This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

PMP9766: Pre charging operation problem

Part Number: PMP9766
Other Parts Discussed in Thread: LP2998, TPS63020,


I recently made this circuit and i am having some weird behavior. I made it as close as the original circuit but i changed the programmed output to reach 5V (R4 and R5, the circuit i need to power runs in 5V). Without load seems to work in most cases but if i had a teensy (80mA more or less) seems like it doesnt reach the 1.8V neccesary to go into the charging stage, It stays around 1.55V. Also the power supply i use says that the circuit consumes around 200mA, but nothing changes. Somehow if increase the voltage to around 5,2V the supercaps start to charge the circuit works as it should, including charging, normal and backup operation.

As i said the only change its the programmed output, but this shouldnt affect because in the pre-charge stage the TPS63020DSJ is not even working. The eFuse should affect this part of the circuit and the LP2998 doesnt show any weird behaviour. Also according to the formula the pre-charge current should be higher than 300mA, but it deosnt go higher than 200mA. There isnt much components involved in the pre-charge stage, and it doesnt seem that the component are broken, in fact the R1, D1 and the LP2998 were replaced and we still got the same results.

Could you help me out with this issue?

Thanks in Advance,


  • Hello Ander,

    Your system voltage is 5V, correct? If this is the case, you need to set the output voltage of the TPS63020 lower than the lowest system voltage that the backup operation is possible, see this paragraph in page 3 of the test report of PMP9766:

    With the feedback resistor divider consisting of R4 and R5, the output voltage of the buck-boost converter must be programmed to a level below the system voltage in normal operation. Ensure that the programmed voltage is below the system voltage including all tolerances of the main power source and the buck-boost converter.

    This would explain as well that the system works as expected when you increase the system voltage by 0.2V.


  • Hello Brigitte,

    First of all thanks for the reply. R4 and R5 are already set up to a lower output voltage, R4=866K and R5=100K. Using the formula the programmed output is around 4.83V, which i could actually measure if i somehow manage the circuit to work (mostly not the conditions i wanted: no load or different input voltage). Maybe it is not low enough? Is there anywhere where it says how low it needs to be?.

    But as i explained my problem is in the Pre-charging stage, the capacitors don't seem to reach 1.8V, so that means the TPS63020 is not enabled. I thought because the enable pin is low (1.55V) this chip should not work and therefore the R4 and R5 shold not have any effect in the Pre-charging stage. I also explained that increasing the voltage that the circuit works (but i get to big of an output) and then that everything works as expalined, the Charging stage after Pre-charging, normal operation and backup operation. For me there is something going on in Pre-charging stage. But maybe i am wrong and this 2 resistors can have an effect in the pre-charging effect. I hope you can help me out.

    Thanks in advance.

    Best regards,


  • Hello Brigitte again,

    It seems like that R4 and R5 values were the problem, apparently they were too high (I still don't fully understand why it was afecting them). After reducing it and getting an 4.6V output the circuit works perfectly. I still will play around and find threshold to get as close to 5V as possible. But if you have any idea why this resistors were affecting the Pre-charge stage i would like to know. 

    Thanks again.

    Best regards,


  • Hi Ander,

    in this circuit, the output voltage of the buck-boos must be lower than system voltage at the beginning. it is to ensure the buck-boost not operate at the beginning. after the voltage at the super capacitor is ready and the the system voltage is gone, the buck-boost will be enabled.

    that is the reason FB pin resistor impact the behavior.