Power Switch

[JohnsonMiller]

Hi Guys,

In openRex design page 28, and A7 sheet area, there are two mosfets used to on/off the OTG power (Q1 and Q2), I was wondering why 1 mosfet is unable to do the job and we have to use two face to face connected mosfets? any comment or help?

[robertferanec]

It's there, because you need to control the current flow from two directions. Inside mosfet, there is a diode which, if you do not use two transistors, will in some cases cause current leaking between external power supply and USB port power.

[JohnsonMiller]

Thank you Robert,
on the same page, there is another MOS in the main power path, but single MOS not double, what is reason? Probably delivering power not sharing between two active sources?
Another related question is the diode, itself. Is it a pasitic component or added on purpose? and can we get rid of it? It looks that sometimes it cause problem.

[robertferanec]

The single mos is used for over voltage protection (it will disconnect power if input voltage is higher than set by TL431). The diode is there to lower voltage to meet PMIC input voltage requirements (max 4.5V).