Sensor LM35 Power

Josh,

Thanks for looking at this.

Here is the schematic of what has been added to the RF430FRL152HEVM board.  The rest of the evm board has not been changed.  The LTC1517-5 is a voltage doubler..

Thanks,

Bernard

I suspect that even with the low current output, that this may be too much to charge up the decoupling caps on the votlage doubler quickly.

Are you planning on powering this system from RF field only?

If you are powering the board using the TRF7970A RF field, I would recommend to test out that you power the RF430FRL152HEVM from the USB power (and selecting the S6 switch position to "supply").  

Let me know if this works or not and we can discuss this further.

Alex

The circuit has to be powered from the RF field only. It also has to use a LM35 temperature sensor because it is part of an existing system.

I powered the board from USB and put S6 in the Supply position. I still get the "Could not write to the device" error when I click the Start Demo button with the above circuit attached.

VDD2X in RF power mode is between 2.0 and 2.3V. In USB mode it is 2.3V. It appears any current draw stops the demo from working.

I was able to power the circuit from VDDH. Unloaded VDDH is ~2.1V, loaded approximately 1.8V. Unfortunately doubled that gives me about 3.6V which is not enough to power the LM35 which has a 4V minimum supply.

Regards,
Paul.

It appears as if the VDD2X power supply is not able to power your circuit.

There is one more thing that we can try.

The VDD2X is only started when there is FRAM access. When a RF command comes in there is access to the FRAM and powering up the voltage double takes time. If there is a load it may miss the command.

Take the NFC only project provided in the examples of firmware and add this line in the main function after all the inits:

RFPMMCTL0 = PMMPW | RFPMM_EN_BATSWITCH_BYTE

This line will permanently enable the voltage doubler. How this may help is that this may remove the startup time on the voltage doubler when an RF command comes in. You may run the part passively.

Let me know if you try this and we can proceed on what to do next.

I made a mistake, please use this line of code:
RFPMMCTL0 = PMMPW |RFPMM_EN_V_DOUB

Hi Alexander,

Thank you for suggestion. Adding the line of code to the main section of the RF430FRL152H_NFC_Only_Project changes the VDDx2 to a constant 3.0V.

However, when I attached my circuit the voltage drops to 2.1V and the demo stops working.

VDDH is 2.1 unloaded, is 1.8V with the circuit attached, and the demo continues to work. This makes me think the VDDH pin is able to supply the current I need, and I just need a different converter.

I'm looking at a TPS60140/1 Regulated 5-V Charge Pump Voltage Tripler that may work in this application. What do you think?

Thanks,

Paul.

Yes, this would be the only option it seems. However keep in mind that the voltage tripler will load the VDDH more but this still may be enough to reach your 5V level.

You could try to look for a temperature sensor that uses less current and can work at a lower voltage than LM35. Have you considered LMT70?

Also, why are you using an external temperature IC? Why don't you use an external thermistor as it is on the EVM?

The LTC1517 that I am using now draws almost 900uA at 1.8V vs. 125uA at 2.7V input, so I think the tripler will work.

Unfortunately I can't change the LM35 because it is part of an existing system. It would make life a lot easier :)

Thanks,
Paul.