LT3032. Regulator output voltage error.

alperen on Sep 20, 2018

Hello,

I'm trying to use LT3032 in my design to regulate +/-15V supply voltages coming from DC/DC swithing regulator.

The output voltage of LT3032 is set to +/-14V with voltage setting resistors. However, output voltage is about +/-12V. I discovered that BYPN and BYPP voltages are a little off.

I measured BYPN is about -78mV and BYPP is about 79mV.

As it is stated in the datasheet these pins are high impedance, sensitive nodes that need attention. I tried to prevent leakage into these pins by clearing the ground and power planes beneath them, but I still get this huge error. What could cause this error?

Thanks in advance!

0 ADIApproved

on Sep 20, 2018 5:24 PM
What are the input voltages and the load currents? And what capacitors did you use in the circuit?
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0 alperen on Sep 21, 2018 1:07 PM in reply to ADIApproved

Load current is about 20mA for each stage. I am feeding in +/-15V and trying to get +/14V, but I get +/-12v instead. No bypass capacitor is used. At the inputs I have 1206 ceramic type 10uF capacitor and and at the inputs I have 6.8uF tantalum cap in parallel with 10uF ceramic capacitor.

I can try different combination of capacitors if you have any suggestion.
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on Sep 21, 2018 5:25 PM in reply to alperen
The capacitance is large enough unless the power supply is far away from the IC. Can you double check the feedback resistors' values? Does the output voltage shift for one board, or all of the boards? Also check the layout. The BYPP and BYPN are high impedance nodes. Any leakage current affects the reference voltage. You can see details on page16 in the datasheet.
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0 alperen on Sep 22, 2018 1:44 PM in reply to ADIApproved

I checked the resistors values they are correct. I tried to check the Feedback voltage at the ADJN and ADJP pins and the voltages was about 1V(-1V for the negative channel). I read the leakage issue in the dataheet and tried to prevent leakge by clearing any copper area beneath these pins..The refrence is off for some reason. I think the error is layout induced.

I only tried one board, but tried a few lt3032 chips to see if there is any change. Suprisingly, the error was really consistent. I thought it was because of contamination in the PCB and I cleaned the board a few times, but no luck.

Any tips on preventing leakage to these pins?
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on Sep 24, 2018 10:20 PM in reply to alperen
Is the board coated? If the board is not cleaned well before it is coated, the leakage is inevitable.
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0 alperen on Sep 25, 2018 6:31 AM in reply to ADIApproved

No It's not coated. It's a 4 layer gold immersion PCB. May I have refrence design files? I've found desing file in the following link

http://www.analog.com/en/products/lt3032.html#product-evaluationkit

It includes a file with .pcb extension. Thus far I failed to open it. Can you tell me which software I should use to open it? I'd like to review before I redesign the PCB.

As I said before I cleaned the a few times it makes literally no difference.
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