Power Good signal indicators for DC-DC Buck converters - Page 1
Forum section: electronics / projects

author: Krotow
author_rank: Regular Contributor
timestamp: December 15, 2024, 02:51:40 am
content: Have a power circuit made from 24V to 5V and 5V to 3.3V Buck converters where both +5V and +3.3V power is required (3.3V is required is for ESP32-S3 and communication transceivers). Quite usual nowadays. Wanted to do power sequencing right by Power Good signals (not an issue too). Also wanted to see if particular voltage is available from Power Good aspect so got the idea to put a LED indicator on PG (Power Good) pin for both converter ICs - LMR51425SDRRR and TLV62569PDRLR. PG pins here are open drain type with LOW as fault status. So pull-up is needed which theoretically allow to connect LED between pulled up PG and GND. Where lit LED indicate existing voltage and unlit - fault/no power. However sink current for TLV62569PDRLR PG pin is only 1 mA and that certainly is too low to drive a signal led with 10-20 mA current. To avoid magic smoke, decided to power indicator LEDs directly from 5V and MOSFET switches. This approach look kinda functional (schematics are attached below). Just wanted to know are this approach will not cause other problems besides slightly increased BOM cost?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 15, 2024, 03:57:43 am
content: Quote from: Krotow on December 15, 2024, 02:51:40 amThat will work, but you do not need 10-20mA these days to drive LEDs.Just find a LED that is not total rubbish, and 1-2mA will be fine.

author: Krotow
author_rank: Regular Contributor
timestamp: December 15, 2024, 05:15:41 am
content: We have a few large bags of 3mm diameter LEDs in office left from previous projects. Not the newest models so certainly will draw more than 1mA.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 15, 2024, 05:36:30 am
content: Quote from: Krotow on December 15, 2024, 05:15:41 amSure, if that is how you manage BOMs and parts counts, by all means add more parts to use old stock

author: Someone
author_rank: Super Contributor
timestamp: December 15, 2024, 05:42:18 am
content: Quote from: Krotow on December 15, 2024, 05:15:41 amHow are you so sure 1mA is not enough current? What level of illumination is needed from a PCB level power good indicator? 20 years ago some LEDs were already low enough leakage and high enough efficiency to only need sub mA drive for indicators. Current LEDs span around 1000x or more intensity between "efficient" and "not efficient".

author: Krotow
author_rank: Regular Contributor
timestamp: December 15, 2024, 12:44:38 pm
content: Yeah, yeah... The PCB is a prototype and I will test the possibility to drive LEDs only by PG signal and pull-up anyway when DC converter ICs arrive. Just want to have prepared backup pads and traces for MOSFET contraption already in place in case if LEDs can't be driven directly like that.Maybe smart guys above can throw an idea how to drive a input voltage (12V or 24V in random possibility) presence LED with minimal component count and keep the same brightness? A single resistor (SMD 0603) end with a very dimly lit LED or resistor itself warm up to +40..50C degrees. Of course there are constant current drivers for LEDs like NCR402U, NCR402T, BCR402R etc. But these will be another overkill plus take more board space than single resistor.Quote from: PCB.Wiz on December 15, 2024, 05:36:30 amWhat is wrong with reusing the old stock for prototyping? We don't want to simply throw them out and make more e-waste.

author: xvr
author_rank: Frequent Contributor
timestamp: December 15, 2024, 03:51:13 pm
content: MOSFETs will light LEDs when its gate will be at 0 (i.e. when 'power NOT good')You should use N channel MOSFETs and change LED and MOSFET connection

author: Krotow
author_rank: Regular Contributor
timestamp: December 15, 2024, 06:21:38 pm
content: Quote from: xvr on December 15, 2024, 03:51:13 pmYep, noticed that and replaced MOSFETs to N-channel type.

author: Krotow
author_rank: Regular Contributor
timestamp: March 02, 2025, 05:24:41 pm
content: Quote from: Krotow on December 15, 2024, 12:44:38 pmFinally made the thing in hardware. For power indicator LED in 12-36V range NCR402UX did the trick. No more burning resistors (or power indicator LEDs).

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 02, 2025, 10:47:37 pm
content: Quote from: Krotow on March 02, 2025, 05:24:41 pmThere is also the PSSI2021SAY, which allows user control of the LED current below 20mA, for less power loss.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 02, 2025, 11:11:27 pm
content: Quote from: Krotow on December 15, 2024, 12:44:38 pmNew parts are getting better.If you want 12-24V Power GOOD to LED lcsc have new parts like CC1108 (SOT23, ~ 7c) and CC1109 (SOT89, ~12c)that can take a 3-5V rated power-good signal, and current regulate a LED with choice of current, and better Temp and PSRR than the older, simpler x402 parts.