Voltage step-up/step-down switching regulator and control method thereof

1. A switching regulator comprising:

a step-down circuit configured to output a stepped-down voltage lower than an input voltage;

a step-up circuit configured to output a stepped-up voltage higher than the input voltage; and

a control unit comprising a voltage detector configured to detect an output voltage,

the control unit being configured to prohibit step-up operation of the step-up circuit while allowing step-down operation of the step-down circuit until the output voltage rises to a first voltage, said first voltage being lower than the input voltage,

wherein in a case that the output voltage rises to the first voltage, the control unit allows the step-down operation of the step-down circuit and the step-up operation of the step-up circuit while the output voltage is (i) higher than a second voltage that is lower than the first voltage and (ii) lower than the input voltage,

wherein the step-down circuit comprises:

a step-down switching transistor that receives the input voltage;

a step-down control circuit configured to control the step-down switching transistor; and

a detection circuit configured to detect a voltage drop across the step-down switching transistor when the step-down switching transistor is on; and

wherein the control unit turns the step-down switching transistor off when the voltage drops across the step-down switching transistor to a voltage below a predetermined value, and the control unit releases an off state of the step-down switching transistor cyclically or every predetermined number of cycles.

2. The switching regulator of claim 1 , wherein the control unit prohibits the operation of the step-up circuit when the output voltage drops to or below the second voltage.

3. The switching regulator of claim 1 ,

wherein the voltage detector comprises a delay circuit, and

wherein the control unit generates an output signal after a predetermined delay time generated by the delay circuit when the output voltage reaches the first voltage or the second voltage.

4. The switching regulator of claim 1 , wherein the output voltage is controlled only by the step-down circuit when the output voltage is less than the first voltage.

5. The switching regulator of claim 1 , wherein the control unit only prohibits the step-up operation of the step-up circuit when the output voltage is lower than the first voltage.

6. The switching regulator of claim 1 , wherein the control unit allows the step-up operation of the step-up circuit when the output voltage rises above the first voltage.

7. The switching regulator of claim 1 , wherein if the output voltage is higher than the first voltage and lower than the input voltage, the control unit allows both the step-down operation and the step-up operation.

8. The switching regulator of claim 1 , wherein if the output voltage is higher than the input voltage, the control unit prohibits the step-down operation of the step-down circuit while allowing the step-up operation of the step-up circuit.

9. A method for controlling a switching regulator comprising the steps of:

detecting an output voltage;

prohibiting a step-up operation of a step-up circuit of the switching regulator while allowing a step-down operation of a step-down circuit of the switching regulator until the output voltage rises to a first voltage, said first voltage being lower than an input voltage, and

in a case that the output voltage rises to the first voltage, allowing the step-down operation of the step-down circuit and the step-up operation of the step-up circuit while the output voltage is (i) higher than a second voltage that is lower than the first voltage and (ii) lower than the input voltage;

detecting a voltage drop across a step-down switching transistor when the step-down switching transistor is on, the step-down switching transistor receiving the input voltage;

turning the step-down switching transistor off when the voltage drops across the step-down switching transistor to a voltage below a predetermined value; and

releasing an off state of the step-down switching transistor cyclically or every predetermined number of cycles.

10. The method for controlling the switching regulator of claim 9 further comprising the step of: prohibiting the step-up operation when the output voltage drops to or below the second voltage.

11. The method for controlling the switching regulator of claim 9 further comprising the step of: generating an output signal after a predetermined delay time when the output voltage reaches the first voltage or the second voltage.