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UC2842: UC2842A Minimum RT resistor

Sulyn ZHANG
Expert 8700 points
Part Number: UC2842
Other Parts Discussed in Thread: , UC2843A, UC2844A, UC2845A

Hi Team,

We are penetrating our UC2842 in my customer and customer plan to evaluate, but they have a question about selecting the RT/CT. In our datasheet, we mentioned that RT should be >=5kohm:

But due customer want to limit the duty cycle, so they want to use RT=680ohm, so is these some issue by using RT=600ohm? Thanks.

Best regards,

Sulyn Zhang

  • 0
    TI__Expert 8700 points

    Hi Team,

    Sorry that it should be UC2842A, not UC2842. And in our UC2842A's datasheet, we have below test specific:

    Customer want to limit the duty cycle, so they want to use RT=680ohm, is these some problem by using RT=680ohm?

    Best regards,

    Sulyn Zhang

  • 0 in reply to Sulyn ZHANG
    TI__Expert 8700 points

    Sorry, attach the picture:

    Best regards,

    Sulyn Zhang

  • 0 in reply to Sulyn ZHANG
    TI__Genius 9130 points
    Sulyn,

    First of all, what is the max duty cycle limit that the customer requires?

    The UC2842A & UC2843A devices are capable of 100% Dmax (or can be programmed for less depending on RT/CT values). But it is not practical to programme max duty cycle <50% with these devices.

    If the required Dmax is < 50%, then it would be better to use either the UC2844A or UC2845A - these are guaranteed 50% Dmax ( they use an internal toggle flip-flop to skip every second cycel - this give 50% Dmax, but also means that Fsw = Fosc/2).

    Again with these devices UC2844A or UC2845A, the choice of R & C for the RT/CT pin can be used to set Dmax less than 50%.


    The graphs in Figs. 6 & 7 show how the Dmax and Fosc vary with RT/CT values, and they show the results even for small Rt down to ~500 ohm. However, the eqn Fosc = 1.72 / (R * C) is only valid for RT >= 5 k, hence the recommended min value for Rt is 5k.


    I hope this helps answer your question, if so, please click the "verify answer" button.

    Thanks,
    Bernard
  • 0 in reply to Bernard Keogh
    TI__Expert 8700 points
    Bernard,
    Many thanks for the support! One question is if Customer use our UC2842 and use RT=680ohm, is these any problem? If the eqn Fosc = 1.72 / (R * C) is only suitable for >=5kohm, is these some eqn suitable for RT=680ohm? Thanks!

    Best regards,
    Sulyn Zhang
  • 0 in reply to Sulyn ZHANG
    TI__Genius 9130 points
    Sulyn

    For UC2842, the datsheet says "never" use RT < 5 k. The osc timing graph in Fig. 10 shows RT >= 5 k always, so it's not allowed to use RT < 5 k.


    As I asked above, what Dmax does the customer require? Why are they using RT = 680 ohm?


    Thanks,
    Bernard
  • 0 in reply to Bernard Keogh
    TI__Expert 8700 points

    Hi Bernard,

    Thanks a lot for your suggestion. Customer want to use duty cycle about 40%~50%, and they tried to use the RT=680 ohm, it works well. So they want to know what's the risk or issue will be with the 680ohm RT? Can you help let me under this, thanks a lot!

    BTW, UC2844A don't have the SOIC-16 package and customer's PCB already fixed, so they have to use UC2842A.

    Best regards,

    Sulyn Zhang

  • 0 in reply to Sulyn ZHANG
    TI__Genius 9130 points
    Sulyn,

    I will enquire about availability of UC2844ADW in wide-body SOIC16, I believe that this is an orderable part number, but let me confirm.

    I will also check inside the product line about using UC2842A with RT = 680-ohm.

    Thanks,
    Bernard
  • 0 in reply to Bernard Keogh
    TI__Genius 9130 points

    Sulyn,

    I had a look in more detail, and unfortunately, the UC2844A is not available in the wide-body SOIC16 DW package. Also, there are no versions of UC284x (non-A version) in that package.

    So the choice of DW package is a big limitation.

    One possibility is the UC2845ADW, can the customer use this? This has 50% Dmax limit already, and is very similar to UC2844A – the only difference is the UVLO thresholds on VCC. Turn-on is 16 V typ for UC2842A/44A, but it is 8.4 V typ for UC2845A. Similarly, turn-off threshold is different, 10 V for UCC2842A/44A, and 7.6 V for UCC2845A.

    If the customer can use the UC2845A, this is the preferred solution. This device can limit to 40-50% Dmax with RT > 5 k.

    The second option is to modify the customer PCB to change the package to either SOIC14 (D package) or SOIC8 (D8 package). This may be difficult, but the benefit is that there is a much bigger choice of pin-compatible controllers in these packages.

    Final option is to use the UC2842ADW as you already have with RT = 680-ohm to limit duty cycle. This value of RT is not ideal, and is well below the recommended min value of 5 k.

    But it should not cause any damage or over-stress to UC2844A.

    However, as can be seen by the way the curve in figure 6 goes non-linear for Rt < 1 k, this indicates that the fixed value external resistor is no longer dominating the frequency setting in that range, internal parasitics and limits are becoming dominant. With low RT value < 5 k, frequency tolerance over lot, device and temperature variation may not be as good as with the recommended RT range > 5 k.

    I hope this helps answer your question, if so please click the "verify answer" button.

    Thanks,

    Bernard

  • 0 in reply to Bernard Keogh
    TI__Expert 8700 points
    Hi Bernard,
    Many thanks for your great support!

    Best regards,
    Sulyn Zhang