Picozed Power Enable

Question

Hello,

I'm designing a picozed carrier card. I'm a bit puzzled about the PWR_ENABLE input. In the FMC design, I see that the switch, when in off state supplies 12 V to the BSS138 (Q3) MOS, and the MOSFET is in open drain. It is said in documentation that till rev C there was an onboard pull up on the picozed itself, but it has been removed from the Rev E. Also, the data sheet of the TPS54618 says that the maximum value of power enable to the IC is only 4 V. So, i cannot follow connecting an external pull up to VIN_HDR, as was the case on previous versions of PicoZed (on board, till rev C) right ?

Secondly, can I connect it to the 3.3 V Power header ? Like 12V_OFF through a RC network for setting the time constant, and a pullup of 10k to 3.3V should be good to brnig up the power enable ?

Guidance would be appreciated ! :)

ctammann · Accepted Answer

I'll try to clear things up a bit, if I go off track let me know and we'll get it sorted out. The PicoZed FMC carrier supports both 7010/20 and 15/30, the TPS54618 is only on the 15/30. On the 15/30, the TPS54618 actually has an internal pullup so to enable the regulator you just have to float the line. If you look at the latest schematics you will note that line is floating in the PicoZed 7015/30 schematic. We tie it low on the carrier until we want it to turn on to keep it in shutdown. The 7010/20 version of PicoZed still has the pullup on the enable since it is needed on the TPS62130 used for the core on that version. We orginally copied the design from the 7010/20 but realized afterwards the pullup limitation on the TPS54618. We also realized it had an internal pullup on enable so rather than put a voltage divider on it we are able to just float it and ground it to shut it down. Let me know if that answers your question. thanks Chris