inverter U₁. The output of the first inverter U₁ is then coupled to the input terminal of the second inverter U₂. The output of the first inverter U₁ also acts as the output terminal of mode control signal generator 10.

[0014] One terminal of the first switch S₁ receives a signal Mode_th which is a first pre-determined threshold, while one terminal of the second switch S₂ receives a signal Mode_th+Mode_Hys which is a second pre-determined threshold. The other terminal of the first switch S₁ and the other terminal of the second switch S₂ are coupled together to the non-inverting input terminal of the comparator U_C. The first switch S₁ is controlled by a signal PWM_EN which is output from the first inverter U₁, while the second switch S₂ is controlled by a signal PWM_ENB which is output from the second inverter U₂. The polarity of inputs of the comparator U_C is not limited to the connection illustrated herein, but they can be swapped by proper adjustment of the polarity of controls for switch S₁, S₂, and thresholds.

[0015] The low-side switch M₂ in the main circuit is controlled by a signal BG, while the sample circuit is controlled by a signal BG_OFF, wherein the signal BG and the signal BG_OFF are complimentarily-phased. When the low-side switch M₂ is turned on, the sample circuit is disabled; when the low-side switch M₂ is turned off, the sample circuit is active and it will sample the sensed input current during M₂'s OFF cycle.

[0016] When the signal BG_OFF is high, the low-side switch M₂ is turned off, the sample circuit is available. The input current I_IN is sensed and amplified by the current sensing amplifier U₀. The output of the current sensing amplifier U₀ is I_IN—sns=A*I_IN, wherein A is the gain of the current sensing amplifier U₀. The signal I_IN—sns is sent to the sample circuit since the sample circuit is available now. The output of the sample circuit I_sns—out is the sampled signal of I_IN—sns, shown as the shadow area of the drawing in FIG. 1, thus it is proportional to the load current I_LOAD. The signal I_sns—out which represents the load current I_LOAD is then sent to the averaging circuit to get an averaged sensed output current I_{out—sns—ave} which is input to the inverting input terminal of the comparator U_C.

[0017] When the output of the comparator U_C is low, the signal PWM_EN is high, and the PWM_ENB is low. Thereupon, the first switch S₁ is turned on, the second switch S₂ is turned off. As a result, the first pre-determined threshold Mode_th is directly sent to the non-inverting input terminal of the comparator U_C. When the output of the comparator U_C is high, the signal PWM_EN is low, the PWM_ENB is high. Thereupon, the first switch S₁ is turned off, the second switch S₂ is turned on. As a result, the second pre-determined threshold Mode_th+Mode_Hys is directly sent to the non-inverting input terminal of the comparator U_C. The comparator U_C compares the sensed output current I_{out—sns—ave} with the first pre-determined threshold Mode_th or with the second pre-determined threshold Mode_th+Mode_Hys. If the sensed output current I_{out—sns—ave} is lower than the first pre-determined threshold Mode_th, the comparator U_C will output a high signal, causing the signal PWM_EN to be low and the signal PWM_ENB to be high accordingly. Thereupon, the system will transition into constant-peak-current PFM mode.

[0018] In contrast, if the sensed output current I_out—sns—ave is higher than the second pre-determined threshold Mode_th+Mode_Hys, the comparator U_C will output a low signal, causing the signal PWM_EN to be high and the signal PWM_ENB to be low accordingly. Thereupon, the system will transition into PWM mode. That is, when the averaged sensed output current is lower than the first pre-determined threshold Mode_th, the switching regulator is in light load condition. When the averaged sensed output current is higher than the second pre-determined threshold Mode_th+Mode_Hys, the switching regulator is in heavy load condition. It can be seen that the signal PWM_EN is the PWM enable signal, the signal PWM_ENB is the complementary-phase signal of the PWM enable signal PWM_EN. When the PWM enable signal PWM_EN is low, the switching regulator enters constant-peak-current PFM mode; when the PWM enable signal PWM_ENB is high, the switching regulator enters PWM mode.

