BQ25628: Output is at 1.0V

Hello,

The schematic is not correct. There is no PMID capacitor which is critical for design. Also, the PMID_GOOD circuit is using an NMOS instead of a PMOS and is connected straight to ground, it is supposed to service OTG load.

Regards,

Mike Emanuel

the PMID output is for the boost converter which I am not really using aside from sourcing low current to an LED. Nor am I using USB OTG. I really only care about the SW output. Unless PMID 5V is used to power internal circuitry I cannot see why the capacitor would be needed. The NMOS transistor is used to turn the LED on or off depending on the state of the PMID_PG pin.

Hello,

The PMID pin is not needed solely for OTG. It is completely necessary to have PMID capacitor as PMID is the input to the converter. The converter will not work without it.

Regards,

Mike Emanuel

Are you talking about the boost converter or the buck converter or both?

PMID is the input to the buck converter.

Michael, I added the 10uF capacitor to the PMID pin and it did not change the output of the BQ25628. I am still getting only 1.0V on the SW output. I suppose this did not surprise me because I am not using the boost converter, only the buck converter which according to the diagram in the datasheet does not need PMID. 

Hello,

The BQ25628 has soft start when enabling the system load. Please wait for system to come up before pulling from it. Please read Section 8.3.3.6. V_SYS_SHORT is typical 0.9 V, right around the 1.0 V observed.

Believe it or not, PMID is the most important node for a capacitor in a buck converter. The Application Diagram Figure 9-1 clearly shows the need for PMID capacitor.

Please disable charging and see if the behavior improves.

Regards,

Mike Emanuel

Charging is already disabled by R252 pullup which I have verified is 1.8V. I removed R243 and R245 from the circuit with no change in behavior observed. As stated before, I already placed a 10uF cap on PMID, although there is a short 3mm wire to the cap. Attached is an image taken at the SW pin prior to the switching inductor. This behavior is periodic as I can trigger on it, however it occurs so slowly I cannot capture multiple pulses (much slower than the switching frequency of 1.5MHz), probably in the 100s of milliseconds or seconds. 

Hi,

On the schematic, the ILIM pin is floating. Is EN_EXTILIM (REGx19[2]) set to 0 to disable ILIM pin function? 

Thanks,

Ning.

Ning, I left the pin floating because as I understood the datasheet you could leave it floating and it would default to the max current. Looks like you have to write to that register first though which we are not doing, we are using it in default mode (no programing). I placed a 1.5K resistor on ILIM and now the output is regulating at 3.75V. Thank you very much. Hopefully that is the only thing I have to do at this point to keep it regulating and stable.