LP5907 – About an acceptable capacitance [uF] and ESR [ohm] value of output capacitor

Hi Team,

Could you please let us know the current status regarding this question?

We would be grateful if you could reply as soon as possible.

Regards,

Kanemaru

" ... Could you please let us know the calculating formula to calculate the phase margin? ..."

There is no formula available for calculating LP5907 phase margin. Ringing of the output voltage due to a load step transistion is used to determine loop stability, and estimate the phase margin.

http://www.ti.com/lit/an/slva381b/slva381b.pdf

http://www.ti.com/lit/an/slva115/slva115.pdf

Hi Donald-san,

Thank you for the information!

I have one more question.

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[Question]

The following content is mentioned in datasheet.

How did you decide the recommended capacitor(1uF to 10uF) and ESR(5mohm to 500mohm) value?

We understood that you have the grounds to decide this value.

For example, we would like to know the following value which is needed for stable operation.

-P(Load) pole =1/(2*pi*Cout*(Vout/Iout)) is form ??kHz to ??kHz

-Z(ESR) = 1/(2*pi*R(ESR)*Cout) is from ??kHz to ??kHz

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So, if this goes on, they can’t design it if there is no information.

Therefore, We would like to know this information.

Could you please let us know it?

I’d greatly appreciate your verification.

Regards,

Kanemaru

 " ... How did you decide the recommended capacitor (1uF to 10uF) and ESR(5mohm to 500mohm) value?..."

The original design team is no longer here to answer these questions about why they designed it the way they did.

I would presume that the output capacitance range and ESR range was a part of the original design target, or perhaps a requirement from a lead customer, and was likely validated via simulation.

The 'Load pole' is an external phenomenon … since we know Vout can be 1.2V to 4.5V, Iout can be 1mA to 250mA, and Cout can be 1uF to 10uF … it’s becomes a spreadsheet exercise to work the range of possibilities.

   10uF, 4.5V, and 1mA gives an Flp of about 4Hz

   1uf, 1.2V and 250mA gives an Flp of about 33kHz

Plus, there may have been some secondary considerations between max Cout capacitance and how to size the automatic output discharge resistor.