Bootstrap circuit and associated direct current-to-direct current converter applying the bootstrap circuit

1. A bootstrap circuit applied to a first transistor of a direct-current (DC) to DC converter, comprising:

a second transistor;

a bootstrapping capacitor, having a first terminal and a second terminal, wherein the first terminal is coupled to a source terminal of the second transistor, and the source terminal of the second transistor is coupled to the first transistor; and

a clamping circuit, coupled between a gate terminal of the second transistor and the second terminal of the bootstrapping capacitor, the clamping circuit being arranged to maintain a voltage drop between the second terminal of the bootstrapping capacitor and the gate terminal of the second transistor;

wherein a drain terminal of the transistor is coupled to a first reference voltage, and a maximum of a voltage level of a gate terminal of the first transistor is greater than the first reference voltage.

2. The bootstrap circuit of claim 1 , further comprising:

a voltage regulating circuit, wherein the voltage regulating circuit is coupled between the drain terminal of the second transistor and the first reference voltage.

3. The bootstrap circuit of claim 2 , wherein the voltage regulating circuit comprises a Schottky diode.

4. The bootstrap circuit of claim 1 , further comprising:

a driving current source, coupled between the first reference voltage and the clamping circuit.

5. The bootstrap circuit of claim 1 , further comprising:

a voltage regulating circuit, coupled between the gate terminal of the second transistor and a second reference voltage.

6. The bootstrap circuit of claim 5 , wherein the voltage regulating circuit comprises a Schottky diode.

7. The bootstrap circuit of claim 1 , wherein the clamping circuit comprises at least one clamping transistor.

8. The bootstrap circuit of claim 7 , wherein a drain terminal of each clamping transistor is coupled to a gate terminal of each clamping transistor.

9. The bootstrap circuit of claim 7 , further comprising:

a voltage source, arranged to provide a second reference voltage, wherein the second reference voltage is greater than a predetermined value, and the predetermined value is a sum of a threshold voltage of each clamping transistor.

10. The bootstrap circuit of claim 9 , wherein when the voltage level on the second terminal of the bootstrapping capacitor is greater than 0 and the second reference voltage is smaller than a sum of the voltage level on the second terminal of the bootstrapping capacitor and the predetermined value, the bootstrapping capacitor is charged with a voltage equal to the predetermined value minus a threshold voltage of the second transistor.

11. A direct current (DC)-to-DC converter, comprising:

a switching circuit, comprising a first transistor and a second transistor, wherein a switching terminal is coupled between a source terminal of the first transistor and a drain terminal of the second transistor, and a drain terminal of the first transistor is coupled to a first reference voltage;

an inductor-capacitor circuit, comprising at least an inductor and a capacitor, wherein the indictor-capacitor circuit is arranged to receive an inductor current from the first reference voltage source via the switching circuit to provide energy to a following loading;

a feedback circuit, coupled to the inductor-capacitor circuit, wherein the feedback circuit is arranged to generate an output voltage at an output terminal and a feedback voltage; and

a bootstrap circuit, comprising:

a third transistor, wherein a source terminal of the third transistor is coupled to the first transistor;

a clamping circuit, coupled between a gate terminal of the third transistor and the switching terminal, wherein the clamping circuit is arranged to clamp a voltage level on the switching terminal; and

a bootstrapping capacitor, wherein the bootstrapping capacitor is coupled between the source terminal of the third transistor and the switching terminal;

wherein a maximum of a voltage level of the gate terminal of the first transistor is greater than the first reference voltage.

12. The DC-to-DC converter of claim 11 , wherein the bootstrap circuit further comprises:

a voltage regulating circuit, wherein the voltage regulating circuit is coupled between the drain terminal of the third transistor and the first reference voltage.

13. The DC-to-DC converter of claim 12 , wherein the voltage regulating circuit comprises a Schottky diode.

14. The DC-to-DC converter of claim 11 , wherein the bootstrap circuit further comprises:

a driving current source, coupled between the first reference voltage and the clamping circuit.

15. The DC-to-DC converter of claim 11 , wherein the bootstrap circuit further comprises:

a voltage regulating circuit coupled between the gate terminal of the third transistor and a second reference voltage.

16. The DC-to-DC converter of claim 15 , wherein the voltage regulating circuit comprises a Schottky diode.

17. The DC-to-DC converter of claim 11 , wherein the clamping circuit of the bootstrap circuit comprises at least one clamping transistor.

18. The DC-to-DC converter of claim 17 , wherein a drain terminal of each clamping transistor is coupled to a gate terminal of each clamping transistor.

19. The DC-to-DC converter of claim 17 , wherein the bootstrap circuit further comprises:

a voltage source, arranged to provide a second reference voltage, wherein the second reference voltage is greater than a predetermined value, and the predetermined value is a sum of a threshold voltage of each clamping transistor.

20. The DC-to-DC converter of claim 19 , wherein when the voltage level on the switching terminal is greater than 0 and the second reference voltage is smaller than a sum of the voltage level on the switching terminal and the predetermined value, the bootstrapping capacitor is charged with a voltage equal to the predetermined value minus a threshold voltage of the third transistor.