[Efi332] HW Question (for JC)

[bowtievette at aol.com]

Hi Paul,
I would include the 47uF on the input side. I don't think you will have any problems with it running from the car's battery/alternator.
On the output side, you're right, ESR values are typically not quoted in catalogs other than to say low ESR, so you usually have to dig up the datasheet(s) to get the tables for it. If you do a search for low ESR capacitor at digikey it will narrow the search to caps that advertise low ESR. Pick a 22 uF one from the selection table presented and follow it to the datasheet. For this application, aluminum electrolytics are common, and occasionally tantalums. Some folks recommend against using tantalums in power supplies since they have a tendency to blow up under rapid transient conditions. With the electrolytics, you may have to watch the temperature variation since the ESR varies quite a bit.
 
-----Original Message-----
From: Paul Helmuth <PaulHelmuth at SprintMail.com>
To: efi332 at diy-efi.org
Sent: Wed, 29 Jun 2005 16:37:06 -0500
Subject: [Efi332] HW Question (for JC)


Jeff,
 
I managed to source a couple of National LM2940T (9volt output) regulators. I am hoping to use this part to power my Phytec development board from the car's 12v battery.
 
The "Typical Application" section of the data sheet shows only two external components. 
 
One is a .47 microfarad capacitor on V-in to ground. An asterisk note indicates that this is required if the regulator is located far from the "power supply filter". Which seems to imply that there should be some filtering in place between the battery and the regulator (though there is no diagram of what such a filter should look like).
 
The second external component is a 22 microfarad (minimum) capacitor on V-out to ground. The interesting note on the output capacitor is that the equivalent series resistance is critical. Reading the performance characteristics table relating to the output capacitor ESR, I see that I need to use a capacitor with an ESR between .1 and 1 ohm. I have never seen ESR given for capacitors. Can you give me any pointers as to how I can determine and arrive at an acceptable ESR for the output capacitor(s)?
 
Thanks,
-Paul
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