TPS53355 schematic confusion

yep, it is the snubber network,

and yes, normally it is connected to GND, but not necessarily...

actually the snubber is a derivator (d/dt) that sense the slope of the switching line (during the transitions) and "shunt" to GND the energy of the resulting ringing... this is the physic behind... the state  of art is to "shunt" the extra energy (that only generates EMI and stresses the FET) to GND, but in this schematic it is readdressed  to out (in a weird temptative to increase efficiency maybe)...

but going back to the main point: yes, I dread it is a Webench error: better to connect it to GND!!

thanks for noted that...

Vincenzo

Hi Vincenzo,

I have done that:

And the resistor R12 (3R) is getting really hot to the point that is cooking.

Any lights on why this is happening?

Thanks,

Mateus Ohse

are you using a common resistor?

the power loss on this R are expected to be high enough to justify a bigger size (typ. 0805 (min size) or 1206)...

basically the energy of the ringing instead of beeing transmitted as unwanted EMI is dissipated on this R: the worse the ringing (that depends on layout, on FET -emebedded- and on how good/fast the FEt is driven) the higher the loss on the little R...

Vins

Hi Vincenzo,

Yes, it's a common 0805 resistor.

I'm currently running the TPS at 970 kHz (RF pin connected to Vreg). If I decrease this switching frequency, will it generate less EMI and decrease the heat of the 3R resistor?

Another thing: I couldn't simulate this frequency at WEBENCH. Could not find out how to change the resistor value. 

Is it possible to do it?

Cheers,

Mateus

yep, it will reduce as well the losses on the snubber R, even if the effect will be "a minor"...

Table 2 of the device datasheet shows the values you have to use to allow device working at a specific FSW...

in webench you should be able to set the FSW as well... send me your design (export it in pdf)...

Vincenzo

5483.JT-D-0161-SB-006-110 - Power Schematics.pdf

Hi,

Please find attached the powers section of my project.

Is there anything that can be optimized?

VIN is from 10V to 15V and Iout up to 10A.

I've been trying to find the TPS53355 at WEBENCH, and couldn't find anymore. Was it taken out?

Cheers,

Mateus

yes.

please refer the attached Webench report for the correct bom values...

if the two LED on the output are minded to indicate the presence of the 3.3V then i would reccomend to use the PGOOD, minded exactly for this purpose...

i.e. close the R8 to PGOOD instead of the GND...

while about D1/2/3 you can use cheap and normal zener in case, in particular something like the CZRQR52C2V7-HF would be more than ok (I assumed the purpose of them is just clamping the voltage of ADC_xxx signals)...

Vincenzo

6607.A9R13F6.pdf

Vincenzo,

Thank you very much for that. :-)

What if I draw 20A from this design? Any changes needed?

I still couldn't find the part at WEBENCH anymore. :-/

Cheers,

Mateus

you are right,

the TPS53353 is actually supported in webench, but web site readdress also TPS53355 to this one (same silicon anyway, just the -55 is 30Amp while -53 is only 20Amp, but control sessio is the same...)

attached the bom for a 19Amp version based on TPS53353: just use the same for the TPS53355 one...

Vincenzo

5165.A9RCC8D.pdf

Hi Vincenzo,

Thanks for your help.

But now they've removed even the TPS53353 from WEBENCH.

Do you know why?

"Design cannot be created with this device.  Please press OK and select a different device from the list"

Cheers,

Mateus

no, devices are still there...

the message

"Design cannot be created with this device.  Please press OK and select a different device from the list"

means that the specific device you selected is not able to meet the specifications you put for various reasons (to high temperature, VIN to close to maximum allowed, or no commercial part can fit as bom)...

what are the specifications you put into exactly?

Vincenzo

Hi,

I go to this page: http://www.ti.com/product/tps53353

Then, on the right side, where it has "WEBENCH TPS53353" window, I click on "Open Design" button, without changing anything.

Will open the Webench, but display that message.

It was working last week, now is not anymore. :-/

Mateus

I see, you are right...

I will double check with Webench Support Team to understand what happened...

I will keep you uptodated, sorry for the inconvenience

Vincenzo

hi

I just wanted to inform you that TPS53353 is once again enabled in WEBENCH®.

Please let me know if you have any other questions or concerns.

Vincenzo