TPS7B4250 - Output Capacitor ESR vs Frequency

Hi Jason,

Thank you for your reply but unfortunately I don’t think my question has been answered.

Your answer is similar to other ambiguous answers regarding ESR and LDO regulators that I reviewed.  

My question was specifically regarding the TPS7B4250-Q1 –

At what frequency(s) should the listed 0.001 to 20 Ohms required ESR of the capacitor be evaluated at?

This is not a general purpose application; it is Automotive, where the liability of a poorly engineered product can be severe.

I do not know what the “working bandwidth” of the TPS7B4250 is as it is not in the datasheet, so I cannot verify your assumption of “several kHz to dozens of kHz” for “most LDO products” statement.

I agree that the specified ESR range of the TPS7B4250-Q1 is wide, but without an associated frequency for reference, it is almost meaningless (to me at least).  I cannot go into a design review and answer “I’m pretty sure there should be no problem” if I am asked if the chosen cap meets the ESR requirements of the regulator.

For reference, please take a look at the datasheet for the Infineon TLE4250 (a part similar to the TPS7B4250-Q1).  The ESR is specified to be a maximum of 3 Ohms - “2) relevant ESR value at f = 10 kHz”.

Thank you

Hi TonyS,

The reason why I did not give you the specified frequency and we did not present Bode Plot in the datasheet is because the loop response is related to the output capacitor you used and the load condition you set. Different output capacitance and different load resistance will result in different poles and zeros in frequency response.

The TPS7B4250-Q1 listed 0.001 to 20 Ohms required ESR of the capacitor is validated in our bench test, and the frequency range from 1kHz to dozens of 10kHz I suggested is used for choosing output capacitor. If the ESR in this range is dropped in the stable region we provided in the datasheet, that indicates the ceramic capacitor could provide a useful zero to prevent LDO from oscillation or instability.

If you would like to understand the deep reason, I would like to recommend two application notes to you,
1. Application Note 1148 Linear Regulators: Theory of Operation and Compensation: www.ti.com/.../snva020b.pdf
2. Application Note 1482 LDO Regulator Stability Using Ceramic Output Capacitors: www.ti.com/.../snva167a.pdf
these two documents explain the control theory behind LDO operation in detail but in an easy-to understand way.

Yes, I agree with you, TLE4250 provided the maximum ESR value at f = 10 kHz, but we do not specify this value. Because we do guarantee a much wider ESR range, from 1mohm to 20 ohm, which most widely used ceramic capacitors (>1uF) should be within.

If you are still concerned about that, you may listed capacitor type selected, we could help you evaluate it.

Best regards,
Jason

Hi Jason,

Thank you very much for your reply.

The two application notes were very helpful to me as I am trying to better understand the details of working with LDO regulators in general.

My goal is to create a stable, reliable, cost-effective, low current, 5 V tracking LDO regulator design.

I would certainly appreciate your help with capacitor selection / evaluation.

So, let's start from the beginning -

The datasheet states that the TPS7B4250-Q1 is stable with a 1 uF to 50 uF ceramic output capacitor.

Since a 1 uF capacitor is most likely the least expensive, is there any reason(s) to pick a larger value?

Note that the output capacitor shown / referenced in various datasheet figures is 2.2 uF.

Thank you,

TonyS

Hi TonyS,

Yes, TPS7B4250 is able to work with 1uF output capacitor.

I am not quite sure which type capacitor you would use. And for many widely used capacitor's, they have similar electrical characteristic.

I will take TDK CGA3E3X5R1H105K080AB capacitor as an example, it is a ceramic capacitor qualified for automotive.

Below is the ESR v.s. frequency plot provided by the manufacturer.  You could find, from 1kHz to 100kHz, the ESR range is from 5mohm to 2ohm, which is dropped in the stable region.

Best regards,

Jason Liu