Wednesday, January 05, 2011

Low Dropout Regulators—Why the Choice of Bypass Capacitor Matters

Widely seen as a panacea for solving noise issues, capacitors deserve more respect. Designers think that adding capacitors will cure noise problems, but give little thought to parameters other than value and voltage rating. But capacitors are not perfect; they possess parasitic resistance and inductance, their value varies with temperature and voltage, and they are sensitive to mechanical effects.


At 3:50 PM, Anonymous Anonymous said...

Very interesting article; good info for those who haven't encountered these issues before. A couple of things puzzle me, though:

To me, Figure 2 and Figure 3 look almost identical, yet the text makes it sound like the voltage trace in Fig. 3 should be nearly flat.

You state that the input capacitor's value should match the output cap's value if you raise the output cap value above the minimum required. ?? I've never seen this before, and I've never encountered an LDO data sheet that mentions this. Could you please explain your thinking behind this?


At 5:18 PM, Anonymous Glenn Morita said...

The is a small difference in the shape of the voltage transient. The larger output capacitor reduces the size of the "dip" and reduces the slew rate of the output transient.

We recommend making the input and output capacitors the same value to improve the transient load response. A larger input capacitor will hold up the input voltage longer than a smaller capacitor thereby allowing energy from the input capacitor to be transferred to the output.


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