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LM2940 and need for a tantalium capacitor

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csdave

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hi all,
I was planning to use an LM2940 to power a 5V circuit off a 6V transformer. However, the datasheet says that the ESR of the output cap is critical (page 13 of http://www.national.com/ds/LM/LM2940.pdf) and suggests using a tantalum cap or combining an aluminum and a tantalum cap for this purpose.

I have a few 0.1uF tantalum caps at home, so I was thinking my best solution would be to parallel three of these.
The thing is that the supply will be powering among other things a relay core. The relay has a 1N4004 in anti-parallel as a flyback.

Is this flyback enough to prevent reverse polarity, or should I worry about my tantalum caps exploding?
 

chuckey

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You should always incorporate a flywheel diode to rid the unwanted back EMF or put a diode across the supply so it can't reverse its polarity which would damage the chip.
As for ESR, there seems to be a minimum value as well, so watch out for adding extra decoupling on the output line! I am not sure that 3 X .1 MF = 22 MF. I would use a 22 Mf tant or a 22MF ali + a .1 MF tant. Seems to be a dodgey chip design to me.
Frank
 

BradtheRad

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Hmmm... does the company that make the device also make ESR meters?

The device exploits ESR in some way to create low dropout.

Ordinarily you want ESR to be close to zero ohms.

ESR has gotten more attention in the last few years. Turns out there are fancy ESR meters and technicians who praise them for flagging faulty capacitors that test good otherwise.

It's not obvious how to measure ESR. Or easy. It tends to get ignored. Anyway 1 ohm ESR in a cap doesn't affect most circuits.

The datasheet calls for 1/10 ohm minimum ESR.

This can be 1 foot of 30 gauge copper wire.
Or 10 ft of 20 ga.

A new capacitor should add very little resistance.

A capacitor labelled 22 uF typically varies by several percent.

It's okay to combine caps, just so the total is 22uF or more.
 

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