TPS562200: Output too high

Hi Katrina, 

Please upload schematic again. now it doesn't show schematic. Could you help to draw a simply block to show the relationship between TPS562200, battery and BQ24650VAR? 

BRs, 

Edwin. 

Here is the schematic and block diagram you requested.

Hi Katrina, 

From the block, battery doesn't have any relationship with TPS562200. 

I think firstly we need to find the reason why TPS562200 output is fluctuating with out battery board. 

From the waveform, when TPS562200 has no pulse, output voltage is pulled high. so i think if there is another power which connects to the output of TPS562200. what's the loading of TPS562200? could you double check the waveform if you can disconnect TPS562200 loading? 

BRs, 

Edwin. 

There are no other supplies after the 3.3VDC.  This power supply powers only onboard circuitry and the only way I could remove the load is to cut traces.  I also measured the input on both instances and it is the expected the +11.5-+12VDC.  The curious thing is that this issue is happening on two separate boards with similar power circuitry, but the +3.3VDC on the other board is the LMR16006YQ3DDCRQ1.  Is there any similarity between these parts that could point me in the right direction?  I wouldn't think the the output voltage would change depending on the load as long as it is within the range of the power supply.  Would there be an issue if the load is very low?  Could the circuit be unstable?

An update--I realized I had too much sink current into STAT 1 and STAT 2--which explains the pulsing since those outputs were toggling.  I removed that issue and powered up a brand new board.  The only issue I have now is that the output of that supply is always sitting at 3.9V

hi Katrina, 

Please double check if the divider resister is right value. could you also share the waveform of 3.9V Vout. please plot Vout, SW and FB voltage. 

please check if FB voltage is still 0.76V. 

BRs, 

Edwin. 

I will take that oscope waveform tomorrow, but in the meantime, I realized that I made a mistake.  I do not have too much current going into the Stat1 and Stat2 pins--I was looking at the wrong side of the circuit (shown below).  But when I removed U16, that is when the output voltage stayed steady at 3.9V.  So--I'm wondering if that is because the load is too low.  I went through the circuitry and when stat1 and stat 2 is low (or when U16 is removed) the total current for 3.3V is 4.2mA.  If one or the other of the stat pins is high-impedance (turning on one FET in U16), the total current is about 20mA.  I haven't been able to prove it yet since I am not in the office right now, but I'm wondering if when the load is low, the output goes to 3.9V and when more current is being drawn, the output starts switching and goes to 3.3V.  Could that be possible?  Also, I did measure the divider resistors and VFB--all looks good.

Hi Katrina,

it's impossible. even if the loading is 0, output voltage should be target output voltage, not higher than target value. 

please help to plot waveform to debug. 

BRs, 

Edwin, 

Hi Katrina Barnes2,

TPS562200 is Eco-mode part so normally the light load voltage will be higher than heavy load condition, but won't higher that much. We may still need the waveform to locate the root cause. Thanks.

I monitored each of the places you asked--pictures attached.  Red is SW, Green is VFB and Blue is Vout.  I also included a zoomed in picture of SW so that you could see it's not a clean pulse.  I'm thinking that at this point it might be best to go with a linear regulator--it's so low current, it's probably a better solution anyway.

Hi，

Of course LDO could be used here as load is pretty low. But if further look into this issue, the VFB seems not 0.76V, could you help confirm?

After monitoring the VFB pin for a while, it seems as though it fluctuates between 0.760V and 0.890V.

Hi Katrina, 

As you said, SW isn't a clean pulse. It's just noise. Because once FB voltage is higher than 0.76V, IC will not have pulse again. Could you capture a waveform of the moment that SW change from clean pulse to unclean pulse. I think at that moment there is something wrong which causes the waveform fluctuate. 

BRs, 

Edwin. 

Here is the picture you requested.

Thank you for your help.  It turned out that an IC on the board that could either supply voltage or be powered by an external voltage was causing the issue.  When I remove that IC, the voltage looks good.  Thanks again!