Hardware debugging - Page 1
Forum section: electronics / beginners

author: Jane
author_rank: Frequent Contributor
timestamp: November 18, 2015, 08:18:20 pm
content: I need to repair a device that uses a dual synchronousstep-down switching regulator controller LTC3850.Datasheet is herehttp://cds.linear.com/docs/en/datasheet/38501fc.pdfLet's suppose it uses the typical application design according to the attached picture.How would you start to find out the faulty part if there is noVout1, nor Vout2? The input voltage Vin is correct.Thank you for you ideas.

author: fivefish
author_rank: Frequent Contributor
timestamp: November 18, 2015, 08:23:24 pm
content: Spend the $5 and try replacing the LTC3850.

author: Jane
author_rank: Frequent Contributor
timestamp: November 18, 2015, 09:13:11 pm
content: I did replace LTC3850 and yet no output voltages

author: Aodhan145
author_rank: Frequent Contributor
timestamp: November 18, 2015, 09:16:45 pm
content: What voltage are you measuring with your DMM?

author: Brutte
author_rank: Frequent Contributor
timestamp: November 18, 2015, 09:26:22 pm
content: I'd do some optical inspection, then test for shorts/opens, values of passives and the transistors, in-circuit. In case nothing suspicious was found, I'd try ramping up supply voltage with current limiter and check voltages. These two regulators are independent so the fault must be close to power supply or the reference (which are common).

author: KL27x
author_rank: Super Contributor
timestamp: November 18, 2015, 11:27:50 pm
content: I'd turn it on and probe the FETs with a scope. Something is obviously wrong, there. Starting at the obvious lack of Vout, work back from there.Vin > top FET > inductor >Vout. There's the first path to check out, in my opinion. Perhaps the bottom FETs are burned/shorted out and the top FETs are ok. That would be one way to get 0V on the outputs.

author: fivefish
author_rank: Frequent Contributor
timestamp: November 18, 2015, 11:36:59 pm
content: Quote from: Jane on November 18, 2015, 09:13:11 pmYou didn't mention that in your 1st post.It's probably not switching, maybe shorted or open transistors, so try replacing all the FETs.Also check inductors if they're open/burnt?Use the fingers... any component getting hot?

author: Jane
author_rank: Frequent Contributor
timestamp: November 19, 2015, 10:10:53 am
content: Thank you all for the ideas. Regarding FETs, do you think the checking by DMM( digital multimeter) is OK?Is it necessary to desolder them first or I can check them in the circuit?

author: KL27x
author_rank: Super Contributor
timestamp: November 19, 2015, 11:28:17 am
content: In circuit, you will probably best to check them with a scope with the circuit powered.You might have to take them out of circuit to make sense of them with DMM. Unless they're burned and you get continuity between the gate and D/S (with the circuit unpowered, of course).IMO, in any sort of switching supply like this, it is usually a good idea to check the FETs, first, before you replace the driving IC. If the FET is smoked, it'll usually kill the IC. An output pin doesn't normally last too long when it's pinned to one of the rails.

author: deephaven
author_rank: Frequent Contributor
timestamp: November 19, 2015, 12:31:39 pm
content: The first thing I would do is (with the power off) check that there isn't a short or low resistance between the output and 0V. There might not be anything wrong with the regulator.

author: Jane
author_rank: Frequent Contributor
timestamp: November 19, 2015, 02:52:49 pm
content: @KL27x:Can you please let me know how I can check FETs ,in circuit,with scope?Thank you