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I have a piece of test gear I am working on designing, in which there I need to drive a number of N channel mosfets, (3 high side, 2 low side)

I found there is a very large range of gate drivers intended for driving 3 phase motors, and thought this would be a good option as it would provide the drivers I need in a cheap, & space efficient package.

The design I ended up using was the Allegro A4919GETTR-3-T to drive NTMFS4C029N mosfets.

I am roughly aware of how the bootstraping circuit works, and the limitations it places on maximum on time. However I expected be able to generate longer than I am. I would like to be able to generate pulses at least 200ms long, but with the current setup only seem to be able to manage about 5ms before the Bootstrap voltage monitor kicks in.

I am running with a 15V supply, and my bootstrap capacitors are 0.1uF

Searching through the datasheet I did find "GHx Passive Pull-Down Resistance" specified at 400kΩ this seems pretty low to me but would potentially explain my problem.

I assume this property is intrinsic to the device and cant be changed, is there anything I can do to increase my maximum on time.

The only thing I can think of is increasing my bootstrap capacitor size(and correspondingly Vreg capacitor). However I don't know what controls the sizes I can use, can I just throw a 20uF capacitor in and call it a day or will this fry the bootstrap diode.

As a further down the road backup option (requiring a PCB redesign) what factors would need to be considered before switching to something like a TMC6200 with an integrated charge pump to allow 100% on time operation.

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  • \$\begingroup\$ Low side PWM must exceed frequency where Z(cap) << R \$\endgroup\$ Sep 17 '19 at 8:55
  • \$\begingroup\$ (From memory): The datasheet provides explicit formulas for calculating bootstrap capacitors and holdup time. What results do you get when you use these as a design guide? \$\endgroup\$
    – Russell McMahon
    Sep 19 '19 at 10:45
  • \$\begingroup\$ I already went larger than the design guide, but couldn't see anything in the notes about holdup time. I suppose I want to know how high I can go on the capacitance without damaging the bootstrap diode. \$\endgroup\$
    – Hugoagogo
    Sep 19 '19 at 10:58
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Before increasing the size of the caps I would check their capacitance at the voltage you are using them. In several cases, the change in the capacitance values with the bias DC voltage can be massive! Check this page to get a better idea of the phenomena.

Now if that is not the issue, I think that increasing the size of the capacitor will be the most reasonable option. If you are afraid of burning the bootstrap diode, you can include a small resistor in series with the cap. I have seen some designs with this modifications and they work fine.

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If you want to generate 200 ms pulses, I would suggest two things:

  1. Use TLP350 type of optocoupler with isolated power supply with each optocoupler. In this way you can generate longer pulses. Because it works in a different way.

  2. Use low ESR capacitors like tantalum capacitors connected in parallel with your current setup. You can use 10uF 16 volt capacitors in parallel.

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