TPS61030: Did I make a design issue with my BQ24075+TPS61030 based power supply

Hi Dolf:

It seems the images are gone. May you upload them one more time? Thank you!

You are quick. I just posted it. They're back now. Copy-paste didn't work.

Hi Dolf:

I have seen the schematics and layout.

Firstly, may you help to check where the issue is? I'll recommend you seperate the IC to locate the issue. How about remove the one of them and redo the test(replace with a DC supply if removing the BQ part)

And i checked the layout of TPS61030. The C14 is too far way from the IC. Please put it as close as possible to the IC and make the outside loop as smallest as possible.  

Thanks Minqiu for the reply. Yes I will indeed separate them (I already asked for a modified PCB for testing). Thanks also for the feedback on C14. On the next iteration I will put it closer. But as far as you can see there are no obvious errors in the design beside the loop for C14 being too large?

Hi Dolf:

If you put the C14 too far, there will be large parasitic inductance in the routing.  It can make the spike on SW high. It will bring the damage risk and EMI noise. 

I'll recommend you use a new board, and remove the BQ part firstly. Then use a DC supply for TPS61030 input. If the TPS61030 works well, then we can focus on debugging the BQ part.

Hi Dolf:

Is there any update? If not, please allow me close this post at first. If there is any update, just reply below. Thank you!

Hi Minqiu,

Sorry for the delay, I was waiting for boards to arrive (Covid doesn't help). I isolated the issue and the issue is indeed with the TPS61030. The BQ24075 works just fine, but as soon as I power the TPS61030 with a 5V power supply, it shorts out internally (it is not the caps or inductor that short out but the chip internally). You think that is indeed caused by the distance of the C14 capacitor and moving it much closer to the output on the 5V rail (on the other side of the tantalum capacitor) would fix it?

Cheers,

Dolf

Hi Dolf:

Is there any loading after TPS61030? You can try startup with no load, and also removing the Tantalum cap.

I just added a ceramic capacitor closer to the TPS chip but haven't powered it on yet so will remove the tantalum cap first and see what happens. It has no load connected indeed. 

Hi Minqiu,

I took off the tantalum capacitor and BQ24075 and used a DC power supply as input for the TPS61030, without any load connected. I did move the 2.2uF capacitor C14 to the other side of the tantalum capacitor, so much closer to the output of the TPS61030.

With a 5V DC input, the output voltage of the TPS61030 initially was a stable 5V, but after around 17 seconds, it started producing a sawtooth that had a continually increasing voltage. It definitely went above 6V before I cut it off in order not to damage parts.

When I changed the input voltage to 3.7V, the output of the TPS61030 started at 3.7V as well (why not 5V?), and again showed the same sawtooth after a few seconds and started to increase the voltage, which I again stopped at 7V to prevent damage. The sawtooth also seems to have an increasing frequency with time.

I attached images of my scope, channel 2 (yellow) is the input voltage while channel 1 (red) is the output voltage. The first image shows the sawtooth right when it started with 5V input voltage, while the 3 after shows the sawtooth at different stages with 3.7V input voltage.

Hi Dolf:

What's the feedback signal for the FB pin?

I now added the tantalum capacitor back, and the sawtooth is smoothed out, but the voltage still keeps increasing above the 7V.

The FB pin is floating, which according to the datasheet should give maximum output power and 5.5V output, right? I tried to measure the signal but the chip shorted out internally again. Not sure what caused it, but it happened before I could see any signal. I now need to wait for the next board which should arrive hopefully tomorrow or the day after

I did check and the FB pin is indeed flaoting. There is no continuity between it and either the 5V output or GND pins next to it.

HI Dolf:

Sorry that I should point it before. I forgot you are using the adjustable version of TPS61030. The FB can't be float for TPS61030. 

Please follow the 11.2 （Fig.13） to add the feedback resistance. 

That is interesting, because figure 19 in 11.2.4.1 it does show an example with a floating FB pin. Is that only applicable to select chips? That is quite confusing to be honest.

On which  chips is the floating FB pin allowed?

Ok, thank you very much for your help, you've been a great help. If I could make a suggestion it would be to make this clearer in the datasheet if possible. I am checking the specs for them to see if we can use them as a replacement without modifying the current PCB design. That would be the best option currently.

Hi Minqiu,

Just to be safe, if I read the datasheet right, the TPS61032RSAR is a drop in replacement for the TPS61030RSAR right? The only difference is that the 61030RSAR is adjustable through the voltage divider on the FB pin while the TPS61032RSAR has a fixed 5V output voltage and can therefore have a floating FB pin. We will stay at or below 1A output current, except for a maybe a very short peak during startup so I think heat won't be an issue.

Regards,

Dolf.