High voltage buck....control circuitry referenced to switching node

[cupoftea]

Hi,
Would you agree the control cctry of the LN320X Buck converter is referenced to the switching node..and so, the actual cct ground should not be used as a ground plane beneath this cctry, since it is "effectively" switching node.

It always seems weird having to keep ground away from signal cctry, in case it makes it malfunction

Page 1 shows schem.

https://www.power.com/sites/default/files/documents/linkswitch-tn2_family_datasheet.pdf

 

[Akanimo]

Hi,

Please rephrase your question. What are you referring to as "the actual circuit ground"? For your buck converter, you just need a little ground patch underneath the IC and all branches to that node are routed individually back to that ground.

Your statement, "It always seems weird having to keep ground away from signal cctry, in case it makes it malfunction", is not very clear.

 

[cupoftea]

Thanks, its HWRect from the mains, so neutral is "actual ground"....ayk, the buck switching node is "control ground".
Actual ground has to be kept away from control cctry.
Its obvious why, but just seems a weird concept.

The Bucks main input electrolytics are however, right beneath the control cctry, and there's nowhere else for them to go unfortunately.

 

[FvM]

There 's no big control circuitry, just a FB node connecting to two resistors. If you keep the node small, it should work even above a DC- plane. Most critical layout detail is keeping the commutation loop (input capacitor to buck diode) small.

 

[cupoftea]

thanks, ayk, those two resistors into the FB pin are effectively the output divider....and as you know, in any SMPS, this is a sensitive trace needing to be routed away from switching node...as far as that FB pin is concerned, the "switching node" is the circuit "actual ground".

 

[mtwieg]

The chip and its internal circuitry is referenced to the switching node of the buck circuit, yes. Whether you refer to that as "ground" is up to you, as the designer (but I would not).

The reference designs seem to give reasonable layout suggestions, including a very small pour for the switching node and its few components.

 

[cupoftea]

it may look small, but as AN-70 by power.com says there can also be a need for soft finish circuit involving a 47uF 35v cap in with that FB pin circuit

 

[mtwieg]

But that soft start capacitor doesn't connect to the switch node. Not sure how it's relevant here.

 

[cupoftea]

thanks, its pllel to the "upper divider res" of the FB pin......so its banging up and down with that divider.
This divider is "grounded" to the switching node.