TPS2121: TPS2121 output drop when switching

I pull ST up to RP1,and output volotage drop reduce 。

But it still makes output volotage drop to 3V in 440us。

How can i reduce output volotage drop？keep output volotage over 4V

4721.TPS2121_XCOMP.pdf

Hi Weifeng,

Your schematic does not seem to line up with your scope shots. If you have 5V on IN1, the the voltage on PR1 will be 5*5.1/(5.1+4.7) = 2.602V.

If you have 5V on IN2, the voltage on CP2 should be 5*5.1/(5.1+10) = 1.688V

Since it seems to be that you are using this device in XCOMP mode, the voltage on PR1 is larger than the voltage on CP2 which means the priority is set to IN1 over IN2. So if IN2(5V_DC) is pulled out, this should not have any change to the device. No channel flip should occur. Channel 1 will continue to pass voltage to the output. 

I am not sure I understand your question.

Hi ，Shreyas Dmello

The priority is set to IN1 over IN2

But when the IN1 pull out，the output volotage drop，the minimum output voltage is 3V

How can i reduce output volotage drop？keep inimum output voltage over 4V

Hi ,Shreyas Dmello

The priority is set to IN1 over IN2.

But when the IN1 pull out,the output volotage drop,like the picture

The output volotage will drop to 3V,which makes the system reset

I want to reduce the output volotage drop,and keep  minimum output voltage over 4V 

My purpose is when IN1 pull out,the IN2 can power to the system without volotage 

But it seems to be that no matter how I debug it there's still a voltage drop

Please help me to check the schematic

Thanks

HI Shreyas Dmello,

Do you have any other contact information?

I am in China.We have time difference

It's one o 'clock in the morning in China

HI Shreyas Dmello,

You are right. IN1 is 5V_USB

I understand what you said,but do you have any other solution?

and are there any problems with my schematic design ?

Thanks

Hi Weifeng,

You have to trigger the switchover at the right time to minimize output drop. To do so you will have to change the resistor network either on PR1 or CP2 as stated before.

As far as the rest of the schematic goes;

1. Your OV is set to 6V. Is this okay for your system?

2. The ILIM is set to 4.94A. Is your system okay with such a high current limit?

Hi Shreyas Dmello

I did the test the way you said.

I find if i change the voltage on CP2 to 2.4V or 2.0V and  the voltage on RP1 is still 2.66V ,the priority of IN2 is over IN1.

At the same time,i test the volotage on RP1,and i find the volotage on RP1 become 1.4V. 

I don't know why the volotage on RP1 will change from 2.66V to 1.4V. 

Thanks

Hi Weifeng,

Could you please take a scopeshot during this "event"?

It would be helpful to have PR1, CP2, ST and VOUT in this scopeshot.

By the way, do you have a TPS2121EVM? 

Hi Shreyas Dmello

I'm sorry I'm off duty. I can't use the oscilloscope.

And i don‘t have the TPS2121EVM

Is there a requirement for the difference between the volotage on RP1 and CP2?

From the test，i find that if RP1-CP2>1V, the priority of IC is correct.

And the ST pin must pull up to RP1? 

I find that the ST pin connect to GND from TPS2121 datasheet. Is right?

HI Shreyas Dmello,

I did the test the way you said.

I find if i change the voltage on CP2 to 2.4V or 2.0V and  the voltage on RP1 is still 2.66V ,the priority of IN2 is over IN1.

At the same time,i test the volotage on RP1,and i find the volotage on RP1 become 1.4V. 

I don't know why the volotage on RP1 will change from 2.66V to 1.4V. 

Thanks

Hi Weifeng,

Please share it when you can. No rush. 

ST can be grounded if unused but it is an open drain output. Some implementations pull up to the PR1 voltage, some implementations pull up to V_out, some implementations pull up to an external compliant voltage source (check the datasheet for max voltage allowed on this pin). But it basically a channel flag. If pulled up to a compliant voltage, when the voltage is high it means channel 1 is enabled, and when the voltage is pulled low, it means channel 2 is enabled. It is a very useful tool.

HI Shreyas Dmello,

But could you tell me the reason

"

I find if i change the voltage on CP2 to 2.4V or 2.0V and  the voltage on RP1 is still 2.66V ,the priority of IN2 is over IN1.

At the same time,i test the volotage on RP1,and i find the volotage on RP1 become 1.4V. 

I don't know why the volotage on RP1 will change from 2.66V to 1.4V. 

" 

Hi Wiefeng,

In this situation CP2 is at 2.4V and PR1 is now at 1.4V so CP2 will have priority which makes sense.

Unfortunately, the question is why is PR1's voltage dropping. This is why I want to take a look at the scopeshots and see the behavior.

Hi,Shreyas Dmello

I tested the RPI,IN1 and Vout with the oscilloscope.

The test results are shown in the picture. IN1 and IN2 exist together

CH1:Vout

CH2:IN1

CH3:RP1

If only IN1 is used,the RP1's volotage will be normal,about 1.8V

But if IN1 and IN2 exist together,the RP1's volotage will be change from 1.8V to 1.0V

Hi Weifeng,

In that scopeshot not only does it look like PR1 is dropping when IN1 is stable, it also shows IN1 rising with VOUT after the switchover I presume. This could be because of no current being pulled from this source now which makes sense.

I am looking into the PR1 drop. This is not expected behavior. What is CP2 in this case? Or IN2? I would say that something is off with your setup because it looks like PR1 is being externally pulled down!

Hi Shreyas Dmello

So now how do I solve this problem?

Could you give a reference design that meets the my requirements？Then I printed a PCB according to the reference design and came back to test

Becsuse i don't have a demo borad of tps2121

Thanks