TPS61200: Why TPS6100 has output at the moment of start?

I need the UVLO and I can understand it will shutdown if the Vin drop under the threshold.   When shutdown, Vout=0. But the datasheet didn't mention the result I observed. My point is that the output will jump to 5V but fall to 0V immediately if I connect a 1.5v battery directly.  Is this a correct phenomenon?   

Another question, I want to measure the quiescent current. I put a serial resistor(20ohm) at the input and measure it's voltage so that Iq=U/R. Is that right?

Thanks

Hi Peng:

Please share the schematic and layout for us, and catch the waveform of below pins: Vin, VAUX,VL and Vout. So that we could try to reproduce the issue in our lab. 

For the quiescent current, I'll recommend you to know what the IQ is with the application note :http://www.ti.com/lit/an/slyt412/slyt412.pdf 

We also have a application note with details guidance for no load measurement:"Calculating and Measuring the No Load Input Current of the Boost Converter "http://www.ti.com/lit/an/slva828/slva828.pdf.   

If you have anything unclear, please do not hesitate to note me

Hi Peng:

May I know if there is any updating?

Do I really need to capture the waveforms of the pins?  As it can output 5v when Vin>3V, I think the DC-DC is working well.  Maybe you can try like this: Directly connect a battery to the input and then see if the output will jump but fall to 0V immediately? After trying, please tell if the same situation happens in your lab. 

Hi Peng:

Sorry I'm not in office these days, and not convenience to do bench test. I'm asking my colleague to support with a bench test. (But haven't checked the schedule with him, as it's weekend now.) Please wait with patience.

The purpose for asking the schematic and waveform is that we could make the setup with much similar condition. It will help to reproduce the issue.

Thank you very much. I will  gather the waveform tomorrow.

Hi Pend,

could you capture the waveform of the VIN and VOUT in the system board? it is possible the input voltage ring above 3V and the IC turns on the VOUT.

No,I think it's not because it rings above 3V as I had tested it with 1.5V battery.  Figure 1: 1.5V battery as the input .  Figure 2: 3.2V DC source as input .   Blue curve is output while yellow is input.

Hi

could you help to check what happen if the UVLO is short GND? do you observe this behavior in several system board? I would like to rule out assembling issue.