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i.mx6ull LDO caps

Question asked by David Luberger on May 23, 2018
Latest reply on May 24, 2018 by igorpadykov

I'm confused about the feature that allows the internal LDOs to be bypassed.  I'm designing an embedded computer using the i.mx6ull processor, and it will be designed as an Intrinsically Safe device. The problem is having is that both the evk reference design and the datasheet call for a lot of "big" capacitors, i.e., 10uF and 22uF.  The evk sodimm board that has the processor, memory, and power supplies has almost 500uF total capacitance, and the datasheet calls for somewhere around 300uF of capacitance connected to the LDO_CAP rails. Unfortunately, in an IS design, any one "power island" can have no more than 100uF of capacitance. 

 

My first thought is to just cut the capacitance on the processor LDO output lines way down and hope things don't get glitchy. I have decent pmic solution that's using a lot less capacitance than the evk switches so I'm good there.

 

Another option I thought of is the datasheet says I can "bypass" the internal LDOs to use my own (TI had some good low noise LDOs that use a noise reduction and voltage reference feedforward caps why are low value but very effective),  but I can't find anywhere a reference design/schematic that shows how this is done.  For example, some of the internal LDOs are used internally, e.g., to power the ARM core. How would I connect an internal device like the CPU core to an external LDO??

 

Any help on any of this would be appreciated. 

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