Abstract:
Methods are proposed for a buck boost voltage regulator to monitor the output voltage or both the inductor current and the output voltage of the buck boost voltage regulator to control the buck boost voltage regulator to reduce the switching times of the power switches of the buck boost voltage regulator to improve the light load efficiency of the buck boost voltage regulator.

Description:
FIELD OF THE INVENTION 
     The present invention is related generally to a voltage regulator and, more particularly, to methods for light load efficiency improvement of a buck boost voltage regulator. 
     BACKGROUND OF THE INVENTION 
     Batteries are unable to continuously supply a stable voltage, so systems using batteries as power source, e.g. portable electronic devices, need to be equipped with a buck boost voltage regulator that is capable of switching between a buck mode and a boost mode.  FIG. 1  is a circuit diagram of a buck boost voltage regulator, in which a power stage  10  has a power switch SW 1  connected between the power input terminal Vin and a node N 1 , a power switch SW 2  connected between the node N 1  and ground GND, an inductor L connected between the node N 1  and a node N 2 , a power switch SW 3  connected between the node N 2  and ground GND, a power switch SW 4  connected between the node N 2  and the voltage output terminal Vout, and a capacitor CL connected between the voltage output terminal Vout and ground GND, voltage divider resistors R 1  and R 2  divide the output voltage Vout to generate a feedback signal VFB, and a control circuit  12  has an error amplifier  14  to amplify the difference between the feedback signal VFB and a reference voltage Vref to generate an error signal VEA, a pulse width modulation generator  16  to generate a pulse width modulation signal PWM according to the error signal VEA, and a control logic circuit  18  to generate control signals VA, VB, VC and VD according to the pulse width modulation signal PWM to switch the power switches SW 1 , SW 2 , SW 3  and SW 4  respectively, to convert the input voltage Vin to the output voltage Vout. U.S. Pat. No. 7,265,524 issued to Jorden et al. proposed a control method for a buck boost voltage regulator, which monitors the output voltage Vout of the buck boost voltage regulator when the buck boost voltage regulator operates in a buck-boost mode, and when the output voltage Vout goes above a high voltage threshold, switches the buck boost voltage regulator to a sleep mode. In this sleep mode, the power switches SW 1 , SW 2 , SW 3  and SW 4  are all open to stop charging the inductor L of the buck boost voltage regulator and thus the output voltage Vout decreases slowly. Till the output voltage Vout goes below a low voltage threshold, the buck boost voltage regulator is switched back to the buck-boost ode in which the inductor L is continuously charged/discharged to regulate the output voltage Vout. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide methods for light load efficiency improvement of a buck boost voltage regulator. 
     According to the present invention, a method for light load efficiency improvement of a buck boost voltage regulator monitors the inductor current of the buck boost voltage regulator and switches the buck boost voltage regulator to a sleep mode when the inductor current goes below a low current threshold. In the sleep mode, the output voltage of the buck boost voltage regulator is monitored and when the output voltage goes below a low voltage threshold, the buck boost voltage regulator is switched to escape from the sleep mode. 
     According to the present invention, a method for light load efficiency improvement of a buck boost voltage regulator monitors the output voltage of the buck boost voltage regulator and switches the buck boost voltage regulator to a pulse skipping mode when the output voltage goes above a high voltage threshold. In the pulse skipping mode, the inductor of the buck boost voltage regulator is charged once whenever the output voltage goes below the high voltage threshold and if the output voltage goes below a low voltage threshold, the buck boost voltage regulator is switched to escape from the pulse skipping mode. 
     According to the present invention, a method for light load efficiency improvement of a buck boost voltage regulator monitors the inductor current of the buck boost voltage regulator and switches the buck boost voltage regulator from a first switch period to a second switch period longer than the first switch period when the inductor current goes below a low current threshold. Under the second switch period, the output voltage of the buck boost voltage regulator is monitored and when the output voltage goes below a low voltage threshold, the buck boost voltage regulator is switched back to the first switch period. 
