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TPS62136: Using 5.6 uH inductor

Antenna_Head
Genius 12650 points
Part Number: TPS62136

Are there any problems using 4.7 or 5.6 uH inductance with the TPS62136?  These values are what is readily available.

The datasheet recommends 2.2 uH.  I know this is a DCS mode device and want to make sure there is enough ripple current for the part to modulate.

The application is an inverting buck boost with a secondary winding providing an isolated supply.  The device is operated in forced PWM mode by pulling MODE pin high.

Asking on behalf of a customer.

Thanks, Best, Steve

  • 0
    TI__Mastermind 26660 points

    Hi Steve,

    I would stick with the datasheet recommended values of 1.5uH/47uF to start with. For an IBB design, increasing inductance will lower the RHPZ and cause instability. It is recommended to have crossover frequency much lower than the RHPZ - you can achieve lower crossover is by increasing output capacitance. However, since you already have a bigger inductor, you may not be able to increase output cap by a whole lot as the resulting power stage will be harder to compensate with the internal zeros of the DCS control. It might be possible to deviate a little bit on the LC combination but this has not been tested especially for an IBB. Hope this helps. Please let me know if you have any more questions.

    Regards,
    Amod 

  • 0 in reply to Amod Vaze
    TI__Genius 12650 points

    Attached is a TINA simulation of the inverting portion of this concept. 

    I put a 100% step load at t=1.1ms.  I do not see any ringing that would indicate instability or other cause for concern.

    Do you have any deeper insights that would invalidate this approach?  We need to use the 4.7 uH or 5.6 uH inductor values that are available in the factory.

    Thanks, Best, Steve

    TPS62136 IBB.TSC

  • 0 in reply to Antenna_Head
    TI__Mastermind 26660 points

    Steve,

    There are really no other issues except stability with the new LC corner frequency. The simulation looks stable. It would be a good idea to evaluate on bench as well. I can probably reconfigure the EVM and check it out in the lab but it will be next week before i can do that. Let me know if that is something you are interested in.

    Regards,
    Amod 

  • 0 in reply to Amod Vaze
    TI__Genius 12650 points

    Amod-

    An empirical bench experiment using a sample of one would be interesting, but I am worried it wouldn't tell us much about how such a design would do in mass production across process variation.

    If the LC corner location is really so crucial, in the actual application having a secondary isolated output there would be no problem reducing C by the same amount L is increased.  The ripple on the Inverted voltage doesn't actually matter.  Energy is coupled to an isolated domain through a winding on the inductor, rectified, and the output capacitor is located there.  (this is actually a fly-buck-boost application)

    What I'm really looking for is some engineering justification why this is a bad idea.  I understand the part wasn't characterized this way, but that doesn't mean it wouldn't work well.

    If we could show that this is a reliable thing to do, I believe it would be a selling point for this part and helpful to many other customers.

    -Steve

  • 0 in reply to Antenna_Head
    TI__Mastermind 26660 points

    Steve,

    Apologies for the delayed response and thanks for your feedback and inputs.

    There are no closed loop equations that can guide here on the inductance choice and if it will be a problem to pick 4.7uH for this design with process variations. The 4.7uH design will need to have sufficient margin for stability parameters like phase and gain margin using a bode plot response. In addition to a single working design (experimental on bench), doing worst case/monte carlo sims may give some insight into the operation and feasibility. 

    Another alternative if the customer is open to use a newer device that is smaller and more efficient. It is a peak current mode part so a higher value of inductance can be used. Please let me know about this and I can provide more details.

    Regards,

    Amod