General > General Technical Chat
Why did my LT3042 die?
giosif:
--- Quote from: coppercone2 on April 03, 2021, 06:44:18 pm ---[...]
They are touchy.
--- End quote ---
Yes, I learned this first hand.
--- Quote from: coppercone2 on April 03, 2021, 06:44:18 pm ---I recommend sticking with 3080 series unless absolutely necessary
--- End quote ---
I will check the specs for the 3080, but I'm after something with very low noise.
Jay_Diddy_B:
--- Quote from: giosif on April 03, 2021, 09:22:28 pm ---
Snip ...
--- Quote from: 2N3055 on April 03, 2021, 06:41:52 pm ---Also LT3042 is a low dropout regulator that can work with 350mV drop on regulator, i.e. for 15V you need 15.35V on input. 5V is way too much. 16V would have given you healthy reserve.
--- End quote ---
Indeed. I should have read the datasheet more carefully, but I was being impatient...
--- End quote ---
giosif and the group,
Be careful, although the LT3042 can operate with only 350mV of input to output differential:
Other parameters will be compromised if operated with a low input to output voltage differential: This is PSRR:
Output noise spectral density:
I would choose a differential voltage 1.3V if I want the best PSRR and noise performance. Typical applications for the LT3042 are normally for lowest noise and highest possible PSRR. To allow for tolerances in a production environment you may need to allow more voltage. If the input supply is 5% and the LT3042 is 2% and the Vset resistor is 1%, a larger differential may be required. If is for home use, you can tweak the circuit.
Best Regards,
Jay_Diddy_B
bingo600:
For an Non EE person...
The headroom (Vin) should be around 1.3v above (Vout) ?
After reading the AN , am i correct in understanding that one should leave the ceramic input cap , and add an additional 47uF Elco in parallel ?
I have a few of these boards , that i expect to use for some OCXO regulation
/Bingo
Jay_Diddy_B:
Bingo600,
Choosing the headroom, is a complex question, the answer is (as always) it depends.
1) You have to consider power loss in the regulator. The power loss is:
(Vin - Vout) x Iout
The maximum dissipation depends on the PCB, but 1 to 1.5W would be a good starting point. This may limit Vin-Vout.
2) You have to decide if you *need* the highest possible noise performance and PSRR. You might be able to operate with a Vin-Vout < 1.3V
3) You may want to consider what rails you have available, You may also be able to choose the output voltage.
4) You may need to consider the tolerances of the LT3042 and the supply being used to feed it. This is to make sure that you get the performance you need under all conditions.
In short, AN88, suggests placing an electrolytic capacitor that is 3 or more time the value of the ceramic capacitor in parallel. The ESR of the electrolytic will dampen any resonances. In most cases a 47uF electrolytic will be fine. It is if you 'hot plug' or hard switch the input supply.
Jay_Diddy_B
giosif:
Coming with a late update: I bought another module like the initial one, installed a 47uF electrolytic cap at the input and am trying to power it with 17V.
With no load, the output stays at around 15V (14.85V, to be exact).
However, as soon as I apply a load of 50 mA, the output goes up to 16.36V. :-//
If I then increase the input voltage, the output tracks the input, until I get to about 18.9V on the input, when the output voltage (under the 50 mA load) becomes exactly 15.00V.
From 18.9V up to 20V on the input (I didn't have the courage to go above that), the output voltage stays the same (i.e. doesn't track the input voltage anymore).
Initially, I thought maybe this was due to the module coming off eBay and the possibility the LT3042 in it might not be genuine.
But (and this is why I took so long for the update), I finally managed to order an LT3042 IC off DigiKey and install it on the board of the initial module from eBay (the one which prompted this discussion) and the behaviour is the same as with my 2nd module from eBay.
I am bit confused at this stage and not sure what I'm doing wrong here.
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