     According to the present invention, a method for light load efficiency improvement of a buck boost voltage regulator monitors the output voltage of the buck boost voltage regulator and switches the buck boost voltage regulator from a first switch period to a second switch period longer than the first switch period when the output voltage goes above a high voltage threshold. Under the second switch period, if the output voltage goes below a low voltage threshold, the buck boost voltage regulator is switched back to the first switch period. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a circuit diagram of a typical buck boost voltage regulator; 
         FIG. 2  is a waveform diagram of a buck boost voltage regulator using a first embodiment of a method according to the present invention; 
         FIG. 3  is a waveform diagram of a buck boost voltage regulator using a second embodiment of a method according to the present invention; 
         FIG. 4  is a waveform diagram of a buck boost voltage regulator using a third embodiment of a method according to the present invention; 
         FIG. 5  is a waveform diagram of a buck boost voltage regulator using a fourth embodiment of a method according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  is a waveform diagram illustrating a first embodiment of a method for light load efficiency improvement of a buck boost voltage regulator, in which waveforms  20  and  22  represent the output voltage Vout and the inductor current I of the buck boost voltage regulator, respectively. Referring to  FIGS. 1 and 2 , this method monitors the inductor current I and when the inductor current I goes below a low current threshold I_L, as shown at time t 1 , the buck boost voltage regulator is switched to a sleep mode in which the power switches SW 1 , SW 2 , SW 3  and SW 4  are all open to stop charging the inductor L and as a result, the output voltage Vout decreases. When the output voltage Vout goes below a low voltage threshold Vth_L, as shown at time t 2 , the buck boost voltage regulator is switched to escape from the sleep mode. Preferably, when it is detected that the output voltage Vout goes below the low voltage threshold Vth_L and the inductor current I goes below the low current threshold I_L, the buck boost voltage regulator will escape from the sleep mode immediately after entering into the sleep mode to increase the output voltage Vout. This method monitors both the inductor current I and the output voltage Vout, sets the low current threshold I_L approximately zero, and reduces the power consumption at light load by controlling the timing of the buck boost voltage regulator entering into and escaping from the sleep mode. 
       FIG. 3  is a waveform diagram illustrating a second embodiment of a method for light load efficiency improvement of a buck boost voltage regulator, in which waveforms  24  and  26  represent the output voltage Vout and the inductor current I of the buck boost voltage regulator, respectively. Referring to  FIGS. 1 and 3 , this method only monitors the output voltage Vout and when the output voltage Vout goes above a high voltage threshold Vth_H, switches the buck boost voltage regulator to a pulse skipping mode in which some pulses of the pulse width modulation signal PWM are skipped to reduce the switching times of the power switches SW 1 , SW 2 , SW 3  and SW 4 . In the pulse skipping mode, whenever it is detected that the output voltage Vout goes below the high voltage threshold Vth_H, as shown at time t 3  and time t 4 , the inductor L is charged once to increase the output voltage Vout; however, if the output voltage Vout still continuously decreases to lower than a low voltage threshold Vth_L, as shown at time t 5 , the buck boost voltage regulator is switched to escape from the pulse skipping mode. 
       FIG. 4  is a waveform diagram illustrating a third embodiment of a method for light load efficiency improvement of a buck boost voltage regulator, in which waveforms  28  and  30  represent the output voltage Vout and the inductor current I of the buck boost voltage regulator, respectively. Referring to  FIGS. 1 and 4 , in this embodiment, the pulse width modulation generator  16  has a short switch period T 1  and a long switch period T 2 . The short switch period T 1  is used in a normal operation mode and when the inductor current I goes below the low current threshold I_L, as shown at time t 6 , the pulse width modulation generator  16  switches to provide the long switch period T 2  to reduce the switching frequency of the power switches SW 1 , SW 2 , SW 3  and SW 4 . Under the long switch period T 2 , when it is detected that the output voltage Vout goes below a low voltage threshold Vth_L, the pulse width modulation generator  16  switches back to provide the short switch period T 1 , so the power switches SW 1 , SW 2 , SW 3  and SW 4  restore the switching frequency of the normal operation mode. This method monitors both the inductor current I and the output voltage Vout, set the low current threshold I_L approximately zero, and reduces the power consumption at light load by reducing the switching frequency of the power switches SW 1 , SW 2 , SW 3  and SW 4 . 
       FIG. 5  is a waveform diagram illustrating a fourth embodiment of a method for light load efficiency improvement of a buck boost voltage regulator, in which waveforms  32  and  34  represent the output voltage Vout and the inductor current I of the buck boost voltage regulator, respectively. Referring to  FIGS. 1 and 5 , this method only monitors the output voltage Vout and the pulse width modulation generator  16  has a short switch period T 1  and a long switch period T 2 . The short switch period T 1  is used in a normal operation mode and when it is detected that the output voltage Vout goes above a high voltage threshold Vth_H, as shown at time t 8 , the pulse width modulation generator  16  switches to provide the long switch period T 2  to reduce the switching frequency of the power switches SW 1 , SW 2 , SW 3  and SW 4 . Under the long switch period T 2 , when it is detected that the output voltage Vout goes below the low voltage threshold Vth_L, as shown at time t 9 , the pulse width modulation generator  16  switches back to provide the short switch period T 1 , so the power switches SW 1 , SW 2 , SW 3  and SW 4  restore the switching frequency of the normal operation mode. 
     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.