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UCC28061: TSET pin resistor

Roy Chou1
Genius 13715 points
Part Number: UCC28061
Other Parts Discussed in Thread: UCC28060

Hi Team,

We face an issue using UCC28061. For fig.1 output couldn't startup successfully when RTSET= 100k and could startup successfully when RTSET=150k.

We would like to know

1. Why modify the RTSET could fix the issue?

2. We calculate the TMIN = 2.48 if I use RTET=150k, it meet the spec showing as datasheet. Does it have any risk when using RTSET = 150k?

3. Why original RTSET couldn't power on successfully?

90Vac/47Hz at Full Load ; 

Vin ; CH1: +12V  ; CH2:+24V

90Vac/63Hz at Full Load

Vin ; CH1: +12V  ; CH2:+24V

Regards,

Roy

  • 0
    TI__Guru 73825 points

    Hello,

    RTESET sets the minimum switching period and maximum on time.  A larger resistor would mean a larger maximum on time.  I wonder if during power up you were hitting peak current limit.

    When the power converter starts up both inductors are in phase and will adjust out of phase with a phase lock loop.  If the on time is to long you could trigger an over current condition.  It this is the case reducing your current sense resistor size may have also resolved the issue. 

    There is also and excel design tool at the following link that you can use to check your design.  It is for the UCC28060 but the calculations are the same for the UCC28061.

    Regards,