TPS61028: Generating an output when VBAT is low

Oscar Zuniga

Part Number: TPS61028

We're seeing something odd when using the TPS61028. We utilize two batteries with our design. One of the batteries goes into VBAT of the TPS61028, which generates an output voltage. We also have a switching circuit that involves using PFETs that allows the overall board to switch from the battery that goes into the TPS61028 to another battery if the battery going into VBAT dies. However, the switching isn't working properly when VBAT goes low because the TPS61028 VOUT seems to be generating an output voltage even though VBAT is low. The datasheet states that the boost regulator will not be able to boost anything if VBAT drops below .9V. We're seeing .2 - .3V on VBAT. Is there a possibility that the regulator is trying to boost even though it is below the minimum requirement? We also have PS tie to ground. Is there also a possibility that power save mode is kicking in when it is dropping below .9V? Any insights as to why we're seeing the boost regulator providing an output under the minimum input requirement is appreciated.

over 3 years ago

0 Minqiu Xie over 3 years ago

TI__Mastermind 21565 points

Hi Oscar:

When in operation and the battery is being discharged, the device automatically enters the shutdown mode if the voltage on VBAT drops below approximately 0.8 V..

May you share the schematic with us?

0 Oscar Zuniga over 3 years ago in reply to Minqiu Xie

Prodigy 120 points

Please see below image:

0 Minqiu Xie over 3 years ago in reply to Oscar Zuniga

TI__Mastermind 21565 points

Hi Oscar:

Thanks for your reply, I'm much clearer with your system now.

"because the TPS61028 VOUT seems to be generating an output voltage even though VBAT is low. "

May you catch the waveform of Vbat, SW, and Vout to help me reproduce this phenomenon in our lab? And what's the load condition?

0 Oscar Zuniga over 3 years ago in reply to Minqiu Xie

Prodigy 120 points

Hi Minqiu,

Unfortunately, I can't get all of the waveforms due to some restrictions and the inaccessibility from the COVID shutdowns. This is the best I can get from my colleague at the current time.

Vout:

Vbat: Measuring .220V.

Load condition should be under 50 uA, but we are seeing a 1 mA spike at times.

0 Minqiu Xie over 3 years ago in reply to Oscar Zuniga

TI__Mastermind 21565 points

Hi Oscar:

Hope you are doing well and stay safe.

I did a test in lab today, and I did't see the phenomenon you mentioned. I use an additional voltage source at output side, and short the En to Vin pin. Once the Vin below 0.8V, the converter stops switching.

The Vout waveform and ripple you observed may caused by other power source. The easiest way to check if the converter still switching is to measure the SW waveform.

0 Oscar Zuniga over 3 years ago in reply to Minqiu Xie

Prodigy 120 points

Thanks, Minqiu for checking on that. We will take a look into the SW waveform. This gives us better confidence that it isn't the TPS61028 that is causing the issue and that the issue might be something else.

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TPS61028: Generating an output when VBAT is low