TMS3705: Getting aleatory response timing on SCIO

Hello Alessandro,

Do you know if the boost circuit could potentially provide more than 5.5V for brief periods of time? If the 5.5V is constant then it should be okay but any small amount over that could damage the chip.

Could you chip swap the TMS3705 and see if the communication improves? If the chip was damaged, then the a new TMS3705 should immediately behave as expected.

You may also want to consider using a voltage divider between the TMS3705 VIN and the Boost output to bring the voltage down slightly (5V probably would be good, but even just 5.3V should give you more flexibility). You could try that first with the current chip and see if that improves any performance before doing a chip swap.

Hello Ralph, thanks for your reply.

Indeed replacing the TMS3705 for a new one solves the problem, at least temporarily I guess.

When the boost is powered on, it goes from 0V to 5.5V and the ripple caused by the switching could reach 5.7V sometimes, but after stabilization the ripple stays at 100mV pk-pk when the TMS demands current.
The equipment stays on for more than 20 days straight, so this transient condition occurs rarely.
However, the datasheet point out the absolute maximum as 7V and I'm never reaching this voltage.

I could change the boost to supply only 5V, but 5.5V gives me a little extra reading range. I really need this extra range because I have a resistor in series with the antenna to limit the energy. I can't change this resistor, but it reduces my reading range drastically.

It should work fine with 5.5V.
Is this an expected behavior? Voltage spikes above 5.5V but below 7V could damage the chip?

thanks.

Hello Alessandro,

It depends on how many spikes there are, how often they occur, if they occur during operating conditions etc.

I can't say for sure if many 100mV peaks above 5.5V would affect the chip but given it operated well for many days and then started to fail, this should be something to take into consideration carefully. Furthermore,TI doesn't guarantee that the device will operate correctly above 5.5V. Generally the TMS3705 is operated at 5V, and the 0.5V tolerance on Vdd provides a buffer for ripple switching on power supplies and it is not expected that the device would be given a 5.5V rail with 100mVpp ripple as that exceeds the Recommend Operating Conditions specifications.

Please note the following about the Absolute Maximum Specifications for 7V: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated under Recommended Operating Conditions is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.

Given the description and that a chip swap solved the issue immediately, it seems to me that it is quite possible that some sort of damage may have occurred to the failed device, and the operating voltage has me suspicious that could be a root cause.

I understand the need to get full read range, so perhaps just adjusting down to 5.4V which will account for the ripple would be sufficient? I agree that the small overshoot before stabilization is not of concern, so just ensuring the maximum values of the switching ripple under operating conditions remains below 5.5V should be sufficient and still provide you better read range than dropping down to 5V.

Another way to amplify the read range would be to use a solution like this, which would let you keep the part at a safe 5V setting and still improve your performance: http://www.ti.com/lit/an/scba031/scba031.pdf