Inverting vs NonInverting Mosfet Driver
in [Design]
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From: Joerg on 11 Feb 2010 12:13 legg wrote: > On Tue, 09 Feb 2010 18:18:54 -0800, Joerg <invalid(a)invalid.invalid> > wrote: > > <snip> >>>>> You have to test drive any prospective part, by any mfr, in-circuit. >>>>> Duals and non-inverters are more prone to misbehaviour. Low voltage >>>>> logic level inputs are a mistake to be avoided, wherever possible, >>>>> even with a 'ground plane'. Some parts are even sensitive to output >>>>> disturbances, never mind ground bounce on the input, regardless of >>>>> sales blurbs or specsmanship. >>>>> >>>>> For non-inversion, bypass Micrel MIC4424 parts, if you want to avoid >>>>> grey hair. Similar lower-powered Maxim parts MAX4427A or Micrel TC4427 >>>>> seemed OK, although I recall a lack of internal UVLO, which required >>>>> vigilance. >>>>> >>>> Never had a problem with Micrel MOSFET drivers. What caused the gray hair? >>> Basically, I had opposing mosfet outputs turning on by themselves on >>> the low side of a full bridge driver. The duration of conduction could >>> be reduced but not eliminated with agressive supply decoupling. The >>> input of the offender scoped slightly negative during the entire drive >>> fault period, following a positive glitche of 100nS duration, >>> possibly generated by it's partner. No other pin-compatible part >>> behaved this way, in the same physical position. >>> >> That almost sounds like a damaged chip. I've mainly used the 4421 but >> AFAIK they are all the same architecture. Ok, they aren't really >> shoot-through protected but they ran nice and cool at a few hundred kHz. >> They do need a really stiff supply with two planes and good X7R caps, >> else all hell can break loose. It also does if you hang too big a gate >> capacitance onto it, which I guess is why they also make 6A, 9A and 12A >> devices. >> > This was not a damaged chip - substitutions and iterations in > documentatiomn of the fault established this. All outputs were only > required to drive the gate limiting resistor, which dominated gate > current control in an assisted switching circuit, where the big fet in > question had it's active capacitances discharged in advance by an > external switch. > > I suspect it might have been the reverse transfer current hitting the > output, before it was required to be active high, that scrambled > adjacent internal logic of it's partner, somehow, but that is just > speculation. I wasn't prepared to rip apart the entire topology in > order to investigate further, with functional substitutes on-hand. Sorry to hear that, it's really strange. I have used Micrel drivers a lot and they always delivered. Typically sans gate resistor because I like to drive FETs with gusto where permitted. >>> In it's 'representative schematic', the 4424 input is depicted as >>> analog, with some kind of current hysterisis introduced to the signal, >>> at the receiver's output, which is just plain barmy, IMHO. >>> >> It actually works, a few hundred mV hysteresis. Not barmy :-) > > It obviously doesn't work well enough to be specified in the part's > datasheet, as such. The concept should have been buried on that basis > alone. > Well, at least they state in in the text: http://www.micrel.com/_PDF/mic4423.pdf Quote "Following the input stage is a buffer stage which provides ~400mV of hysteresis for the input, ..." -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |