PF3000 PMIC: SWBST coming up as 4.3V not 5V

[benjaminblanton]

We are having an issue on our boards with the SWBST boost output is coming up initially as 4.3V and not 5V.  When the kernal boots; we program the PMIC to 5.15V and this seems to happen correctly, but the initial turn at 4.3V is causing issues across our system.  Any thoughts what could cause this?

[reyes]

Hi,

Make sure SWBST1MODE and SWBSTBYMODE bits are ‘10’ (Auto (default)), otherwise, you will see a parasitic leakage path for a boost regulator causes the SWBSTOUT and SWBSTFB voltage to be a Schottky drop below the input voltage whenever SWBST is disabled.

Voltage drop at Schottky diodes of around 1 mA is in the range of 0.15 V to 0.46 V.

Regards,

Jose

[benjaminblanton]

We are seeing the part come up at 4.3V on power on;  Does setting those bits make sense in this situation?  Around 2.5 seconds after power on, the PMIC gets written to and the power output goes to 5V.  Why are we seeing it below that before registers are written?

[reyes]

Hi,

I understand now, thanks for the clarification, it is not that you are not seen 5V on this regulator, but the problem is that it take around 2.5 sec after power-on to get to this voltage, correct?

Then this is normal behavior and it is caused by your start-up sequence configuration, take for example OTP A5 of table 42 of the datasheet, the ‘OTP_SWBST_SEQ’ is 6, which means that SWBST is programmed to turn on SEQ_CLK_SPEED * 6, where  SEQ_CLK_SPEED is 2ms, so, 2ms * 6 = 12ms.

Check what is your ‘OTP_SWBST_SEQ’ configuration.

Also, this is calculated with the typical external components mentioned in table 60 of the datasheet and no load on SWBST output (ISWBST = 0.0mA), turn on time can be affected if different external values are used and with large current load are attached to the regulator.

Regards,

Jose

[benjaminblanton]

Can we decrease this delay by programming the OTP?

[reyes]

Hi,

Yes, you can decrease the powerup sequence of the SWBST by OTP programming the PMIC using ‘OTP_SWBST_SEQ’.

Regards,

Jose