ISO5851: RC Filter on the Power Rail

Hi Euan,

This is a really good question.

The RC filter in both designs uses 10ohm resistor/0.1uF cap, so the 3dB cutoff frequency is ~160kHz, and the 0.1uF cap serves double duty as the bypass capacitor for the input side of ISO5451 and ISO5851.

The buck converters supplies both the LDO (used for input side on gate driver) and the SN6505a push-pull converter used for the high-side drivers. 

This push-pull converter operates at ~400kHz. By its nature as a transformer-based switching converter, it could necessarily cause some interference on the 5V line, especially since it supplies the output-side of the gate drivers, which have high peak current draw.

So, SN6506a could inject some interference at ~400kHz (it operates 350-420kHz) onto the 5V rail that supplies the LDO. Why would the LDO pass it through? Unforunately, the LDO has a pretty poor PSRR at these frequencies of around 30dB, pretty bad.

ISO5451 and ISO5851 can both operate in the same frequency range that SN6505a push-pull converter does, so they want to make sure that the 3.3V supply is filtered to attenuate those frequencies, because its not super capable to do it on its own. You don't want your 3.3V supply rippling at the same frequency as your inputs!

Thats the reason they are using an RC filter. The cutoff-frequency of 160kHz was chosen based on the main source of interference, which in this case is SN6505a push-pull converter. You want the filter cutoff to be <=1/2 of the expected ripple/interference. 160kHz is less than half of SN6505a's operating frequency range (350-420kHz), so thats good.

I think that they just used the standard recommended bypass cap size and selected the resistor from that. More than likely, they just wanted to use nice round numbers which is why 10Ohms was used rather than 7,8, 9 etc. You would not want a very high resistance so you dont drop the DC voltage very much.

If in your design, you know exactly the input switching frequency to the gate drivers and its not close to ripple frequency you expect, then you most likely do not need an RC filter in your design. In these reference designs, its a general design where they don't know how the user's input frequency, so they put it there just in case the user wants to operate at higher frequencies.

Because the RC filter incorporates the decoupling capacitor, its really only one extra footprint on the PCB for each driver. If you are concerned that you may or may not want an RC filter, you can always put the footprint there and populate it with a 0ohm jumper, and change to a resistor later if you see the need.

Another option to ensure a clean supply to your gate driver input is to select an LDO that maintains high PSRR across the ripple frequencies to reject the ripple.

I hope this helps. If this answered your question, let me know by pressing the green button. And if you have any more questions, please let us know!

Best

Dimitri