UCC29950: Pin Configuration

Hello Cagatay

The first question I would ask is how do you plan to get the system to start ? - if you don'd use the start up FET then you will need a small separate bias supply to supply power to the IC.

If you are not using the start-up FET then you are correct to leave SUFG open. SUFS supports an X-CAP discharge function in both Self Bias and Aux Bias modes. If you are not using this feature then you should tie SUFS to VCC.

If you want to use the device in Aux Bias mode then the MD_SEL pin should be kept below VMODE_SELSB (1.1V) for TMODE_SELSB (10ms) after VCC is applied. You have tied this pin to VCC so the device will start up in Self Bias mode. You should be able to select Aux Bias mode with a simple RC at the MD_SEL pin, choose R and C to meet the V and timing limits above. For testing it is also easy to hold MD_SEL pin low with a mechanical switch.

You can see how this pin can be used in the schematic for the UCC29950 ECM

Please let me know if you need any more information.

Regards
Colin

Hello Colin,

Firstly, thanks for the information which gave you about configuration. I just need one final step to understand. I used auxiliary winding(self bias mode) from LLC transformer, then I rectified and regulated it to +15V. Is this enough for supply IC without start-up FET or any other external control signal for MDSEL pin? I also connected MDSEL pin to +15V via a resistance.

Hello Cagatay

If you want to use the device in self bias mode then you are correct to tie MD_SEL to VCC - this can be a hard connection, you don't need a resistor.

In Self Bias mode the device assumes that there is a Start-Up FET in place - as shown in the simplified schematic on page 1 of the DS.

Please feel free to send me your schematic - I'm at colingillmor@ti.com and I can review it to make sure you have the device configured correctly.

Any information you send me will of course be treated as company confidential.

Regards

Colin