I use TPS65131 as a dc-dc converter from 5V to +/-12 V. While I set the output voltage according to Equation 2 page 24 (datasheet),
for the negative output voltage, I get a larger deviation. I use a R3=1M Ohm, R4=97K, which should output a negative voltage:
But I got a much high voltage, which is higher than 16V, even I change R4 to 118K, it still get 14V. Is this caused by FBN leakage current?
It is said that R4 can up to 200K in the datasheet. I thought this maybe caused by something on the pcb which lower the R4 value, so I cleaned the PCB board,
but did not get any better.
Could you give me a clue?
Please check the voltage on VREF pin. You can check the voltage on one of the pins of R4. I expect that this voltage will differ from 1.213V given in the datasheet.
R4 is directly loading Vref and Vref is just defined for 10uA.
I measured the Vref voltage, which is not drifted much, 1.215V. Do the positive and the negative rail use the same reference? The positive rail output not differ too much from the equation value.
I also found that this output voltage deviation is related to load. When load is disconnected, the output is exactly what the equation gives (regardless the input voltage). The load is about consumes 60mA current. When the load added, the negative output voltage begin to go lower as the input voltage increase. In my test, when input voltage is 4.2V, the output obtained the specified -12.5 V output, after that the negative voltage become lower.when input is 5V, the output is -15.1V.
Correct, both output voltages use the same reference voltage.
It seems that your output voltage is around -3 times the input voltage. What load are you using? How does the feedback voltage of the negative converter look like when the output voltage is too negative? Normally the feedback pin should show 0V.
Please share your schematic and layout to discuss more in details your application.
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