UCC28950-Q1: UCC28950-Q1 Burst mode to Shutdown mode

Hello Kura

The controller duty cycle is controlled by the  voltage on the COMP pin, which of course is part of the feedback network. it's useful to think of the voltage at the COMP pin as being a current demand signal. As the load current decreases from full load it passes the DCM threshold (set by the user) at which point the SR drives are stopped and rectification is through the body diodes of the SR. As the load is reduced further the output inductor current goes discontinuous. Up to this point the converter duty cycle is held constant because in continuous conduction mode Vo = N * D * Vin where N is the transformer turns ratio. As the load is reduced even further the duty cycle starts to reduce and eventually it reaches the TMIN threshold (also set by the user).  The controller outputs (OUTx) are set to zero and so will not output an on time of less than TMIN - this means that TMIN is the minimum on time applied to the transformer. If the load is reduced further past the TMIN threshold then the OUTX outputs are turned off (set LO). This means that there is no energy transfer into the secondary and the result is that the output load causes the output voltage to fall. This then propagates through the feedback loop and the COMP voltage increases - to demand more current. When the COMP pin reaches a level that corresponds to TMIN the OUTx drives are turned on again. Given that the load is unchanged, the output voltage increases as a result of the OUTx drives being turned on. As Vout exceeds its regulation setpoint the COMP pin drops below the TMIN threshold again and the OUTx outputs are turned off. This process repeats until such time as the load increases enough so that continuous operation at duty cycle corresponding to TMIN or greater becomes necessary. In order to help avoid transformer saturation the controller logic always applies an even number of pulses to the transformer during a burst. Yhe length of each burst is not really predictable neither is the interval between bursts and you will find that the burst activity is apparently chaotic. The only thing that can be said is that, in general, as the output load decreases the length of each burst reduces and the interval between bursts increases.

The use of the term 'shutdown' in the data sheet is probably a bit misleading - it refers only to the fact that the OUTx outputs are set to LO. There is no decrease in ICC during this 'shutdown' phase - other than that due to the absence of gate drive currents.

I hope that this answers your question - if it does not then please repost.

Regards
Colin

Hello Kura-san

You are completely correct. The device continues to run in burst mode until the load increases enough to make continuous operation necessary to maintain regulation. There is no shutdown mode associated with burst mode. As the load decreases the burst length typically gets shorter and the interval between bursts gets longer, but as I said the length of individual bursts and the interval between them is not well controlled.

The only shutdown modes in the device are UVLO and Thermal shutdowns.

Regards

Colin