VCC：3.5V
PG:0V
Why is the output only 0.6V?
Hello, I am using the MP8759 chip with an input of 12V and a target output voltage of 5.2V. Currently, I have found that the output voltage is only 0.6V after being powered on. I have tried replacing chips from multiple suppliers, but the output voltage is only 0.6V May I ask what could be causing this phenomenon? The EN voltage measurement is 3.1V, VCC voltage is 3.5V, PG voltage is 0V, MODE voltage is 3V The schematic and board diagram show FB voltage below 0.05V. Could you please help analyze the possibility of the problem and provide further design suggestions? Thank you!
The actual welding resistance of R35 is 1.5MR
We have attempted to purchase chips from multiple suppliers and have also removed normal chips from other reference boards for testing, with outputs of 0.6V
Hello!
Welcome to the MPS Forum. Sorry to hear you are having issues with output voltage regulation with the MP8759.
From your measurements, it sounds like the part does not properly turn on. The 0.6V is the typical reference voltage. Please check the resistor values for R34 and R38. Since FB is 0.05V, it leads me to believe this part could have a bad solder connection or an external resistor value has been derated. You can also try using the values reccomended by the DS for a 5V output (R1 = 41.6k, R2 = 5.6k).
Can you please provide waveforms of Vin, Vout, BST, and SW? This can give indication if the part is properly switching and possible protections.
Is there anything else supplying the chip besides VCC_12V?
From your schematic, the pin connections look OK, and the appropriate pins are pulled to their respective voltages. I would note that the output capacitance looks a bit large. Can you try reducing C29 to a lower capacitance (47 uF)? What is the ESR of that capacitor? The internal ramp control is dominated by ESR when using an electrolytic capacitor.
For layout, connections look good and are in accordance with the recommended layout. Can you do a continuity test between pins to check for any shorts?
There are some differences in the schematic recommended values and your schematic. When debugging I would recommend following the DS and EVB as closely as possible and compare with MPS data. Here are links for those: