Zero current detecting circuit and related synchronous switching power converter

1. A zero current detecting circuit, comprising:

a first zero current comparator for determining current variation on an inductor of a synchronous switching power converter so as to accordingly turn off a down-bridge transistor of the synchronous switching power converter;

a second zero current comparator for determining whether the first zero current comparator turns off the down-bridge transistor too early or too late, and outputting a comparison result;

a counter coupled to the second zero current comparator for ascending or descending a control bit according to the comparison result; and

an adjustable voltage source coupled to the first zero current comparator and the counter for adjusting a offset voltage according to the control bit.

2. The zero current detecting circuit of claim 1 , wherein the counter ascends the control bit when the comparison result is at a low level and descends the control bit when the comparison result is at a high level.

3. The zero current detecting circuit of claim 1 , wherein the adjustable voltage source decreases the offset voltage when the control bit is decreased and increases the offset voltage when the control bit is increased.

4. The zero current detecting circuit of claim 1 , wherein the first zero current comparator comprises:

a positive input terminal coupled to a first terminal of the down-bridge transistor for detecting current variation on the inductor;

a negative input terminal coupled to the adjustable voltage source; and

an output terminal for outputting a zero current signal according to the current variation on the inductor and the offset voltage to turn off the down-bridge transistor.

5. The zero current detecting circuit of claim 1 , wherein the second zero current comparator comprises:

a positive input terminal coupled to a first terminal of the down-bridge transistor for detecting current variation on the inductor;

a negative input terminal coupled to a reference voltage; and

an output terminal coupled to the counter for outputting the comparison result to the counter according to the reference voltage and the current variation on the inductor.

6. A synchronous switching power converter with zero current detecting, comprising:

an up-bridge transistor comprising:

a first terminal coupled to an input power supply;

a second terminal; and

a third terminal for receiving a turned-on signal;

a down-bridge transistor comprising:

a first terminal coupled to a ground;

a second terminal coupled to the second terminal of the up-bridge transistor; and

a third terminal for receiving a turned-off signal;

an inductor coupled between the second terminal of the down-bridge transistor and an output capacitor; and

a zero current detecting circuit comprising:

a first zero current comparator for determining current variation on the inductor of a synchronous switching power converter so as to accordingly turn off a down-bridge transistor of the synchronous switching power converter;

a second zero current comparator for determining whether the first zero current comparator turns off the down-bridge transistor too early or too late and outputting a comparison result;

a counter coupled to the second zero current comparator for ascending or descending a control bit according to the comparison result; and

an adjustable voltage source coupled to the first zero current comparator and the counter for adjusting an offset voltage according to the control bit.

7. The synchronous switching power converter of claim 6 , wherein the counter ascends the control bit when the comparison result is at a low level and descends the control bit when the comparison result is at a high level.

8. The synchronous switching power converter of claim 6 , wherein the adjustable voltage source decreases the offset voltage when the control bit is decreased and increases the offset voltage when the control bit is increased.

9. The synchronous switching power converter of claim 6 , wherein the first zero current comparator comprises:

a positive input terminal coupled to a first terminal of the down-bridge transistor for detecting the current variation on the inductor;

a negative input terminal coupled to the adjustable voltage source; and

an output terminal for outputting a zero current signal according to the current variation on the inductor and the offset voltage to turn off the down-bridge transistor.

10. The synchronous switching power converter of claim 6 , wherein the second zero current comparator comprises:

a positive input terminal coupled to a first terminal of the down-bridge transistor for detecting the current variation on the inductor;

a negative input terminal coupled to a reference voltage; and

an output terminal coupled to the counter for outputting the comparison result to the counter according to the reference voltage and the current variation on the inductor.

11. A zero current detecting method for a synchronous switching power converter, comprising:

determining current variation on an inductor of a synchronous switching power converter to accordingly turn off a down-bridge transistor of the synchronous switching power converter;

determining whether the down-bridge transistor is turned off too early or too late and outputting a comparison result;

ascending or descending a control bit according to the comparison result; and

adjusting an offset voltage according to the control bit.

12. The zero current detecting method of claim 11 , wherein ascending the control bit when the comparison result is at a low level and descending the control bit when the comparison result is at a high level.

13. The zero current detecting method of claim 11 , wherein decreasing the offset voltage when the control bit is decreased and increasing the offset voltage when the control bit is increased.