LM5122: Canceling the output cap ESR Zero

Kelvin Le THS

Part Number: LM5122

Hi,

In the LM5122 datasheet (pg 26) it recommended the high frequency compensation pole location beyond (greater than) the crossover frequency :

"The high frequency pole should be placed beyond the crossover frequency since the addition of CHF adds a pole in the feedback transfer function."

My comment:

This can be possible only if the ESR is low like in the datasheet design example where the Fcrossover=5.3khz and the Fz_ESR=8kHz (ESR=20mohm, Cout=990uF).

Questions:

What about the case where I have a high ESR AL output cap and the Fz_ESR is < Fcrossover? Should I still place the compensation pole to cancel the Fz_ESR even when it is < Fcrossover? Does the Fcrossover target dictate my Cout selection in a way so the the Fz_ESR > Fcrossover?

Thanks

-Kelvin

over 5 years ago

0 Richard Fu22 over 5 years ago

TI__Mastermind 30895 points

Hi Kelvin,

1) In order for your Fz_ESR < Fcrossover, you would need a very big ESR. Usually, this case does not appear since output capacitance usually consists of a large non-ceramic capacitor in parallel with ceramic capacitors, which will reduce the ESR.

2) Crossover frequency is dictated by the lower of (1/5 switching frequency) or (1/3 RHPZ). Which can affect your Cout selection in the following ways:

2.1 You can find the output capacitance required based off of output ripple, or

2.2 you can find output capacitance based off of load transient (based primarily off principle that Vout_transient = Iout_Transient*Zc, where the frequency required is your crossover frequency.

2.3 You can also select output capacitance off the principle of making the low, low frequency pole 1/(Rout*Cout) to be less than 1/10 of the RHPZ. Typical equation is:

Cout = K*(Lm)/[(Vin/Vout)^2*Rout^2], where K can be a large number such as 200.

Thanks,

Richard

0 Kelvin Le THS over 5 years ago in reply to Richard Fu22

TI__Expert 7845 points

Richard,

Thank you for the explanation.

A hybrid cap network with a high ESR and a low ESR cap will create two zeros. The high ESR zero will still be inside the crossover frequency even with ceramic cap in parallel. This was my take-away from this TI app note:

https://www.ti.com/lit/an/slvae26a/slvae26a.pdf?ts=1594221692507&ref_url=https%253A%252F%252Fwww.google.com%252F

This was also mentioned in this app note:

https://www.ti.com/lit/an/slva452/slva452.pdf?ts=1594174512551&ref_url=https%253A%252F%252Fwww.google.com%252F

-Kelvin

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LM5122: Canceling the output cap ESR Zero