Basic TinySwitch-II and TinySwitch-III Operation
Attention: For the complete description of device functionality and specifications, see the TinySwitch-II and TinySwitch-III data sheets.
TinySwitch-II and TinySwitch-III terminates the primary Switch conduction period when current limit has been reached. As output loading is reduced, the TinySwitch-II and TinySwitch-III will maintain regulation by skipping subsequent conduction cycles. To accomplish this, the EN/UV pin is sampled at the beginning of each conduction cycle. If current out of the pin exceeds the EN/UV pin turnoff threshold current (IDIS, see the appropriate TinySwitch datasheet for the actual value and more information), a ”disable” signal is generated and the conduction period is skipped. If current out of the pin is less than IDIS, an "enable" signal is generated and the primary Switch is turned on for that cycle. This process is repeated at the beginning of each conduction cycle.
The TinySwitch-II and TinySwitch-III both contain an internal state machine that counts sequential disable and enable events. After a certain number of consecutive events occur, either disable or enable, a predetermined transition between one of four operating states is triggered. Each state corresponds to a discrete current limit ranging full from current limit to a very much reduced current limit.
The following example illustrates the operation of the state machine: assume operation in High state (100% current limit). Upon removal of output load, a number of consecutive disable events will trigger a transition from the highest current limit to the Upper Medium current limit. More consecutive disable events will trigger a transition from Upper Medium to Low state. If the load is increased and a number of enable events are recorded, a transition back to Lower Medium state will occur.
Avoid the audible noise problems by reducing the current limit in discrete steps while maintaining a minimum operating frequency coupled with the lower flux density in the transformer.