Laying components under switching regulator? - 2"x1.5" board. Space at premium.

[kyledickerson]

Laying components under switching regulator? - 2"x1.5" board. Space at premium.

I'm designing a 2"x1.5" board. It has a dual switching regulator based on the LT1944 for 24V and 5V, post-switching linear regulators, analog circuitry with ac coupled single ended in to diff-out, microcontroller, usb, usb lipo charger, and bluetooth. The 24V lines are for biasing sensors on the analog side. Approximately 9mA of current runs through them. I've produced a 2 layer test board using our in-house prototyping machine with just the power supply, microcontroller, and analog circuitry. It works flawlessly. The 24V lines to the sensors are point-to-point wired as to simulate them being on their own layers. The space under the regulator, on the opposite side of the board (approximately 0.8"x0.75"??) is empty as I'm afraid of switching noise being induced into other parts of the circuitry. If I use a 4 or even 6 layer board, can I place components and route lines on the opposite side of the board under the power supply? Ideally, I'd like the usb connector and lipo charging circuitry to be there as it will be close to the lipo battery. The power supply section only needs 2 layers - top layer and ground layer. Any help is appreciated.

 

[barry]

Re: Laying components under switching regulator? - 2"x1.5" board. Space at premium.

I don't think you have to worry about running usb or charging traces under the regulator, just be careful to keep your analog circuitry away from there. A multilayer board will definitely help protect against unwanted noise, etc. But if cost is a real concern, try the 2-layers and see how it works. Good luck.

 

[marce]

Re: Laying components under switching regulator? - 2"x1.5" board. Space at premium.

Avoid components or copper (other than 0V/GND) under the switching nodes and inductors/transformers.
Again a schematic would help.

 

[kyledickerson]

Re: Laying components under switching regulator? - 2"x1.5" board. Space at premium.

Right. Only ground is under the switching nodes. I'm concerned about sharing the ground directly under the inductors and switching nodes with USB lines and lipo charging components on the opposite side of the board.

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Also, if I were to route a line from the micro to the regulators shutdown pin, how would I accomplish this without noise being injected into the micro, through the internal pull-up resistor and into DVCC?

 

[barry]

Re: Laying components under switching regulator? - 2"x1.5" board. Space at premium.

You don't have to really worry about noise being injected into the micro. Digital signals are a lot more immune to noise than analog. If you're really concerned you could filter your digital lines with an RC or a ferrite bead.

 

[poorchava]

Re: Laying components under switching regulator? - 2"x1.5" board. Space at premium.

If you have an SMPS on top layer, then a solid GND undeaneath, you should be fine placing and routing on bottom layer. I'd keep the sensitive analog stuff out of that area though. If this is not possible and you notice noise from smps on your analog stuff, then perhaps you could measure the noise spectrum and filter the most offending frequencies.

 

[marce]

Re: Laying components under switching regulator? - 2"x1.5" board. Space at premium.

A ground plane will not screen the magnetic interference from the inductor, again do not place components under the switching nodes, minimise these areas and keep them clear.
The high di/dt also means there is often a high frequency component, any traces or pads will have capacitive coupling.

 

[kyledickerson]

Re: Laying components under switching regulator? - 2"x1.5" board. Space at premium.

A ground plane will not screen the magnetic interference from the inductor, again do not place components under the switching nodes, minimise these areas and keep them clear.
The high di/dt also means there is often a high frequency component, any traces or pads will have capacitive coupling.

This is what I thought to be the standard rule. However, I was hoping there'd be an exception for extremely compact designs. I think I might try putting the charging circuitry under the power supply and any lines that go into the power supply section will have a ferrite bead between the power supply section and other sections. Hopefully this will work. I'm hurting for space, here.

Also, I'm working with analog signals under 50kHz. Not sure if this additional information helps.

 

[marce]

Re: Laying components under switching regulator? - 2"x1.5" board. Space at premium.

I have put two SMPS's on a very small board with sensitive analogue circuitry that all fitted in a headphone earshell, with no problems. But I do specialise in this sort of layout. Any more info, pictures etc you could give us would help. The main thing is minimising the size of the switching nodes and any effects the switching may have on other circuitry. without knowing the design though it is hard to give any more advice than has been given, apart from reading all you can about SMPS layout. Ti and Linear app notes are good.