Electronics > Beginners
Linear Technology LT1963 1.5A LDO - Output current no more than 100mA!
hgg:
Hi,
I am having a problem with an LDO I bought from TME. It is supposed to be capable of 1.5A
but I cannot get it to supply more than 100mA. My input voltage is 4.2V and the output voltage
setup 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.pdf
When I connect a load to its output, a 3W LED in this case, it outputs from 70mA up to 100mA and
the voltage drops down to 1.9V-2.0V Power supply is set with a current limit of 600mA. The LED
when 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 wrong :-//
Probably I am doing something stupid but I cannot find the cause.
Any help would be appreciated.
Thanks.
Psi:
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
Zero999:
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.
Psi:
--- Quote from: Zero999 on April 07, 2019, 02:25:17 pm ---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.
--- End quote ---
That 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.
hgg:
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 240mA
and 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.
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