LM2937 voltage regulation insability problem
So I soldered 3 voltage regulators on a small proto board. On testing, I found both 7805 and 7809 working OK, but LM2937-ET3.3 has an output 12V. I though the device is burnt because my soldering time was too long. So I replaced a new one, but still no luck. I read the datasheet again and guessed that perhaps I was too lazy to add the by pass capacitor. So I added the capacitor and everything works well. The datasheet actually says that the capacitor is very important. But when I used a scope to check the output, I did not find any oscillation, though the output is still 12V.
The datasheet talks about ESR, and advises on the uses of tanalum and electrotic capacitors. I don't nothing about the theory of ESR, but I think I just use a big cap and the problem will go away.
.END
*** LM2937 datasheet extract ***
Application Hints
EXTERNAL CAPACITORS
The output capacitor is critical to maintaining regulator stability,
and must meet the required conditions for both ESR
(Equivalent Series Resistance) and minimum amount of capacitance.
MINIMUM CAPACITANCE:
The minimum output capacitance required to maintain stability
is 10 μF (this value may be increased without limit).
Larger values of output capacitance will give improved transient
response.
ESR LIMITS:
The ESR of the output capacitor will cause loop instability if
it is too high or too low. The acceptable range of ESR plotted
versus load current is shown in the graph below. It is essential
that the output capacitor meet these requirements,
or oscillations can result.
It is important to note that for most capacitors, ESR is specified
only at room temperature. However, the designer must
ensure that the ESR will stay inside the limits shown over the
entire operating temperature range for the design.
For aluminum electrolytic capacitors, ESR will increase by
about 30X as the temperature is reduced from 25°C to
−40°C. This type of capacitor is not well-suited for low temperature
operation.
Solid tantalum capacitors have a more stable ESR over temperature,
but are more expensive than aluminum electrolytics.
A cost-effective approach sometimes used is to parallel
an aluminum electrolytic with a solid Tantalum, with the total
capacitance split about 75/25% with the Aluminum being the
larger value.
If two capacitors are paralleled, the effective ESR is the parallel
of the two individual values. The “flatter” ESR of the Tantalum
will keep the effective ESR from rising as quickly at low
temperatures.
.END
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