Linear regulator proximity to switching inductor - Page 1
Forum section: electronics / beginners

author: cowana
author_rank: Frequent Contributor
timestamp: October 17, 2014, 07:25:03 am
content: I'm laying out a PCB based on several PT4115 switching LED drivers. These operate around 500kHz, with a maximum frequency of 1MHz. The board also uses a 78L05 regulator to provide a 5v line, then further 3v3 regulators for the digital electronics.The PCB has to fit in a small enclosure, so space is rather tight. I've laid out the switching regulator, using thick tracks, short paths for the critical signals, and a big ground plane under everything with lots of vias. It would suit the design to place the 78L05 regulator on the underside of the PCB (it is a two layer design) - however I notice that is close to the inductor on the other side of the board.The 78L05 has the required caps on it's input and output, and the 5v supply it generates does not need to be particularly low noise. Logic is telling me the inductor's magnetic field is almost entirely internal, would be partially shielded by the groundplane, and the linear regulator shouldn't be that sensitive to it anyway.However I'd appreciate some thoughts from anyone with a bit more experience with switching regulators - should I treat the inductors as dangerous areas and keep my linear regulator away? Or am I worrying too much about this and the separation will be sufficient? The die of the 78L05 (SOIC package) will be around 8mm from the inductor (and on the other side of the board) - a couple of the pins might be directly underneath it.CheersAndy

author: Simon
author_rank: Unknown
timestamp: October 17, 2014, 07:28:23 am
content: I'm certainly no expert but I'd give it a go, if you can move it away just a bit, remember double the distance reduces signal by a square or cube so much to gain from a little distancing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 17, 2014, 07:34:01 am
content: 2 layer or 4 layer board?I'd guess it's fine.Tim

author: cowana
author_rank: Frequent Contributor
timestamp: October 17, 2014, 07:43:47 am
content: It's a two layer board (with ground fill top and bottom wherever there aren't traces).

author: Simon
author_rank: Unknown
timestamp: October 17, 2014, 07:44:45 am
content: Well copper is an RFI blocker so should be fine

author: cowana
author_rank: Frequent Contributor
timestamp: October 17, 2014, 07:49:45 am
content: Quote from: cowana on October 17, 2014, 07:43:47 amHaving said that, there may be some small areas with no ground plane - for example where there is one of the inductor's pads on the top side, and the regulator's pads on the bottom side. The regulator's body will have ground plane under it though.

author: Simon
author_rank: Unknown
timestamp: October 17, 2014, 07:50:58 am
content: I can't see it being a problem if your running digital stuff, if it was low noise opamps I'd be worrying.

author: David Hess
author_rank: Super Contributor
timestamp: October 19, 2014, 02:59:24 pm
content: Quote from: cowana on October 17, 2014, 07:25:03 amThe copper ground plane is not going to shield against the magnetic flux in any useful way. The changing flux will induce voltages into any copper it intersects.The line regulation of the linear regulator falls with frequency so this by itself cannot be relied on to remove switching regulator noise.QuoteUse an inductor which is self shielding like a toroid or pot core. Avoid inductors which use open construction like bars and bobbins. Avoid running high dV/dT signals where they can capacitive couple into other circuits. Return high dI/dT currents to their origin without excessive loop area.