LMS1585A: Capacitor and ripple

Hi Nick,

Since it's a production design we are not able to migrate to a new part.

In datasheet page 5 Electrical characteristics, RMS output noise is given as 0.003% of Vout. For 3.3V, it will be 0.099mV. Can this be considered as max noise/ripple?

If not, can you pls share how to calculate max ripple of this IC?

Thanks

Hi Nadini,

"Ripple" is commonly used as a catch-all term that includes noise, ripple from the input supply that is attenuated by PSRR, and overshoot/undershoot from load/line transients. I'm not a fan of this terminology because (as maybe you are seeing now) it is not clear what is meant by "ripple" if that is the only requirement. 

The noise spec is only in the range 10Hz to 10kHz, which is not even the standard 10Hz to 100kHz range, and so there will certainly be more noise than the 0.003%VOUT listed in the noise spec. This is a major drawback of using these quite old legacy devices that are poorly specified - there is basically no data in the datasheet besides the EC specs, which are all only at specific conditions. With that said, I highly doubt that there will be greater than 30mV of ripple just from noise. It would be much closer to single-digit mVRMS at the most. Assuming that the requirement for "ripple" also includes load/line transients, the noise is probably the least of your worries because these old devices often do not have very good transient performance, and even many modern devices will struggle to meet 0.9% tolerance when transients are included. 

Can you help me understand the context of this inquiry? If this design is already in production, I wonder why these requirements are now being looked at because in my mind there should have been a lot of questions about this device in the design phase since it is so poorly defined. 

Regards,

Nick

Hi Nick,

Thank you for the elaborate explanation. We already have pilot production going on and recently debugging some issues which cropped up at this stage and are ensuring that the voltage supplies meet the sink requirements as a part of debug effort. Unfortunately, I don't have the history of how this device was chosen in the first place by the previous team.

Hi Nandini,

Got it. How can I help then? We don't have an EVM that fits the pinout of this device but I could probably make something work on an EVM that we do have. I'm not sure how much I can help besides doing some load transients, but I suspect that they would just show that the device will not meet a 30mV tolerance with transients included. 

Regards,

Nick