TPS543C20: TPS543C20 Voltage drops to zero when loaded

The switching waveform for when is loaded it not very good, the duty cycle is changing constantly and that is why the have overlap. The duty cycle increases and decreases

Often times duty cycle issues is related to stability or ramp selection. Please try decreasing the ramp resistor or please share the full schematic along with capacitor part details including esr and derating

Here is the Schematic. I added more capacitors in the output and input, but still nothing. The weird part is that it does not turn on when it is under more than 2A load. Or when I do step changes from 0.5ohm to 0.25ohm, it shuts down and stays off and doesnot turn on!

Let me know if decreasing the ramp resistor resolves it. I will double check the schematic in the meantime

Best regards,

Britton

I used 100k for ramp and still the same problems and not working

I also noticed that when the input is set to 12V, it doesnot drive the load and IC gets shut down! but when I decrease the input to around 5-6V, it turns on and drives the load successfully!!!!!

Hello, 

Can you confirm which inductor you are using on the board? The excel tool listed a 1uH inductor but the schematic lists a 330nH.

The reason that 5VIN works successfully and 12VIN does not is that as VIN is increased, the transconductance increases proportionally. This results in a higher loop gain and can cause instability issues. 

Assuming that the issue is related to stability, decreasing the ramp capacitor, increasing the switching frequency, increasing the inductor or some combination of these will help.

The other potential issue that would be more impactful at 12VIN than 5VIN is SW or VIN coupling to feedback. This is prevented by having appropriate shielding GND layers between switching planes and feedback path. If the above suggestions (lower ramp, then increase fsw, then increase inductor) does not work, you can attach your layout here for a review

Best regards,

Britton

Hi Britton

Hope you are doing great

Thanks for your reply. I used a 0.33uh inductor there. When I removed R29 and R30, it works.

There is another problem that I am facing now! I am using a bench supply that can provide up to 30V, 3A. When I power up the DC source instantly from 12V, the buck does not work, and when I gradually increase it, it works. It is due to surge current that either the supply cannot provide or DC DC controller shut down due to surge current protection. I already 4x22 uf MLCC and 1x330uf AL-cap as my input cap, and I have 3 hybrid 470 Caps and 4x47uf MLCC as my output caps. I tried to decrease the cap and increase the SS to maximum (32ms) but still it has that weird behavior. All the time I should increase the input source gradually until it starts working!

When power is applied to the input, the input current that TPS543C20 draws should be proportional to the load. For example, the TPS543C20 should not be drawing any more current from the power supply than it requires so the device influencing the a surge is not likely. This may require a power supply solution. One thing I am thinking about - does the same issue occur if the TPS543C20 is disabled? If so, then it needs to be addressed on supply side, if it does not then you can consider adding some enable logic so that the device is enabled after power is provided

Best regards,

Britton

I changed the supply and it a little bit helped, I assume! I thought it is just because of the inrush current that prevents the IC to be turned on! Hence, I removed Ilim resistor and it still does the same! It doesnot turn on! I added more cap at the input and still nothing! when I turn on the DC source at 12V, IC looks trying to turn on but it does not. So,l I have to lower the voltage to 11V until it is on and after that it can work with 12V
I donot know if this kind of behavior is normal for all ICs but I just to make sure the design is fine

Try disabling the IC, then turning on the 12V supply and then enable the device. This should have similar effect as turning on to 11V then increasing to 12V. And just for more information, The current limit resistor is just monitoring the output current so should not have any effect on this behavior assuming there is no load or normal load

Dear Britton

Thanks for your reply. Yes I agree with tou, Ilim resistor just controls the load current.

It is very weird that why this topolgy just works with lower voltage. I mean when I set the dc source at 5-8V, even with 40W load, it works and the moment I increase the voltage to 12V, sometimes it works and sometimes not

Hello,

Britton is out of office. Please expect a couple weeks delay in response.

Thanks,

Calan

Hey,

Just checking in here - did you get a chance to try tuning some of the knobs that help with the stability? For example, decreasing the ramp capacitor, increasing the switching frequency, increasing the inductor. 

If needed, I can put together a BOM that should work for the 12V application but before I do so, we will also need to double check your layout to make sure its BOM related and not layout related.

Best regards,
Britton

Hi Britton

I did some tuning with the feedback and stability, and nothing happened! I increase the frequency from 500khz to 1Mhz in it worked. I didnot want to increase to switching frequency due to loss and heat! I was hoping to get it to work with a lower switching frequency

Just to make sure we are on the same page here: the issue that we are trying to address is the device failing under load? 

Since its working at higher switching frequency, we can likely rule out SW coupling.

I will need the output capacitor part numbers in order to truly evaluate the stability including ESR and derating, without that - I can only provide estimates

In addition to sharing the capacitor part numbers, you can also try a combination of 121kOhm RAMP and a 0.47uH inductor (our maximum recommended inductance for this condition)

Best regards,
Britton