TM4C123 doesn't start

[B@tto 51]

Hi guys,

 

I have a simple problem (I hope) : I have designed a custom board with a TM4C123, and I implemented a simple USB port with a linear regulation on 5V to generate 3.3V for the MCU. As there is another source of power on this board for 3.3V, I added a schottky diode to prevent short circuits between supplies. 

 

My problem is when I power my board with USB, It doesn't start, but with the main supply it's okay. I checked voltages : from main = ~3,3V and from USB = ~3V  So I guess that 3V is not enought.

 

What can I do against that ? I tried unsuccessfully to disable BOR but I'm not sure at all of what I tried (not a lot of information on the web). My IDE is Energia but as it's not a problem directly link to it, I posted here.

 

Thanks

[chicken 630]

Chapter 24.3 of the datasheet specifies 3.15 to 3.63 V for recommended operating conditions.

http://www.ti.com/lit/ds/symlink/tm4c123gh6zrb.pdf

 

Also see chapter 24.6.2:
 

The POK (Power-OK) monitor is used to keep the digital circuitry in reset until the VDD power supply is at an acceptable operational level. The digital Power-On Reset (Digital POR) is only released when the Power-On Reset has deasserted and all of the Power-OK monitors for each of the supplies indicate that power levels are in operational ranges. The BOR0 (Brown-out reset) and the BOR1 monitors are used to generate a reset to the device or assert an interrupt if the VDD supply drops below its operational range. The BOR1 monitor's threshold is in between the BOR0 and POK thresholds.

[chicken 630]

PS: You could try to put the diode between 5V and the LDO. Won't work with all LDOs but worth a try.

 

I regularly use the MIC5504, which works in that setup as long as EN is connected to VIN.

[B@tto 51]

Thanks for your answer.

 

I think you're right, even if table 24-10 seems to show that I'm ok versus power-ok. I cannot add any other component as my pcb was design like this.

 

I will try without diode for tests and if it's that issue I will try with another model of diode

[B@tto 51]

Ok so I tried without diode and it's okay. I did another test with a PTC fuse instead of the diode, it does not work too. So for now I directly connected the 3V3 from USB to the main 3V3 power but now I know that I should choice an ajustable regulator to compensate the voltage drop

[LiviuM 43]

You can try to compensate the diode voltage drop by putting another (series) diode (same type as the protection one) between the GND pin of the 3.3 V LDO and the GND of your circuit board.

In this way you will raise the LDO's reference with the same amount you loose on the protection diode.

[B@tto 51]

Yep but the board from china was already in my hand and GND tab directly connected to the ground plane so hard to modify in this case xD

 

But you're right it's a good way to do the job