Linear Technology LT1963 1.5A LDO - Output current no more than 100mA! - Page 1
Forum section: electronics / beginners

author: hgg
author_rank: Frequent Contributor
timestamp: April 07, 2019, 02:00:04 pm
content: Hi,I am having a problem with an LDO I bought from TME. It is supposed to be capable of 1.5Abut I cannot get it to supply more than 100mA. My input voltage is 4.2V and the output voltagesetup by a resistor divider at the ADJ pin of the LDO is 2.6V The resistor values are R1 = 892Ω,R2 = 1040Ω and it outputs a stable 2.6V, which is what the ADJ pin formula gives as well.The reference voltage is 1.21V as it is supposed to be.https://www.analog.com/media/en/technical-documentation/data-sheets/1963fc.pdfWhen I connect a load to its output, a 3W LED in this case, it outputs from 70mA up to 100mA andthe voltage drops down to 1.9V-2.0V Power supply is set with a current limit of 600mA. The LEDwhen connected to 2.6V directly from the power supply draws 650mA.I have used low ESR output capacitors either a 10uF or 330uF 40mΩ, but nothing changes.What am I doing wrongProbably I am doing something stupid but I cannot find the cause.Any help would be appreciated.Thanks.

author: Psi
author_rank: Super Contributor
timestamp: April 07, 2019, 02:20:41 pm
content: My first guess would be that you have some vdrop in your wires or connectors.As the current increases the input voltage drops and prevents any higher current from flowing.Connect up the LDO so it should be powering the LED at full power but isn't.Then measure what voltage across the lab psu at the lab psu's terminals compared to the voltage at the pins of your LDO.You may find that your LDO is not getting the voltage you think it is.Breadboards are notorious for bad connections

author: Zero999
author_rank: Super Contributor
timestamp: April 07, 2019, 02:25:17 pm
content: The ICs could be overheating as there doesn't appear to be sufficient heatsinking. There's nowhere for the heat to go to, so the regulator heats up and triggers the thermal overload protection which limits the current.

author: Psi
author_rank: Super Contributor
timestamp: April 07, 2019, 02:27:15 pm
content: Quote from: Zero999 on April 07, 2019, 02:25:17 pmThat occurred to me as well.But i think, if that were the case, he would get full brightness at first then it would dim after some seconds as the chip got hot and thermal protection kicked in.If that was happening i think he would have said so.

author: hgg
author_rank: Frequent Contributor
timestamp: April 07, 2019, 02:32:06 pm
content: The IC is stone cold.Psi I think you are right. I moved the output ground jumper wire to a different place and I got 240mAand a 2.1V output. Still not there, but it seems that I have a faulty chinesium breadboard...I will try right now a different one and see what happens.

author: Kjelt
author_rank: Super Contributor
timestamp: April 07, 2019, 02:34:12 pm
content: If that does not help try an ohmic (electronic) load first!Leds can be tricky at voltage control, you should always use current control or add a series resistor for the led.

author: hgg
author_rank: Frequent Contributor
timestamp: April 07, 2019, 03:10:12 pm
content: I've just tried a second breadboard and I got to 315mA but no higher.Maximum temperature of the IC was 40 degrees C.If I press on the jumper wires it goes a bit higher.Especially on the ground wire.So, it looks like that the problem lies with the breadboards.But I get half the current current that the LED normally draws.What do you think?

author: Zero999
author_rank: Super Contributor
timestamp: April 07, 2019, 03:26:42 pm
content: Yes, I think the breadboard connections are an issue, but now you've increased the current, thermal effects could also be an issue.How did you measure the temperature of the IC? The IC has a thermal pad underneath it, so most of the heat should be released from the bottom, on the other side of the board. There's no way the IC can dissipate a decent amount of power, when mounted like that: a few Watts at most.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: April 07, 2019, 03:32:06 pm
content: You still fighting with those same crappy breadboards George .

author: hgg
author_rank: Frequent Contributor
timestamp: April 07, 2019, 03:42:43 pm
content: I don't know if you remember, but this is the second time Ι got cooked from the same cause...I swear, I am never going to bye a chinesium breadboard ever again!..and I've just ordered a 3M one.>> Yes, I think the breadboard connections are an issue, but now you've increased the current, thermal effects could also be an issue.This IC does not have any thermal pad underneath from what I see. I measured the temperature with a FLIR camera.40 degrees. This is a SOT23 package. I thing that the DPAK version has a metal pad underneath.This is still a problem though because I am going to pull 1.2A from this guy and I am not sure how I amgoing to dissipate the heat...

author: Nerull
author_rank: Frequent Contributor
timestamp: April 07, 2019, 04:55:22 pm
content: That's a SO-8 package, and its rated at about 70C/W, if it has 2500mm^2 of copper heatsinking on the board.That little breakout board is not going to put out 1.2A.

author: hgg
author_rank: Frequent Contributor
timestamp: April 07, 2019, 05:01:08 pm
content: Yes sorry, SO8.The breakout is only for testing.When I finish the prototype I am going to design a small 30x25mm pcb.Is it possible?...

