System and method for controlling synchronous switch for a synchronous converter

1. An apparatus, comprising:

a synchronous converter for providing a regulated output voltage responsive to an input voltage, a control PWM signal to a control switch of the synchronous converter and a synchronous PWM signal to a synchronous switch of the synchronous converter; and

a first circuit for generating the control PWM signal and the synchronous PWM signal responsive to a PWM control signal, wherein the first circuit limits a maximum duty cycle of the synchronous PWM signal to a predetermined level, wherein the first circuit further comprises:

a transistor for enabling provision of the synchronous PWM signal at a first logical level when the transistor is in an ON state and for enabling provision of the synchronous PWM signal at a second logical level when the transistor is in an OFF state;

an AC coupling circuit for controlling the ON state and the OFF state of the transistor responsive to the PWM signal; and

a second circuit for generating the synchronous PWM signal at a first logical level that turns off the synchronous switch of the synchronous converter during startup of the synchronous converter to maintain the synchronous converter in a diode mode of operation.

2. The apparatus of claim 1 , wherein upon completion of the startup, the second circuit further controls the synchronous PWM signal to increase the synchronous PWM signal duty cycle from 0 to a steady state value to enable a transition for the diode mode of operation to a synchronous mode of operation responsive to a soft start signal.

3. The apparatus of claim 2 , further including a single ended PWM controller for generating the PWM control signal and the soft start signal.

4. The apparatus of claim 2 , wherein the second circuit further comprises:

a first transistor for enabling provision of the synchronous PWM signal at a first logical level when the transistor is in an ON state and for enabling provision of the synchronous PWM signal at a second logical level when the transistor is in an OFF state;

an AC coupling circuit for controlling the ON state and the OFF state of the first transistor responsive to the PWM signal; and

a second transistor for maintaining the first transistor in the ON state when the second transistor is in an ON state responsive to the soft start signal and for providing control of the first transistor to the AC coupling circuit in an OFF state.

5. The apparatus of claim 1 , wherein a discharge time of the AC coupling circuit controls a length of the ON state of the transistor.

6. The apparatus of claim 5 , wherein the discharge time is controlled responsive to at least one capacitive value and one resistive value in the AC coupling circuit.

7. A circuit for controlling operation of a synchronous converter, comprising:

a first input for receiving a PWM signal;

a second input for receiving a soft start control signal;

a first circuit for generating a control PWM signal and a synchronous PWM signal responsive to the PWM signal, wherein the first circuit limits a maximum duty cycle of the synchronous PWM signal to a predetermined level, wherein the first circuit further comprises:

a transistor for enabling provision of the synchronous PWM signal at a first logical level when the transistor is in an ON state and for enabling provision of the synchronous PWM signal at a second logical level when the transistor is in an OFF state; and

an AC coupling circuit for controlling the ON state and the OFF state of the transistor responsive to the PWM signal; and

a second circuit for generating the synchronous PWM at a first logical level that turns off the synchronous switch of the synchronous converter during startup of the synchronous converter to maintain the synchronous converter in a diode mode of operation.

8. The circuit of claim 7 , wherein upon completion of the startup the second circuit further controls the synchronous PWM signal to increase the synchronous PWM signal duty cycle from 0 to a steady state value to enable a transition for the diode mode of operation to a synchronous mode of operation responsive to a soft start signal.

9. The circuit of claim 8 , wherein the second circuit further comprises:

a first transistor for enabling provision of the synchronous PWM signal at a first logical level when the transistor is in an ON state and for enabling provision of the synchronous PWM signal at a second logical level when the transistor is in an OFF state;

an AC coupling circuit for controlling the ON state and the OFF state of the transistor responsive to the PWM signal; and

a second transistor for maintaining the first transistor in the ON state when the second transistor is in an ON state responsive to the soft start signal and for providing control of the first transistor to the AC coupling circuit in an OFF state.

10. The circuit of claim 7 , further including a single ended PWM controller for generating the PWM signal and the soft start control signal.

11. The circuit of claim 7 , wherein a discharge time of the AC coupling circuit controls a length of the ON state of the transistor.

12. The circuit of claim 11 , wherein the discharge time is controlled responsive to at least one capacitive value and one resistive value in the AC coupling circuit.

13. A method for controlling startup of a synchronous converter, comprising the steps of:

initiating start up of the synchronous converter;

turning off a synchronous switch of the synchronous converter;

starting the synchronous converter in a diode mode of operation;

determining if a start up process of the synchronous converter is completed;

transitioning the synchronous converter from the diode mode of operation to a synchronous mode of operation;

providing a synchronous PWM signal at a first logic level responsive to an ON state of the synchronous switch in the synchronous mode of operation;

providing the synchronous PWM signal at a second logic level responsive to an OFF state of the synchronous switch in the synchronous mode of operation;

controlling the ON state and the OFF state of the synchronous switch in the synchronous mode of operation responsive to the synchronous PWM signal; and

generating the synchronous PWM signal at a first logical level that turns off the synchronous switch of the synchronous converter during startup of the synchronous converter to maintain the synchronous converter in a diode mode of operation.

14. The method of claim 13 wherein the step of transitioning further comprises the step of increasing a duty cycle of the PWM switch from 0 to a steady state value over a predetermined period of time.