STM32L152VC: LCD segments not all 5V tolerant

sheldon · ‎2014-10-21

Posted on October 21, 2014 at 19:36

The STM32L152VC includes an internal voltage regulator with contrast control from 2.60 to 3.51V. Those voltages then get used to drive the individual segments. In my case, I'm running the processor at 2.6V (VD). What I found, however, is that SEG2 (PA3), SEG5 (PB0), and SEG21 (PC3) are only standard 3.3V I/O. Therefore, when these segments are used with an LCD voltage greater than VD, the protection diode on the pin is activated and the processor draws excessive power.

Thankfully I'm not using all possible segments so I can change my segment pins on the next PCB layout to avoid the problem pins. I think this should be listed in the processor errata, otherwise maybe this comment will help someone else avoid the same problem.

Danish1 · ‎2014-10-22

Posted on October 22, 2014 at 11:08

Protection diodes are ordinary bipolar diodes.

People regard diodes as having a turn-on voltage of about 0.6 V.

This is an oversimplification; when there is 0.3 V across the diode there is current through it, but it is small enough to be generally neglected. This is why in many ''absolute maximum'' ratings of chips you will see the maximum voltage on an input pin to be Vdd + 0.3 V

Put another way, if you're only dropping 0.21 V (the difference between Vdd at 3.3 V and a contrast setting of 3.51 V) then the current flow is going to be extremely small. I would expect it to be well under 1 uA.

As you're using Vdd of 2.6 V this is an issue for you. But if ST were to include warnings for all such complications then the reference manual and data sheet would be twice their current size!

Regards,

Danish