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I am currently looking at the LTC4444 MOSFET driver.

It states that the rise and fall time of the gate driver for 1nF load. Is this 1nF load the Ciss value or Qg/Vgs of the MOSFET?

Thanks

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2 Answers 2

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It is the Cgate of the driven load, in your case the gate capacitance.

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  • \$\begingroup\$ So would that be Qg/Vgs? \$\endgroup\$
    – Daniel
    Oct 26, 2012 at 22:34
  • \$\begingroup\$ Not sure why you are using such odd terminology, dimensionally what have written is units of Farads but Gate Capacitance is more than just that calculation. As far as the driver is concerned it cares about the apparent capacitance/load that it must drive. So yes, that is AN element of the Cload. \$\endgroup\$ Oct 26, 2012 at 22:59
  • \$\begingroup\$ What values from the MOSFET datasheet do I need to fully calculate the total gate capacitance then? \$\endgroup\$
    – Daniel
    Oct 26, 2012 at 23:04
  • \$\begingroup\$ It shouldn't be that sensitive, my comments about were more of a comment on clarity. Keep in mind that Gate capacitance is non-linear and the conditions for quasi-static conditions may not be met (discrepancy between measurement and use) so just use the maximum gate capacitance from the data sheet to be safe. \$\endgroup\$ Oct 26, 2012 at 23:25
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The data sheet will list the gate capacitance of a MOSFET. This is also one of the prime parameters in most parametric search engines online (for example, Digi-Key search.)

The symbol for gate capacitance is typically "Cgate" or "Cg" or "Ciss" (for "input capacitance.")

If you look at a typical MOSFET data sheet, you will see the gate capacitance listed. For small-signal discrete MOSFETs, it can be as low as 10 pF; for large power MOSFETs, it can be as high as 13000 pF (which is 13 nF.)

For example, the BUK9515-60E has an input capacitance of 2651 pF at 25V. The parametric search for this component can be found here: http://www.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&keywords=BUK9515-60E

The data sheet for this component can be found here: http://www.nxp.com/documents/data_sheet/BUK9515-60E.pdf

The Ciss is listed on page 6/13 of that data sheet.

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