Low dropout regulator and the over current protection circuit thereof

1. An over current protection circuit for a low dropout regulator, the low dropout regulator comprising a power transistor, the over current protection circuit comprising:

a sense transistor configured to sense a current flowing through the power transistor;

a sense resistor coupled to the sense transistor, the sense resistor sharing the same current flowing through the sense transistor;

an operational amplifier configured to output a control signal according to a voltage across the sense resistor and a reference voltage; and

a first transistor configured to control the power transistor according to the control signal.

2. The over current protection circuit of claim 1 , wherein the power transistor, the sense transistor and the first transistor are all NMOS transistors.

3. The over current protection circuit of claim 2 , wherein the source electrode of the sense transistor is coupled to the source electrode of the power transistor, and the gate electrode of the sense transistor is coupled to the gate electrode of the power transistor.

4. The over current protection circuit of claim 2 , wherein one end of the sense resistor is coupled to a common node of the drain electrode of the power transistor and a supply voltage, and the other end is coupled to the drain electrode of the sense transistor.

5. The over current protection circuit of claim 2 , wherein the inverting input terminal of the operational amplifier is coupled to the drain electrode of the sense transistor, and the non-inverting input terminal of the operational amplifier is coupled to the reference voltage.

6. The over current protection circuit of claim 2 , wherein the drain electrode of the first transistor is coupled to the gate electrode of the sense transistor, the gate electrode of the first transistor is coupled to the output terminal of the operational amplifier, and the source electrode of the first transistor is grounded.

7. The over current protection circuit of claim 1 , wherein the reference voltage is provided by a reference voltage circuit, the reference voltage circuit comprising:

a first resistor with one end coupled to a supply voltage, and the other end coupled to a non-inverting input terminal of the operational amplifier; and

a current source coupled to the non-inverting input terminal of the operational amplifier.

8. An over current protection circuit for a low dropout regulator, the low dropout regulator comprising a power transistor, the over current protection circuit comprising:

a sense transistor configured to sense the current flowing through the power transistor;

a sense resistor coupled to the sense transistor, the sense resistor sharing the same current flowing through the sense transistor;

a current source;

a first current-mirror circuit coupled to the current source so as to form a first part of a first current path and a first part of a second current path;

a second current-mirror circuit coupled to the first current-mirror circuit so as to form a second part of the first current path and a second part of the second current path;

a first resistor coupled to the second current-mirror circuit so as to form a third part of the first current path; and

a first transistor configured to control the power transistor according to voltages across the sense resistor and the first resistor.

9. The over current protection circuit of claim 8 , wherein the power transistor and the sense transistor are both NMOS transistors.

10. The over current protection circuit of claim 9 , wherein the source electrode of the sense transistor is coupled to the source electrode of the power transistor, and the gate electrode of the sense transistor is coupled to the gate electrode of the power transistor.

11. The over current protection circuit of claim 9 , wherein one end of the sense resistor is coupled to a common node of the drain electrode of the power transistor and a supply voltage, and the other end is coupled to the drain electrode of the sense transistor.

12. The over current protection circuit of claim 9 , wherein the first current-mirror circuit comprises:

a second transistor with its drain electrode coupled to an output terminal of the current source, with its gate electrode coupled to its drain electrode, and with its source electrode grounded;

a third transistor with its drain electrode coupled to the second current-mirror circuit, with its gate electrode coupled to the gate electrode of the second transistor; and with its source electrode grounded; and

a fourth transistor with its drain electrode coupled to the second current-mirror circuit, with its gate electrode coupled to the gate electrode of the third transistor, and with its source electrode grounded;

wherein the second transistor, the third transistor and the fourth transistor are all NMOS transistors, and their size ratios are substantially the same.

13. The over current protection circuit of claim 12 , wherein the second current-mirror circuit comprises:

a fifth transistor with its drain electrode coupled to the drain electrode of the third transistor, and with its source electrode coupled to the first resistor; and

a sixth transistor with its drain electrode coupled to a common node of its gate electrode and the drain electrode of the fourth transistor, with its gate electrode coupled to the gate electrode of the fifth transistor, and with its source electrode coupled to the drain electrode of the sense transistor;

wherein the fifth transistor and the sixth transistor are both PMOS transistors, and their size ratios are substantially the same.

14. The over current protection circuit of claim 13 , wherein the first transistor is an N type transistor with its drain electrode coupled to the gate electrode of the sense transistor, with its gate electrode coupled to the drain electrode of the fifth transistor, and with its source electrode grounded.

15. A low dropout regulator with an over current protection mechanism comprising:

an NMOS power transistor with its drain electrode coupled to a supply voltage, and with its source electrode coupled to a feedback circuit;

an error amplifier with its non-inverting input terminal coupled to a reference voltage, with its inverting input terminal coupled to the feedback circuit, and with its output terminal coupled to the gate electrode of the power transistor;

a sense transistor configured to sense a current flowing through the power transistor;

a sense resistor coupled to the sense transistor, the sense resistor sharing the same current flowing through the sense transistor;

a current source;

a first current-mirror circuit coupled to the current source so as to form a first part of a first current path and a first part of a second current path;

a second current-mirror circuit coupled to the first current-mirror circuit so as to form a second part of the first current path and a second part of the second current path;

a first resistor coupled to the second current-mirror circuit so as to form a third part of the first current path; and

a first transistor configured to control the power transistor according to voltages across the sense resistor and the first resistor.

16. The low dropout regulator of claim 15 , wherein the sense transistor is an NMOS transistor.

17. The low dropout regulator of claim 16 , wherein the source electrode of the sense transistor is coupled to the source electrode of the power transistor, and the gate electrode of the sense transistor is coupled to the gate electrode of the power transistor.

18. The low dropout regulator of claim 16 , wherein one end of the sense resistor is coupled to a common node of the drain electrode of the power transistor and a supply voltage, and the other end is coupled to the drain electrode of the sense transistor.

19. The low dropout regulator of claim 16 , wherein the first current-mirror circuit comprises:

a second transistor with its drain electrode coupled to an output terminal of the current source, with its gate electrode coupled to its drain electrode, and with its source electrode grounded;

a third transistor with its drain electrode coupled to the second current-mirror circuit, with its gate electrode coupled to the gate electrode of the second transistor; and with its source electrode grounded; and

a fourth transistor with its drain electrode coupled to the second current-mirror circuit, with its gate electrode coupled to the gate electrode of the third transistor, and with its source electrode grounded;

wherein the second transistor, the third transistor and the fourth transistor are all NMOS transistors, and their size ratios are substantially the same.

20. The low dropout regulator of claim 19 , wherein the second current-mirror circuit comprises:

a fifth transistor with its drain electrode coupled to the drain electrode of the third transistor, and with its source electrode coupled to the first resistor; and

a sixth transistor with its drain electrode coupled to a common node of its gate electrode and the drain electrode of the fourth transistor, with its gate electrode coupled to the gate electrode of the fifth transistor, and with its source electrode coupled to the drain electrode of the sense transistor;

wherein the fifth transistor and the sixth transistor are both NMOS transistors, and the size ratios thereof are substantially the same.

21. The low dropout regulator of claim 20 , wherein the first transistor is an N type transistor with its drain electrode coupled to the gate electrode of the sense transistor, with its gate electrode coupled to the drain electrode of the fifth transistor, and with its source electrode grounded.

22. The low dropout regulator of claim 15 , wherein the feedback circuit comprises:

a second resistor with one end coupled to the source electrode of the power transistor, and the other end coupled to the inverting input terminal of the error amplifier; and

a third resistor with one end coupled to the inverting input terminal of the error amplifier, and the other end grounded.