How to separate ground planes for Vin and Vout for LDO voltage regulator? - Page 1
Forum section: electronics / beginners

author: tylerssims
author_rank: Contributor
timestamp: December 05, 2022, 02:31:32 am
content: Hi all,I have never laid out a regulator before. Reading through the datasheet it says that,"TI recommends designing the board with separate ground planes for VIN and VOUT, with the ground plane connected only at the GND pin of the device. In addition, the ground connection for the output capacitor should be connected directly to the GND pin of the device.In the TI forum, I have seen them say that "Both GNDs can be on the same layer. They should come together at a point near or at the GND pin of the IC to improve AC performance."I have also seen this thread (https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/312612/separate-ground-planes-for-vin-and-vout-in-ldo) but the diagrams don't show copper pours.Here is my layout alongside the relevant screenshots from the datasheet:https://imgur.com/a/SWByAtUHere's the regulator I'm using:https://www.mouser.com/ProductDetail/595-TLV1117LV33DCYTHow wrong am I? Thanks for the help.

author: ataradov
author_rank: Super Contributor
timestamp: December 05, 2022, 03:27:09 am
content: Don't overthink it, just use a normal ground plane as you normally would. You will do more harm by trying to accommodate this nonsense requirement. 1117 is a pretty standard LDO, it is hard to make it do something weird.Your capacitors seems to be very small (physically). It looks weird, but I see no specific reason why it would not work.

author: tylerssims
author_rank: Contributor
timestamp: December 05, 2022, 11:18:21 am
content: Thank you for the advice. I will switch to one large ground pour. The footprints are small because only had 0603 caps but I just ordered some 0805 ones, which are a bit bigger.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 05, 2022, 11:20:40 am
content: Just use a singleplane as ataradov says. Keep your layout tight. Caps right at the regulator pins with short gnd paths between.

author: tszaboo
author_rank: Super Contributor
timestamp: December 05, 2022, 12:11:03 pm
content: Quote from: tylerssims on December 05, 2022, 11:18:21 amThe TLV1117 isn't designed to work with ceramic capacitors, which are typically in 0805 or 0603 packages. You need electrolytic capacitors for this regulator.Also don't split the ground as others said.

author: ataradov
author_rank: Super Contributor
timestamp: December 05, 2022, 03:53:43 pm
content: Quote from: tszaboo on December 05, 2022, 12:11:03 pmIt says right in the features "Stable With Cost-Effective Ceramic Capacitors: – With 0-Ohm ESR".

author: David Hess
author_rank: Super Contributor
timestamp: December 05, 2022, 04:20:09 pm
content: Quote from: ataradov on December 05, 2022, 03:27:09 amIt is not exactly a nonsense requirement, but it probably does not matter in this case. Using a single point ground *instead* of a ground plane prevents return currents from modulating the ground/common of the LDO which will spoil the output regulation. In most cases when a ground plane is used, the impedance will be so low that return currents will not have a significant effect.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 05, 2022, 04:35:27 pm
content: The point with separating the gound / using a single point of contact is because of possible ripple currents from the input side to couple to the ground of the output side. Much of the current would normally be through a filter capacitors.The TLV1117 is not for large power (so trace resistance is not that relevant) and it depends on the power source if the ripple current at all (not a problem with a battery).

author: mariush
author_rank: Super Contributor
timestamp: December 05, 2022, 04:52:33 pm
content: A lot of 1117 regulators require an output capacitor with ESR between 0.1 ohm and 1 ohm , hence why in some datasheets they recommend a 22uF tantalum capacitor for the output capacitor (tantalum caps have low esr, but generally above 0.1 ohm). Pretty much all 10uF electrolytic capacitors will have ESR above 0.1 ohm , but from around 47uF there's some capacitor series that can do better than 0.1 ohmSome manufacturers make them ESR stable but if you plan to release the design or think you may make more in the future, it may be worth adding a 0.1 ohm (or higher) resistor in series with the output ceramic capacitor, just in case. If you know the regulator is stable, then you could just put a blob of solder between the resistor footprint pads.Or just use a different regulator that's doesn't have specifications that vary from manufacturer to manufacturer.

author: ataradov
author_rank: Super Contributor
timestamp: December 05, 2022, 05:00:03 pm
content: Quote from: mariush on December 05, 2022, 04:52:33 pmYes, this is one of the annoying things about 1117. Substitutions may bite you in the ass if you are not careful. And yes, you often see a real series resistor.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 05, 2022, 05:26:40 pm
content: Quick reminder:* High accuracy DC signals (microvolt levels etc) -> ground splits can be beneficial* EMC and AC signals -> ground splits do more harm than goodThis is because DC and AC behave differently; DC return current emphasizes the path of least resistance, AC return current takes the path directly below the trace which is optimal already, so you can easily make it worse.Some tens of kHz is still "DC" in this regard, so audio circuits often use star grounding or splits.Power electronics and digital circuits -> almost always one plane.

author: magic
author_rank: Super Contributor
timestamp: December 05, 2022, 07:25:55 pm
content: This is not a 1117, but a complete redesign in CMOS technology with 500 less quiescent current, 0 ESR stability (not clear if it's stable with high-ESR), and guaranteed no-load operation (presumably Iq flows to GND rather than OUT).It's probably better not to apply any generic 1117 wisdom to this chip.

author: tszaboo
author_rank: Super Contributor
timestamp: December 07, 2022, 11:17:55 am
content: Quote from: ataradov on December 05, 2022, 03:53:43 pmYou are right, this is the LV version which is stable, because it's a completely different part than the TLV1117 named the same way.