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I'm trying to design a PCB including a TPS5450 step down converter.

For the boost capacitor, the datasheet asks for a "low ESR ceramic capacitor":

7.3.5 Boost Capacitor (BOOT) Connect a 0.01-μF, low-ESR ceramic capacitor between the BOOT pin and PH pin. This capacitor provides the gate-drive voltage for the high-side MOSFET. X7R or X5R grade dielectrics are recommended due to their stable values over temperature

But what does that mean exactly? Below how many ohms should the ESR be? (I suppose at the switching frequency, i.e. 500 kHz)

And then, how do I find a corresponding capacitor, knowing that most datasheets I looked into had no information about ESR?

Or does it just mean that I shouldn't use electrolytic capacitors but that any MLCC is fine?

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2 Answers 2

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Most MLCCs will be fine, though of course there's the voltage derating to deal with. They will typically have ESRs of under 10 mOhm, where electrolytics will typically be much higher.

In this case the datasheet specifies the capacitor type, dielectric and value, so presumably they are already taking the voltage derating into account.

If you use capacitors from Murata they have an online tool called SimSurfing that will give you the ESR as well as a bunch of other data on their MLC capacitors. Other manufacturers may have similar tools, but I think Murata's is the most comprehensive.

If your datasheet specifies the dissipation factor, dissipation factor is the ratio of ESR over the capacitive reactance at the test frequency (which may not be close to the frequency of interest, but it gives an idea or approximation of the ESR).

For a bootstrap capacitor I don't think twice about picking a general purpose X5R/X7R ceramic capacitor from a quality vendor. I'd probably make sure the voltage rating is at least 2X the gate drive voltage.

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  • \$\begingroup\$ Thanks. Good to know that I can just pick a general purpose X7R. I had a quick look on SimSurfing for one of the capacitors in stock at LCPCB : GCM188R71H103KA37D. Do I understand right that I have to look at the "R" curve (résistance vs frequency)? Is so, ESR is only 0.08 ohms at a few MHz, but already 0.5 ohms@500kHz (my switching frequency), and 100 ohms at 1kHz : for me, its looks more like the norm of impedance than the ESR. \$\endgroup\$
    – Sandro
    Commented Jan 4, 2022 at 22:27
  • \$\begingroup\$ @Sandro Yes, theoretically the R curve should be the ESR vs frequency, but the waveform applied to the cap isn't sinusoidal so it's not so straightforward. However you can be confident that a general purpose X5R/X7R ceramic cap with correct voltage rating will work fine in a bootstrap application. \$\endgroup\$
    – John D
    Commented Jan 4, 2022 at 22:37
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For the boost capacitor, almost any ceramic surface mount capacitor (MLCC) is low ESR. For the dielectric, X7R will suffice and is a common dielectric for capacitors under 0.1uF.

For ESR information, many major capacitor manufacturers have information on ESR for the particular part number. A few sites with ESR and other information:

AVX/Kyocera
Kemet
Murata

You may be able to find similar information on other manufacturer's sites.

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