Apparatus and method for monotonic start up of a synchronous switching regulator

• US 7,109,688 B1
• Filed: 11/05/2004
• Issued: 09/19/2006
• Est. Priority Date: 11/05/2004
• Status: Active Grant

• Current Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Original Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Inventors: Chiu, Kwok-Fu, Culpepper, Barry James
• Primary Examiner(s): Vu, Bao Q.
• Application Number:
  US10/981,892
• Time in Patent Office: 683 Days
• Field of Search: 323/222, 323/282, 323/284, 323/286
• US Class Current: 323/222
• CPC Class Codes: H02M 1/36 Means for starting or stopping converters
• CPC Class Codes: H02M 3/1588 comprising at least one synchronous rectifier element H02M3/1582, H02M3/1584 take precedence
• CPC Class Codes: Y02B 70/10 Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

A controller for a synchronous switching regulator is arranged to control a switch with a control signal, and to control a synchronous switch with a synchronous switch control signal. The controller disables the synchronous switch control signal at power-up so that the regulator operates with asynchronous rectification. Also, the controller is arranged to detect whether the regulator is in discontinuous or continuous conduction mode by determining whether a switch node voltage at the switch node a switch node stays negative. The regulator keeps operating with asynchronous rectification until the switch node voltage stays negative for the entire off period of the switch, which indicates the regulator is operating in continuous conduction mode. At this point, the controller enables assertion of the synchronous switch control signal, so that the regulator operates with synchronous rectification. The controller is arranged to enable the output voltage to rise monotonically when driving a prebiased load.

Claims

1. A circuit for regulation, comprising:

a pulse modulation circuit that is arranged to provide a pulse modulation output signal based, in part, on an output signal; and

a synchronous switch control circuit that is arranged to provide a synchronous switch control signal such that the synchronous switch control signal is based, in part, on the pulse modulation output signal;

assertion of the synchronous switch control signal is disabled, substantially during a discontinuous conduction mode associated with a switch node current at a switch node; and

such that the assertion of the synchronous switch control signal is enabled, substantially during a continuous conduction mode associated with the switch node current, wherein the pulse modulation circuit and the synchronous switch control circuit are configured to operate with a first switch circuit and a synchronous switch circuit coupled together at the switch node to enable regulation of the output signal.

2. The circuit of claim 1, wherein the synchronous switch control circuit is configured to determine whether the switch node current is associated with the discontinuous or the continuous conduction mode by determining whether a voltage at the switch node is greater than approximately zero approximately while the first switch circuit is substantially closed open.

3. The circuit of claim 1, wherein the synchronous switch control circuit includes:

a comparator circuit that is arranged to perform a comparison based, in part, on a comparison input voltage that is based on a voltage at the switch node.

4. The circuit of claim 1, wherein the synchronous switch control circuit includes:

a comparator circuit that is arranged to compare a comparison input voltage that is based on a voltage at the switch node with a substantially zero voltage, and further arranged to provide a comparison output signal based on the comparison.

5. The circuit of claim 4, wherein the synchronous switch control circuit further includes:

a timing window logic circuit that is configured to provide a CCM signal that is based, in part, on the comparison output signal; and

further based, in part, on the pulse modulation output signal, such that the CCM signal is asserted, approximately when the switch node current is associated with the continuous conduction mode.

6. The circuit of claim 4, wherein the first switch circuit includes a first switch transistor that includes a gate, and wherein the synchronous switch control circuit further includes:

a timing window logic circuit that is configured to provide a CCM signal that is based the comparison output signal, and further based, in part, on the pulse modulation output signal, such that the CCM signal is asserted if a voltage that is associated with the voltage at the switch node is less than approximately zero during substantially a time that a voltage at the gate of the switch transistor corresponds to an unasserted logic level for a cycle.

7. The circuit of claim 6, wherein the synchronous switch control circuit further includes:

a switch logic circuit that is arranged to provide the synchronous switch control signal based, in part, on the CCM signal and the pulse modulation output signal.

