automotive regulators

[Chris Conlon]

At 08:59 AM 8/29/00 -0700, Garfield Willis wrote:

>On Tue, 29 Aug 2000 10:15:18 -0500, steve ravet <sravet at arm.com>
>wrote:
>
>>...the power supply is just a 7805.
>
>One improvement you can make right way is to swap that for an
>LM2940-5, which has all the automotive protection features, such as

One thing that has not come up is that the LM2940, LM2936 and LM2931
are all LDOs. (Low Dropout Regulators). This means that they can keep
providing regulated voltage even when the input voltage drops to only
a little abouve the output voltage. This is normally a good thing,
BUT, LDOs are almost universally subject to oscillation depending on
the output capacitor and the load they're driving. The exact type and
size of the output capacitor could be critical, or merely very
important, and stability over the full load/temperature range can also
be a bit tricky if the load varies a lot. Circuits that present a
lot of high frequency/high power load components can make the situation
worse. You might think that just adding more capacitance would help,
but that's not true either. The LDO will also oscillate if the ESR
goes too *low*.

Each of the 3 parts mentioned above has a good section in the
datasheet about how to choose the output capacitor, and a graph of
the stable area vs. output cap ESR. I suggest that anyone looking to
use these parts read those sections carefully, especially if you're
new to LDOs. They are not drop-ins for a 780x, in many if not most
cases an output cap suitable for a 780x will not work with an LDO.
Even worse would be the situation where the cap is marginal, so the
LDO works fine until the temperature or load goes to an extreme.

I'm not saying don't use them... I'm just saying read the directions
carefully.


   Chris C.

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