Backlight boost vs buck-boost configuration? - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: November 04, 2024, 08:28:18 pm
content: Hi,in a PCB I am designing I need to power two different types of displays (both datasheets attached)but only 1 at the time - i.e. two products each with just one display and both using the same PCB design (with jumpers for config/voltage setting)- with the following backlight characteristics:- Product A uses Display 1 with Backlight needing 18V @ 80mA- Product B uses Display 2 with Backlight needing 30V @ 40mAOn the PCB the only voltages available are 5V and 24V.So I was thinking of using:a) either a boost converter boosting from 5V (datasheet of a possible IC attached - but it does not provide guideline on maximum output voltage generally nor at any given current).If any of them can boost from 5V to 30V at 50mA then that solves it.But from reading some online posts that is pushing it to its limits.b) or a buck-boost fed from the 24VThe buck-boost ICs are all relatively large (working with 0402 because of limited space).So a couple of questions:1. Are there better solutions?2. is the 5V boost to 30V configuration feasible? that is 6x voltage boost. Is it reliable?3. generally speaking, out of the two solutions (a) and (b), I assume solution (b) is more reliable?Thank you

author: Andy Chee
author_rank: Super Contributor
timestamp: November 04, 2024, 08:39:28 pm
content: My suggestion is a multi output flyback:You don't have to use this particular device, but the basic circuit topology is the general idea.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 04, 2024, 08:50:09 pm
content: Thank you Andy,sorry, my OP was not clear, I meant only 1 display is connected at any given time to the PCB. I just updated the OP.So the same PCB drives either the display with 30V backlight or the display with the 18V backlight. Two products each with just a single display (a different size each) but the same PCB is used for both.Thank you

author: langwadt
author_rank: Super Contributor
timestamp: November 04, 2024, 09:30:13 pm
content: https://jlcpcb.com/partdetail/LegendSi-LGS63042B5/C5123978figure 6C ?

author: ricko_uk
author_rank: Super Contributor
timestamp: November 04, 2024, 09:52:55 pm
content: Thank you @langwadtI don't think that would suit my application. In the datasheet it shows a wide input voltage but only a couple of LEDs in series on the output.Also the datasheet is not in english so cannot read any of the notes/specs. So would not take such risk.Any other part perhaps?Thank you

author: Someone
author_rank: Super Contributor
timestamp: November 04, 2024, 10:14:37 pm
content: Quote from: ricko_uk on November 04, 2024, 08:28:18 pmThe datasheet tells you straight up what its output voltage range is.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 04, 2024, 10:25:21 pm
content: You are correct,but it does not mean it can push 5V to 30V.Also the datasheet is not in English so I don't want to use it to find out there are some specs/details I should have known but couldn't read because of the language.Also just below Fig 6c the only english comment is (VIN<V LED VIN>VLED) which makes no sense.Or do you just mean that - with reference to one of my original questions - any switcher can boost from 5V to 30V? Not sure I understand if so.If so, could anybody suggest any english datasheets?Thank you as always

author: langwadt
author_rank: Super Contributor
timestamp: November 04, 2024, 11:10:42 pm
content: Quote from: ricko_uk on November 04, 2024, 10:25:21 pm(VIN<V LEDorVIN>VLED) so it can do both 24V to 18V and 24V to 30V, just pick the right current set resistor

author: Salitronic
author_rank: Contributor
timestamp: November 05, 2024, 01:48:10 am
content: You should check out the MPS MP3363:https://www.monolithicpower.com/en/mp3363.htmlI think it would be a perfect fit for your application.

author: Zipdox
author_rank: Regular Contributor
timestamp: November 05, 2024, 02:17:52 am
content: Would it be an option two have a PCB that allows populating different parts based on the screen used?

author: ricko_uk
author_rank: Super Contributor
timestamp: November 05, 2024, 04:02:34 pm
content: Thank you all for your replies.@Zipdox not ideal, but which parts were you thinking about?Thank you

author: Zipdox
author_rank: Regular Contributor
timestamp: November 05, 2024, 07:30:49 pm
content: You could have two drive circuits in the PCB design for the two displays but only populate the one you need.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 05, 2024, 10:18:42 pm
content: Yes, but as mentioned in the OP the PCB has dimensions constraints.Thank you

author: xvr
author_rank: Frequent Contributor
timestamp: November 06, 2024, 06:33:12 pm
content: Quote from: ricko_uk on November 04, 2024, 09:52:55 pmGoogle can translate pdf with quite good quality:

author: mariush
author_rank: Super Contributor
timestamp: November 06, 2024, 09:07:34 pm
content: IS31BL3508Bhttps://www.digikey.com/en/products/detail/lumissil-microsystems/IS31BL3508B-STLS2-TR/5319745Supply voltage: 2.7V~5.5V• Built-in soft start• Drive series strings of 10 LEDs or 3×8 LEDs• PWM or DC voltage dimming• Frequency of PWM is 500Hz ~ 500kHz• 1.0MHz high speed switching frequency• 0.2V feedback voltage• Integrated high power MOSFET• Over voltage protection• -40°C ~ +85°C working temperature range• SOT-23-6 package1.2A switch current ... boosting 5v to 30v at 40mA shouldn't go above 1ATPS92360DCKR is about same specs as the Lumissil one above :https://www.digikey.com/en/products/detail/texas-instruments/TPS92360DCKR/15856560Or Richtek RT4533, same specs :https://www.digikey.com/en/products/detail/richtek-usa-inc/RT4533GJ6/6676703

author: ricko_uk
author_rank: Super Contributor
timestamp: November 06, 2024, 10:31:17 pm
content: Thank you both!@mariush those are great and what I was looking for.