8. The circuit of claim 6, further comprising:

a first switch driver circuit that is arranged to provide the voltage at the gate of the first switch transistor based on a first switch control signal, wherein the first switch control signal is based, in part, on the pulse modulation output signal; and

a synchronous switch driver circuit that is arranged to drive the synchronous switch circuit based on the synchronous switch control signal.

9. A circuit for regulation, comprising:

a comparator circuit that is arranged to compare a comparison input voltage that is associated with a voltage at a switch node to a ground voltage; and

a synchronous switch logic circuit that is arranged to provide a synchronous switch control signal to a control input of a synchronous switch circuit such that the synchronous control signal is based, in part, on an output signal; and

further based, in part, on the comparison, wherein the comparator circuit and the synchronous switch circuit are configured to operate with a first switch circuit and the synchronous switch circuit coupled together at the switch node to enable regulation of the output signal.

10. The circuit of claim 9, further comprising:

a pulse modulation circuit that is arranged to provide a pulse modulation output signal based, in part, on the output signal.

11. The circuit of claim 10, wherein the synchronous switch logic circuit includes:

a timing window logic circuit that is configured to provide a CCM signal based on the comparison, and further based, in part, on the pulse modulation output signal, such that the CCM signal is asserted, approximately when the switch node is in continuous current mode.

12. The circuit of claim 10, wherein the synchronous switch logic circuit includes:

a timing window logic circuit that is configured to provide a CCM signal that is based the comparison output signal, and further based, in part, on the pulse modulation output signal, such that the CCM signal is asserted if a voltage that is associated with the voltage at the switch node is less than approximately zero during substantially a time that a voltage at the gate of the switch transistor corresponds to an unasserted logic level for a cycle.

13. The circuit of claim 12, wherein the synchronous switch logic circuit further includes:

a switch logic circuit that is arranged to provide the synchronous switch control signal based, in part, on the CCM signal and the pulse modulation output signal such that assertion of the synchronous switch control signal is enabled or disabled based on the CCM signal.

14. A method for starting up a synchronous switching regulator, comprising:

when the synchronous switching regulator is powered up, disabling an assertion of a synchronous switch control signal, wherein the synchronous switch control signal is employed to control a synchronous switch of the synchronous switching regulator;

determining whether the synchronous switching regulator is operating in continuous conduction mode; and

if the synchronous switching regulator is operating in continuous conduction mode, enabling the assertion of the synchronous switch control signal.

15. The method of claim 14, wherein determining whether the synchronous switching regulator is operating in continuous conduction mode includes:

determining whether a voltage at a switch node is greater than approximately zero approximately while the switch circuit is substantially open.

16. The method of claim 15, wherein determining whether the voltage at the switch node is greater than approximately zero while the switch circuit is substantially open includes:

determining whether a voltage at a current limit node is greater than approximately zero approximately while the switch circuit is substantially open.

17. The method of claim 15, comprising:

providing a pulse modulation output signal based, in part, on an output signal; and

providing, to a first switch, a first switch control signal based, in part, on the pulse modulation output signal, wherein first switch is coupled to the switch node, and wherein the synchronous switch is coupled to the switch node.

18. The method of claim 17, wherein determining whether the voltage at the switch node is greater than approximately zero while the switch circuit is substantially open includes:

comparing a ground voltage with a voltage that is based on the voltage at the switch node;

asserting a comparison signal if the voltage that is based on the voltage at the switch node is greater than the ground voltage;

determining whether at least one timing condition is satisfied;

asserting an intermediate signal if the comparison signal is asserted and the at least one timing condition is satisfied;

de-asserting the intermediate signal if at least one other timing condition is satisfied; and

asserting the CCM signal if the intermediate signal is not asserted substantially during a rising edge of the pulse modulation output signal.

19. The method of claim 18, wherein determining if at least one timing condition is satisfied includes determining if the pulse modulation output signal is low after the pulse modulation output signal begins oscillating.

20. The method of claim 18, wherein the first switch includes a first switch transistor including a gate, and wherein determining if the at least one other timing condition is satisfied includes determining if a falling edge occurs in a voltage of the gate of the first switch transistor.