TPS610986: Device stability when transitioning between pass through and boost mode.

A few more comments:

In our design the CC1310 is the only active device on the VMAIN bus. We use the CC1310 to control the mode pin which enables the VSUB for a number of sensors. When the CC1310 sleeps we see approx 1.3 uA of current consumption. When the CC1310 turns on VSUB, we see appox 15mA during sensor readings and then 26mA for the active transmit. The device then transmits data, turns off VSUB, and goes back to sleep. Thus we expect the supply to provide 1.3uA most of the time and then be called on to supply 26mA for active transmit.

The stability issue seems to be exacerbated when the Vin is near the pass through threshold 3.35V for the '986.Based on my calculations the current demand between 1.3uA to 26mA, should not be an issue. I calculated 120mA with 80% eff and vin / vmain of 3.3V. 

Our questions are:

1. Is the current requirement of 1.2uA to 26mA an issue?

2. Is the current requirement of 1.2uA to 26mA an issue with the mode pin set low?

3. It is not clear what else the mode pin actually does? When the datasheet says low power mode is this really just defined as VSUB being turned off? Does mode pin being low have any affect on the'986's operation? does it affect boost operation or continuous / discontinuous operation.

4. Like stated above if the device is running completely in boost mode the CC1310 is stable. If we are well above the passthrough range i.e. 3.5V the cc1310 is stable. 

Thanks

Hi Ken

Thanks for asking. Please check the datasheet for the spec of Vout in datasheet page 7. You could see the range of the Vout is from 3.2V to 3.4V. It's a normal behavior for TPS610986 get higher Vout when the load is light or Vin is close to Vout. Showed in Fig 54,"Burst mode"  

I hope I could give you some other chip recommended, but firstly please let I know what the Vsub use for, and what's the Vin range you prefer?  

Hi Minqiu,

Thank you for your reply, but I don't believe you really addressed the question. Our main question is the behavior of the mode pin as it relates to "LOW POWER MODE". It is obvious that it turns VSUB off, but is that it?

We believe that this device has a stability issue around the Pass through threshold. As mentioned earlier, the circuit behaves as expected when below pass-through threshold operating in boost mode and when 300mV above pass-through. Hysteresis of .1V does not appear to be accurate. I should note that the voltage level supplied is correct and as you mentioned slightly larger scale. That said this is not the issue. We see a significant increase in noise riding on VMAIN (150mV to 200mV) when the TPS device is transitioning between pass-through threshold and about 300mV above pass-through threshold. Our testing shows the CC1310 begins to mis-behave when powered in this region. Again, we can operate the device without issue when above or below this region. Also using a external supply (bypassing the TPS) the CC1310 operates as expected. Additionally, we confirmed our results using the TI EVM (TPS610981EVM-674) observing the additional noise while in the region i.e. (boost to out of pass-through). We have discovered that by greatly increasing the circuit capacitance we are able to mitigate the CC1310 issues, but it is far beyond what the datasheet recommends. 

Regards,

Ken

Hi Ken:

Thanks for your reminder. Sorry as it's not a face to face talk here, I'm not clear about your system some time. So please allow me check you system information at first, otherwise we will easily misunderstand each other. (As I hope I can give you answer at my first reply, but there is too much uncertainty. )

As I not see your waveform about the output voltage, I'm not sure it's noise or it caused by the burst mode(PFM). For the burst mode, the Vout ripple will be much higher than normal operation as I mentioned before. As the phenomenon you describe at first is very close to PFM behavior. I assume the issue is caused by the PFM, so choose a device without PFM maybe a better and easier solution. That's the purpose I asked about what is load for the Vsub, the requirement of the sensor. Before that, we still need to confirm if it's really caused by PFM. Therefore, it's recommended to share the Vout ripple with 3.3V offset with us to have a double check.

For your questions. 

1 . What is the "1.3uA" for? If it's output current. Then no problem. If you want it's for input current, I afraid it couldn't. 

2. I'm confused if there is no power for Vsub, where is the "26mA" come from?

3. And for “Mode” pin, it will cut down the power supply to the Vsub, but no influence to Vmain directly. (For example, Vmain has no load, but Vsub has load. Once cut down the Vsub, the total load will be decrease and the converter will enter the PFM mode. It will influence the Vmain, but not directly)

4.  As I mentioned, the PFM mode have large ripple than other mode.

Hi Ken:

For better understand your issue, please share the schematic and the waveform for me. Thanks for your kind understanding. 

Minqui,

I would like to PM you a document regarding this issue.

Regards,

Ken