[0019] Referring to FIG. 2, waveforms of different signals in circuit 100 shown in FIG. 1 is depicted. As shown in FIG. 2, before time t₁, the load current I_LOAD is low, the inductor current I_L is discontinuous. The sensed output current I_{out—sns—ave} is lower than the first pre-determined threshold Mode_th, the output of the comparator U_C is high. Thus the signal PWM_EN is low, and the signal PWM_ENB is high accordingly. The low signal PWM_EN turns off the first switch S₁, while the high signal PWM_ENB turns on the second switch S₂. Thereupon, the second pre-determined threshold Mode_th+Mode_Hys is directly sent to the non-inverting input terminal of the comparator U_C, and the system is on constant-peak-current PFM mode. When the load current begins to increase at time t₀, the time between each switching cycle gets shorter and pulses of inductor current I_L are getting closer. The sensed output current I_{out—sns—ave} is increased proportionally. When at time t1, the sensed output current I_{out—sns—ave} is higher than the second pre-determined threshold Mode_th+Mode_Hys, the comparator U_C outputs a low signal, causing the signal PWM_EN to be high and the signal PWM_ENB to be low accordingly. The system transitions into PWM mode. From time t₂, the system stays in the heavy load condition. The operation of switching regulators in heavy load condition is known to those skilled in the art, which will not be illustrated herein.

[0020] Continuing to refer to FIG. 2, at time t₃, the load condition changes again, the load current I_L begins to be decreased, the sensed output current I_{out—sns—ave} is decreased accordingly. When at time t₄, the sensed output current I_{out—sns—ave} is lower than the first pre-determined threshold Mode_th, the comparator U_C outputs a high signal, causing the signal PWM_EN to be low and the signal PWM_ENB to be high accordingly. Thus, the first pre-determined threshold Mode_th is directly sent to the non-inverting input terminal of the comparator U_C, and the system transitions into constant-peak-current PFM mode. The inductor current I_L becomes discontinuous. From time t₅, the system stays in the light load condition. The operation of switching regulators in light load condition is known to those skilled in the art, which will not be illustrated herein.

[0021] FIG. 2 illustrates that the mode transition is very smooth and well defined.

[0022] The present invention further provides a mode control method for switching regulators. The method comprises: receiving an input signal; amplifying said input signal to get an amplified signal; sending said amplified signal to a sample circuit to get a sample signal; delivering said sample signal to an averaging circuit to get an averaged sensed output current; comparing said averaged sensed output current with a first pre-determined threshold or an second pre-determined threshold to get a PWM enable signal; and determining mode based on said PWM enable signal.

[0023] The first pre-determined threshold indicates the switching regulator is in light load condition. The second pre-determined threshold indicates said switching regulator is in heavy load condition. When the averaged sensed output current is lower than said first pre-determined threshold, the PWM enable signal is low, the switching regulator enters constant-peak-current PFM mode accordingly. When the averaged sensed output current is higher than the second pre-determined threshold, the PWM enable signal is high, the switching regulator enters PWM mode accordingly. When the low-side switch in the main circuit is turned on, the sample circuit is disabled. When the low-side switch in the main circuit is turned off, the sample circuit is active. When the averaged sensed output current is higher than the first pre-determined threshold, the switching regulator is in light load condition. When the averaged sensed output current is higher than the second pre-determined threshold, the switching regulator is in heavy load condition.

[0024] As described above, circuit 100 realizes good efficiency in both heavy load condition and light load condition. The switching regulator switches between constant-peak-current PFM mode at light load and PWM mode at heavy load. The PFM mode has lower switching frequency which reduces the switching loss, and lower quiescent current consumption resulting in the better standby efficiency. The PWM mode has high efficiency and good regulation at heavy load. Therefore, coupled with the invention, it offers high efficiency through the entire load range and provides the smooth transition automatically between the modes.

[0025] While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.