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MC33063A work under over 100kHz Fosc

Alex Yeh
Expert 5435 points
Other Parts Discussed in Thread: MC33063A

Hi team,

My customer is developing a VFD (VACUUM FLUORESCENT DISPLAY) driver (should be boost application) with MC33063. But they found there are some strange results as below. Could you please comments how to deal with this problem?

1. Original design with MC34063(DIP package) with Ct=220pF, Fosc=88kHz, the VFD brightness is uniformly

2. Change to use MC34063 (SO8 package) with Ct=220pF, Fosc=154kHz, the VFD brightness is nuiformly

3. Use MC33063(QFN package) with Ct=220pF, Fosc=91KHz, the VFD brightness is not uniformly

4. Use MC33063(QFN package) with Ct=120pF, Fosc=143KHz, the VFD brightness become uniformly

 

Customer decide to use MC33063 with QFN package but the key problem is that MC33063/MC34063 only supports the Fosc up to 100kHz.

 

Schematic is here: 

I need your help to clarify what is the causes of this problem and please kindly provide your comments to solve this question.

Thanks!

Cheers,

Alex