author: David Hess
author_rank: Super Contributor
timestamp: April 07, 2019, 06:55:08 pm
content: It may also be oscillating with the capacitors so far away. Move the capacitor connections to a spot immediately adjacent to the regulator or even solder them onto the interposer.

author: hgg
author_rank: Frequent Contributor
timestamp: April 07, 2019, 07:11:25 pm
content: I've managed to get 800mA. Its the breadboard. Genuine Chiniseum.The trace on the oscilloscope is completely flat with non existent ripple or spikes even in the 2mV range.

author: hgg
author_rank: Frequent Contributor
timestamp: April 07, 2019, 07:35:06 pm
content: I have another problem...I have connected the FDS6898A N-Channel MOSFET in the circuit as in the schematic below :The 2.6V output of the LDO is very stable but when I connect the gate of the MOSFET to the 2.6V rail,the 2.6V rail becomes a 3.2V rail...Its like something is changing the ADJ pin voltage divider.Normally I measure the ADJ pin at 1.1V and when I connect the gate of the MOSFET directly tothe 2.6V rail, I measure 1.4V on the ADJ pin.What am I doing wrong?

author: bson
author_rank: Unknown
timestamp: April 07, 2019, 07:36:01 pm
content: Shouldn't you connect the output - rail to ground?

author: hgg
author_rank: Frequent Contributor
timestamp: April 07, 2019, 07:38:52 pm
content: It is connected to ground when the MOSFET is ON and because the gate is connectedto 2.6V, its always ON.

author: Kjelt
author_rank: Super Contributor
timestamp: April 07, 2019, 08:33:02 pm
content: Is it me or is your diode reversed ?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: April 07, 2019, 08:42:33 pm
content: Hi,In this picture I can't see a connection between the two negative rails and the two positive rails:You have to connect them like this:Regards,Jay_Diddy_B

author: hgg
author_rank: Frequent Contributor
timestamp: April 08, 2019, 06:08:45 am
content: kjelt, you are right, I have updated the schematic, but in reality the LED is properly connected.Jay_Diddy_B The upper rail is the 4.2V power supply rail and the lower one is the LDO 2.6V output rail.I have only connected the grounds together. Here is another photo with the MOSFET in circuit:The MOSFET works as expected, gate to 2.6V LED is ON and gate to ground LED is OFF.But when the MOSFET is on, the 2.6V power rail becomes a 3.2V rail...Does it affect the LDO ADJ pin voltage divider?Because the ADJ voltage changes as well to a higher value.Strange. Please help...

author: Psi
author_rank: Super Contributor
timestamp: April 08, 2019, 09:23:50 am
content: I call more breadboard shenanigans going on

author: hgg
author_rank: Frequent Contributor
timestamp: April 08, 2019, 09:28:32 am
content: Hmm, ok, I think I will wait for the 3M breadboard to arrive and continue from there...I am also going to buy this one as well from TME :WISHER ENTERPRISE WBU-204-1https://www.tme.eu/gr/en/details/wbu-204-1/universal-pcbs/wisher-enterprise/I've read that they are good as well.

author: bd139
author_rank: Super Contributor
timestamp: April 08, 2019, 09:51:50 am
content: Both are good. I have wisher and 3m boards.However I would solder something up for this sort of test on a bit of FR4.

author: hgg
author_rank: Frequent Contributor
timestamp: April 08, 2019, 09:59:33 am
content: Good to know.I don't have a desoldering gun and these multiple pin connectors are a bugger to desolder!I might try it though because this problem is really strange even with these wicked breadboards.Maybe I will solder them higher from the board and when I finish, cut them.

author: bd139
author_rank: Super Contributor
timestamp: April 08, 2019, 10:05:56 am
content: If I'm doing prototyping with the SMD adapter boards, I usually use a piece of FR4 as a ground plane, don't use the pins, glue the SMD adapters down with hot snot and then wire them direct with 1/0.6 wire. I also got some SMD prototype boards made up which make this stuff a lot easier:(unfortunately i cant find the gerbers I used otherwise I'd post them here)Quick fugly example. RC filter to get rid of some noise in something.

author: hgg
author_rank: Frequent Contributor
timestamp: April 08, 2019, 10:09:07 am
content: Sweet!

author: hgg
author_rank: Frequent Contributor
timestamp: April 08, 2019, 11:55:39 am
content: The problem was too weird to wait for the breadboard to arrive and I solderedeverything on a breadboard layout PCB I had. PROBLEM SOLVED!Psi you were right again.For the love of God, never ever buy a chinesium breadboard again!You will forget what you already know..Thanks guys.

author: bd139
author_rank: Super Contributor
timestamp: April 08, 2019, 12:23:26 pm
content: Worth reading this through as a review:http://www.mit.edu/~6.331/an47fa.pdfAN47-24 onwards on ground planes and breadboarding.While that looks like it works now, it depends on the load. LDOs are rather tetchy and can break out into oscillation.

author: hgg
author_rank: Frequent Contributor
timestamp: April 08, 2019, 02:11:54 pm
content: I've tested it up to 1.2A and its stable, no problems.Thanks for the link. I will have a look.