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LMG1210EVM-012: -200V operation

Saiga
Prodigy 140 points
Part Number: LMG1210EVM-012
Other Parts Discussed in Thread: LMG1210

If I give -200V at input i.e. V(HS) will be -200 and PGND will be connected to GND.

In this case I will need to remove the bootstrap diode and give V(BST) from an external isolated supply

1)What should the value of V(BST) be?From datasheet 5V should be fine

2)Can V(BST) be continuous of should I control it externally so that it turns on when low side driver is on?

3)If I need to switch V(BST)  externally can I use the LO output signal to control a FET?

4)What are the other ways of controlling V(BST)?

  • 0
    TI__Genius 12990 points

    Hi Saiga,

    Thanks for reaching out on e2e about LMG1210.

    You will actually need to remove the bootstrap diode like you said however apply the floating supply to HB not BST. BST can remain floating at 5V since it is connected to the LDO output through a Vds BST switch resistor. HB is the source of the high-side drive just like VDD is the source of the low-side drive so the floating supply needs to be connected to HB. BST is a bootstrap switch that turns on during low-side on time and helps to prevent the bootstrap cap from overcharging above 5V by not being on during dead-times when the HS node goes most negative. Since the floating supply does not need a bootstrap it can be continuously replenishing the HB-HS cap. The supply may need a current limiting resistor or a zener diode to keep the HB-HS voltage under control. Section 7.3.1 and section 7.3.6 from the LMG1210 datasheet describe this in more detail. Let me know if this helps answer your questions and if you have any more.

    Thanks,

  • 0 in reply to Jeffrey Mueller
    Prodigy 140 points

    Hi Jeff thanks for the quick reply

    I went through sections 7.3.1 and 7.3.6 in the datasheet but I still have a few questions regarding the current limiting resistor or the diode as you suggested

    1)In case I am using a current limiting resistor , one end will connect to isolated 5V and other end to HB pin.

    In this case how do I decide the max current I have to limit it to,I didnt understand from the datasheet

    2)In case of zener, should Vz be 5V and in which direction should I connect anode and cathode

    What should max Iz value be

    and do i need a current limiting resistor in case of zener also?

  • 0 in reply to Saiga
    TI__Genius 12990 points

    HI Saiga,

    Thanks for your follow up questions,

    For an example schematic of a isolated supply with the option of limiting the current with a series resistor (R6/R7).Check out this tida -

    Also for deciding the max current to limit to check out section 3.4 from this bootstrap app note - the app note refers to the bootstrap however the current limiting resistor operates similarly. 

    Adding a zener is not always necessary and is typically only needed when over/under-shoot happens. A current limiting resistor can be used with out a zener like in the case of the tida. The zener should have a sharp knee for low currents. The current through the limiting resistor should be more than the worst case leakage current to keep the HB-HS cap charged and is no more than a few mA. The same goes for the current though the zener, a few mA or less of breakdown current should be all that is needed to keep HB at 5V.

    let me know if this helps answer your question or you have any more.

    thanks,