TPS63020: Power Save mode and Synchronization issue

Hello Muruga,

Please share the measurements of the output voltage you are seeing. Please measure at the same time the input voltage and please share your schematic. You can send it to lbs_request@list.ti.com if you do not want to place it here.

Hello Muruga,

Thank you very much for the schematic and the measurement.

Is it possible that the conductivity measurement is taking high current from the 3.3V output of TPS63020? How much?

The measurement you sent looks like a transient response to a high current load with a current limit on the input. Is it possible that something in the input path limits the maximum current to the TPS63020?

Hi,

The maximum stable current during conductivity is about 120mA as measured. The spike is observed when we switch ON the PWM for conductivity, once it is ON it is stable. The PWM is switched ON/OFF multiple times, also we measure four ADC channels. Sometimes it is noted the rail drop below 3V when it is doing the 3 channel or channel (there is a drop during the 1st and 2nd but not significant enough to reset the micro).

The same is not noted when the supply 3.3V is provided using the external power supply. e.g. TI launchpads (any power using a USB). Retrating the point as we have not seeing the same behavior except the TPS63020 is isolated. We tried simulation of the condition with just the power supply and we noted when we have a combination of inductor and capacitor load is suddenly added at the output.

During my discussion with TI engineer for ADS1256, he suggested to remove some the capacitors on the board along with the feritte beads, the understanding is that it might be resonating. We have removed all ferrite beads and reduced the capacitors as much as possible.

Another points is the PS/SYNC pin is connected to VINA and not to VIN as done in the eval board. (We referred the datasheet during the design).

Regards,
Muruga

Hello Muruga,

The connection of PS/SYNC to VINA is not a problem. There is just a resistor from VIN to VINA in this device.

As mentioned, I think that at the moment when you observe the voltage drop, there is a sudden increase in load current. Please check the load current on VCC_OUT and AVCC_OUT when the voltage drop occurs. In addition, please check the supply current from VBAT at this moment.

The problem looks as if there is a current limit on the battery.

Hi,

Sorry did not understand AVCC_OUT, are you refereing to the ADS1256. If so, we have already verified it the load current is not due to ADC, we have isolated the section and verified.

The sequence of operation is like this

1. Power ON section of the Analog Circuit (Condutivity)

2. Change the Channel of the ADC to respective channel

3. Start timers to generate PWMs.(Totally 3 PWM are used at 10KHz (PWM1 & PWM2 is polarity is inversed every 1.2s)

4. Measure the ADC

We measure 4 channel of ADC for conductivity, 1 for temp, 1 for pH and 2 for Turbidity.

Sequence of Measurement

1. Temperature (All other section power is switched off)

2. pH (All other section power is switched off)

3. Conductivity (Channel 1) (All other section power is switched off)

4. Conductivity (Channel 2)

5. Conductivity (Channel 3)

6. Conductivity (Channel 4)

7. Turbidity Channel 1 (All other section power is switched off)

8. Turbidity Channel 2 (All other section power is switched off)

The surge is observed during the 3rd step when the PWM1 and PWM2 is started or when polarity inversion is done. We like to know if the inductor selection of the TPS63020 would impact, currently we are using 1.3A saturation current. Could this be the issue.

We have gone through the layout and did not find any significant issue. Could share the layout files if required on the email.

Regards,

Muruga

Hello Muruga,

It is possible that a 1.3A inductor causes such a problem, if the load current on the TPS63020 is high enough to get it into saturation.

Please replace it experimentally with one with a higher saturation current to check it.

An inductor going in saturation will not be able to transfer energy to the output which might be observed as a voltage drop on the output voltage.

To know how much saturation current the inductor needs to have, it would be absolutely necessary to measure the current drawn by the system in all of the above cases. You have R1 in the output path which you could remove to add a current probe there. Do you have a current probe for your oscilloscope?

Hi,

We have ordered for the Inductors with high saturation, Meanwhile we are testing the power supply section and noticed that the quiescent current is 6mA at 3.6V VIN and Vout = 3.3V and Iout - 0mA. As per the datasheet the quiescent current is supposed to be few uA. We have isolated the output and testing with U2 depopulated.

Regards,

Muruga