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From: Joerg on 3 Aug 2008 17:20
JosephKK wrote:
> On Fri, 1 Aug 2008 14:05:06 +0200, "Henrik [7182]"
> <not.valid(a)nowhere.dk> wrote:
>
>> <langwadt(a)fonz.dk> skrev i en meddelelse
>> news:cae888a6-c839-4051-9f70-6ad28b0bfaf1(a)k30g2000hse.googlegroups.com...
>>> The datasheet I just checked had an example with 5V/1A it uses a
>>> 50mOhm Rsense
>>> With a typical Vsense at 165mV thats 3.3A and it that must be lower
>>> than the
>>> maximum current rating for the inductor.
>>>
>>> So why did you choose 7mOhm ?
>>>
>>> -Lasse
>> Actually I been trying to experiment a little with Rsense today, and what
>> happes does not make me happy at all :-(
>>
>> Everything above 20mR will not yield the 24V output when the load is a 24R
>> resistor and the input fall below 24V. With 7mR i can obtain my 24V output,
>> but at the cost of the insanely hot construction :-)
>>
>> With no lad, the output is fine, but when I load the output, the voltage
>> drops.
>>
>> Something else is wrong with my setup I think, but I cannot figure it out. I
>> have some ugly spikes at the FB pin and suspect that these might have
>> something to do with my problems, but I don't really know how to move on
>>from here.
>> Best regards
>> Henrik
>>
>
> One thing that has bothered me is the high frequency ringing on the
> source trace. Is it a measurement issue or is something else going
> on?
>

Looks like the usual. The Vsense scope plots from my clients almost
always look like that until I visit and show them. You are measuring
millivolts while the FET a few millimeters away sloshes tens of volts
around. You are trying to measure the buzz of a bee behind a roaring jet
engine.

Plain old crosstalk into the scope probe assy. A direct and correctly
terminated coax connection and a good measure of ferrites on that cleans
that up quite well.

--
Regards, Joerg

http://www.analogconsultants.com/

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