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TPS560430-Q1: Boostrap resistor

Manuel Marquez
Prodigy 10 points
Part Number: TPS560430-Q1

Tool/software:

We are usign TPS560430YQDBVRQ1 DCDC converter with input voltage of 9V to 16V and output voltage 3,3V and 0,24A output current. Fsw=2,1Meg.
We are facing an EMC problem with the NFA test (Near field antenna) and we would like to evaluate increasing Rboot in series with Cboot.
In the following application note  we see that it is recommended Cboot 100nF and Rboot  75 ohms. However we would like to know if we could increase Rboot and which would be our limit. App note:
We have the following questions:
1.-Which is the Qg of the HS(high side) mosfet?

We understood that with Qg and Tsw we could estimate the Igate, that would be the discharge current of bootstrap cap. And also calculate which is the time of this Tcharge.
For instance Qg=5nC, Tsw=476,2ns-->Tcharge=47,6ns( considering 10% sw)
Igate=5nC/47,6ns=105mA

2.-Does the boostrap cap have another discharge/leakage current that should be taken into consideration apart of Igate of HS mosfet

3.- Which is the min voltage acceptable to be maintained on Cboot?
that means which is the mosfet Vgs voltage threshold?

4.-75ohms Rboot is the max value ? 
how we could calculate which could be the limit?
5.-Do you have some suggestion for ltspice simulation of Cboot, Rboot?
  • 0
    TI__Expert 5610 points

    Hi Manuel,

    1: Sorry, we don't share the Qg of our devices publicly.

    2: No, you can just take into account the powering of the gate of the HS FET during turn on.

    3/4. Let me talk to my team about this and I will get back to you as soon as I can.

    5.For simulation you can just use a standard cap/resistor model.

    Thanks,

    Dino Maslic

  • 0 in reply to Dino Maslic
    TI__Expert 5610 points

    3. We do not share the VGS of our devices publicly. 

    4. Please see this application note if you'd like more info on choosing a boot resistor, titled "Design Consideration on Boot Resistor in Buck Converter". Datasheet does not recommend boot resistor so not going to find much info on it there.

  • 0 in reply to Dino Maslic
    Prodigy 10 points

    Without information of Qg and other internal parameters of mosfet it is not feasible to calculate Rboot.  We need to know tcharge that depends on internal capacities and with Qg we could estimate Icharge. This value is needed to know how capacitor should be discharge. 

  • 0 in reply to Manuel Marquez
    TI__Expert 5610 points

    Hi Manuel,

    That information is not something that we share on a public forum.

    If it is necessary for your design, we recommend you reach out to your local TI representative.

    Best Regards,

    Dino Maslic