[Diy_efi] PCB layout/circuit design
Bevan Weiss
kaizen__
Mon Feb 26 06:17:59 UTC 2007
What's the ESR and ESL of the 1 farad capacitor? I generally only use
around 1 or 2 1000uF caps for bulk decoupling.
Then 1uF or 2u2 ceramic 0805 capacitors for more local decoupling.
Finally 100nF 0805 ceramics for chip level decoupling.
Does the board have a TVS device on the power supply input? It also
assume it is fused suitably, preferably with a quick blow fuse.
Bevan
> We have a 1 farad - ie 10x or more your need in front of this
> already :-)
>
> On Sun, 2007-02-25 at 17:50 -0800, Heitor Lima wrote:
>
>> Bill
>>
>> We need to use two capacitors , one big as 100uF ,
>> electrolytic type and another one , 100nF , ceramic
>> type.
>> Regards
>>
>> Heitor
>>
>>
>> --- Bill Washington <bill.washington at nec.com.au>
>> wrote:
>>
>>
>>> James,
>>> You said "all ICs should have at least a100uF
>>> cap as close as
>>> possible to the supply " do you mean 100nF?
>>> Steven,
>>> In PCB layout it is good practise to have a
>>> large reservoir Cap
>>> (your 1 Farad) close by plus smaller (ceramic 100nF
>>> and sometimes 10nF
>>> as well) caps right on the IC supply pins (as close
>>> as you can get
>>> them). The reason for this is that the large
>>> reservoir cap has some
>>> inductance and resistance in its construction which
>>> slows down its
>>> response time - the ceramic Caps (which very low
>>> ESR) on the IC supply
>>> pins handle the sharp rising and falling edge surges
>>> and minimise ground
>>> bounce which can cause all sorts of problems ....
>>> Regards
>>> Bill
>>>
>>>> Subject:
>>>> [Diy_efi] RE: More ECU progress (Steven P.
>>>>
>>> Donegan)
>>>
>>>> From:
>>>> "James Holland" <j_holland at btopenworld.com>
>>>> Date:
>>>> Sat, 24 Feb 2007 22:41:30 -0000
>>>> To:
>>>> <diy_efi at diy-efi.org>
>>>>
>>>> To:
>>>> <diy_efi at diy-efi.org>
>>>>
>>>>
>>>> I have been following this project with some
>>>>
>>> interest. I had a look on the
>>>
>>>> website but you seem to have the same schematic
>>>>
>>> for the datalogger and I/O
>>>
>>>> board. I presume the I/O board schematic hasn't
>>>>
>>> been posted. I've designed
>>>
>>>> electronics for military automotive issues so I'm
>>>>
>>> aware of the problems of
>>>
>>>> designing for this kind of environment.
>>>> I like the idea of this but I think that you will
>>>>
>>> have some noise issues
>>>
>>>> with the design as it stands. The PCB design lacks
>>>>
>>> a good ground plane and
>>>
>>>> the decoupling could do with being improved, all
>>>>
>>> ICs should have at least a
>>>
>>>> 100uF cap as close as possible to the supply. The
>>>>
>>> PIC should have two, one
>>>
>>>> on each supply pin. I'm a bit surprised that the
>>>>
>>> PIC doesn't have a separate
>>>
>>>> Analogue supply for the A/D as it stands the
>>>>
>>> analogue and digital returns
>>>
>>>> are mixed which isn't good. The A/D inputs should
>>>>
>>> really have anti-aliasing
>>>
>>>> filters close to the inputs, a simple RC would do
>>>>
>>> and only the C really
>>>
>>>> needs to be up by the input. The R would also
>>>>
>>> provide some input protection
>>>
>>>> to the PIC. Input voltage clamping is also a good
>>>>
>>> idea. The PIC is pretty
>>>
>>>> well protected but voltages outside of its supply
>>>>
>>> rails throw the A/D
>>>
>>>> readings all over the place.
>>>> It looks like you are using the 5V supply as the
>>>>
>>> reference for the A/D, what
>>>
>>>> supply are the I/O lines referenced to?
>>>> The digital I/O could do with some resistance in
>>>>
>>> line as well, a couple of
>>>
>>>> hundred ohms would provide output short circuit
>>>>
>>> protection and give some
>>>
>>>> filtering on the inputs. If your digital inputs
>>>>
>>> are coming off another board
>>>
>>>> then you could see some ringing on them, that
>>>>
>>> could be more of a problem on
>>>
>>>> PortB which doesn't have Schmitt inputs.
>>>> The 5V regulator has plenty of protection, that's
>>>>
>>> good. Automotive supplies
>>>
>>>> are horribly noisy. The 28V military stuff I
>>>>
>>> design has to be protected
>>>
>>>> against +280V/-130V spikes. The EMC test involves
>>>>
>>> 100V for 100mS. You could
>>>
>>>> reduce the protection needed by using a regulator
>>>>
>>> specifically designed for
>>>
>>>> automotive use, there are a few of them out there.
>>>>
>>> I have used the LM2940S
>>>
>>>> in the past and that works well but I think the
>>>>
>>> best spec only comes in an
>>>
>>>> SMD package.
>>>> I hope this doesn't sound too harsh, I don't see
>>>>
>>> any reason why it won't
>>>
>>>> work but I think you will lose some A/D resolution
>>>>
>>> by having to digitally
>>>
>>>> filter the inputs. I learnt the hard way with a
>>>>
>>> few board redesigns along
>>>
>>>> the way. The Microchip website has a very good on
>>>>
>>> line seminar about PCB
>>>
>>>> layout and design for A/Ds, its well worth
>>>>
>>> checking out.
>>>
>>>> HTH
>>>> James
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>> ------------------------------------------------------------------------
>>
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>>
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