LM1117: ESR of Cout

Fandy

Expert 2285 points

Part Number: LM1117

Hello,

The ESR of the output capacitor should range between 0.3 Ω to 22 Ω.

Capacitor datasheet shows that ESR depends on frequency, so the minimum demand on ESR should base on which frequency ?

over 5 years ago

0 JCHK over 5 years ago

TI__Guru 50090 points

Hi Fandy,

The minimum ESR is what we use for simulation.

Please note we have an improved device the TLV767 which is ceramic stable and a much better all-around performance device.

I hope this answers your question.

0 Fandy over 5 years ago in reply to JCHK

Expert 2285 points

Hi John,

what's the meaning of Fripple ? I want to check how to select Cadj

0 JCHK over 5 years ago in reply to Fandy

TI__Guru 50090 points

HI Fandy,

This is an example of how we can make our datasheets clearer. fRIPPLE means the frequency of ripple. Please note, the above example is just looking at PSRR (Power Supply Ripple Rejection) at 120Hz. This is the 2nd harmonic of 60Hz AC line frequency. This was used in very old Linear regulators as it was easy to measure.

I hope this answers your question.

0 Fandy over 5 years ago in reply to JCHK

Expert 2285 points

Hi JCHK

so how I to understand it:"just looking at PSRR at 120Hz ", it have any influence for all other frequency PSRR ?

0 JCHK over 5 years ago in reply to Fandy

TI__Guru 50090 points

Hi Fanny,

Yes. See the below plot:

Using CADJ helps across the frequency spectrum shown.

I hope this answers your question.

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LM1117: ESR of Cout