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From: Joerg on 9 Feb 2010 17:34
legg wrote:
> On Mon, 8 Feb 2010 13:01:17 -0800, D from BC <myrealaddress(a)comic.com>
> wrote:
>
>> I have some fuzzy recollection of reading to the effect that
>> noninverting mosfet drivers are more prone to instability due to layout
>> effects.
>>
>> Confirm?
>>
>> Just doing a quick sim in my head:
>> With poor layout, when a noninverting mosfet driver turns on (sourcing),
>> the signal ground pops up due to the mosfet gate capacitance (iirc
>> called ground bounce?) then the driver sees a valid (actually invalid)
>> '0' threshold to turn the mosfet drive off.. The driver oscillates.
>>
>> Is it best to use a noninverting mosfet driver in a smps design?
>
> 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?

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: legg on 9 Feb 2010 20:59
On Tue, 09 Feb 2010 14:34:33 -0800, Joerg <invalid(a)invalid.invalid>
wrote:

>legg wrote:
>> On Mon, 8 Feb 2010 13:01:17 -0800, D from BC <myrealaddress(a)comic.com>
>> wrote:
>>
>>> I have some fuzzy recollection of reading to the effect that
>>> noninverting mosfet drivers are more prone to instability due to layout
>>> effects.
>>>
>>> Confirm?
>>>
>>> Just doing a quick sim in my head:
>>> With poor layout, when a noninverting mosfet driver turns on (sourcing),
>>> the signal ground pops up due to the mosfet gate capacitance (iirc
>>> called ground bounce?) then the driver sees a valid (actually invalid)
>>> '0' threshold to turn the mosfet drive off.. The driver oscillates.
>>>
>>> Is it best to use a noninverting mosfet driver in a smps design?
>>
>> 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.

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.

RL

From: Joerg on 9 Feb 2010 21:18
legg wrote:
> On Tue, 09 Feb 2010 14:34:33 -0800, Joerg <invalid(a)invalid.invalid>
> wrote:
>
>> legg wrote:
>>> On Mon, 8 Feb 2010 13:01:17 -0800, D from BC <myrealaddress(a)comic.com>
>>> wrote:
>>>
>>>> I have some fuzzy recollection of reading to the effect that
>>>> noninverting mosfet drivers are more prone to instability due to layout
>>>> effects.
>>>>
>>>> Confirm?
>>>>
>>>> Just doing a quick sim in my head:
>>>> With poor layout, when a noninverting mosfet driver turns on (sourcing),
>>>> the signal ground pops up due to the mosfet gate capacitance (iirc
>>>> called ground bounce?) then the driver sees a valid (actually invalid)
>>>> '0' threshold to turn the mosfet drive off.. The driver oscillates.
>>>>
>>>> Is it best to use a noninverting mosfet driver in a smps design?
>>> 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.


> 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 :-)

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: miso on 10 Feb 2010 00:10
On Feb 9, 2:34 pm, Joerg <inva...(a)invalid.invalid> wrote:
> legg wrote:
> > On Mon, 8 Feb 2010 13:01:17 -0800, D from BC <myrealaddr...(a)comic.com>
> > wrote:
>
> >> I have some fuzzy recollection of reading to the effect that
> >> noninverting mosfet drivers are more prone to instability due to layout
> >> effects.
>
> >> Confirm?
>
> >> Just doing a quick sim in my head:
> >> With poor layout, when a noninverting mosfet driver turns on (sourcing),
> >> the signal ground pops up due to the mosfet gate capacitance (iirc
> >> called ground bounce?) then the driver sees a valid (actually invalid)
> >> '0' threshold to turn the mosfet drive off.. The driver oscillates.
>
> >> Is it best to use a noninverting mosfet driver in a smps design?
>
> > 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?
>
> --
> Regards, Joerg
>
> http://www.analogconsultants.com/
>
> "gmail" domain blocked because of excessive spam.
> Use another domain or send PM.

Micrel is a bit of a floor sweeping company. I'm amazed they got as
far as they got in standard products, mostly ripping off Maxim and LTC
designs. [In the dark ages, they used to be a foundry.]

I had a device that was killing the internal nicads, ripped it open,
and found a Micrel LDO, the kind that sucked current in dropout since
it used a bipolar pass device. PFET pass device took care of the
problem.
From: legg on 10 Feb 2010 22:15
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.
>
>> 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.

RL
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