nRF52DK power supply circuit

[maher]

Hi there,

I have been investigating the schematics of nRF52 DK for a while, especially the power supply circuit. I am very confused how this could work!

The question is, in the case of no USB power and no External power connected but Coin Cell battery is connected. How would the the battery power go through and load switch? 

The AND gates should give 0 and they not even powered! The are supplied by Vsupply and Vsupply should be Zero volts as there is no USB or external power and Coin cell battery is not connected yet to Vsupply!!!

Second question, why you connect the load switch in opposite direction? Normaly, I would expect the battery to be connected to Vin and Vout is connected to Vsupply.

Third, why do you make a load switch and then a MOSFET, is not one enough?

Fourth, Why do you connect the VBUS" through a MOSFET first before the 3.3V regulator?


https://infocenter.nordicsemi.com/pdf/nRF52_DK_User_Guide_v2.x.x.pdf page 11

Thanks,

[Kazi Afroza Sultana]

Hello,

I have asked about revision/version number of your Dk because the power circuitry changed at version 2.0.0 so I wanted to make sure that you had a corresponding development kit.

1. The question is, in the case of no USB power and no External power connected but Coin Cell battery is connected. How would the the battery power go through and load switch? 

2.The AND gates should give 0 and they not even powered! The are supplied by Vsupply and Vsupply should be Zero volts as there is no USB or external power and Coin cell battery is not connected yet to Vsupply!!!'

The load switch U11 is reversed so we can utilize the body diode inside it as well as the body diode of the P-MOS. Then we get voltage VBAT = VSUPPLY - diode voltage drop. This VBAT then will turn VREG_DETECT and VEXT_DETECT high and enables the AND gates U12 and U13, which will fully open U11 and Q5B. So. these two diodes will power the AND gates which will power the VSUPPLY by enabling MOSFETS. (These answers cover your both q1 and q2)

3. Why do you make a load switch and then a MOSFET, is not one enough?

Having both a load switch and MOSFET is for protection. Both the U11 and P-MOS act as a reverse voltage protection for the battery if USB is connected. In case if one of the body diodes shorts out, we will get VREG_OUT or the external supply into the battery, which is not acceptable. 

4. Fourth, Why do you connect the VBUS" through a MOSFET first before the 3.3V regulator?

V5V can be used as an input to power the nRF52832 device, and the body diode in the P-MOS prevents current from going to VBUS' and USB_DETECT which again would power the Interface MCU (IMCU, a.k.a. the separate onboard debugger chip). This just gives us the option to power the nRF via V5V without powering the IMCU.

So, this is to make sure we do not get voltage on VBUS that will lead to get high level on USB_DETECT.

I hope it helps. If you would like to know more, feel free to write us.

Thanks.

Best regards,

Kazi Afroza Sultana

[maher]

Thank you for the very well-explained and detailed answer.