LM5175: LM5175 Issues with MOSFETs

Ali Tatari38

Part Number: LM5175
Other Parts Discussed in Thread: CSD19538Q2, CSD17571Q2, , LM5176

Hi ,

Please see attached simulated design using Webench for Lm5175 . On our board we changed the FET on Boost side from CSD19538Q2 to CSD17571Q2 and the system has stopped working. Vref is measured between 800 to 940 mV . PGOOD is like a square wave. Switching all the FETS is off too. I am attaching webench design to understand what would cause this issue. Also some screen shots are attached. another difference from Simulation is that we have DMC mode with no hiccup. We need to get more current for USB type C and that is why we made this change8764.LM5175_simulation.pdf

over 4 years ago

0 Ali Tatari38 over 4 years ago

Intellectual 470 points

0 Ali Tatari38 over 4 years ago in reply to Ali Tatari38

Intellectual 470 points

0 Youhao Xi over 4 years ago in reply to Ali Tatari38

TI__Guru 68495 points

Hi Ali,

I am afraid the IC is damaged. The TI FETs are very good but their Qg is too small, and the LM5175 has a strong driver, this may results in a very fast di/dt or dv/dt, and ringing may be resulted if your layout is not optimized. My recommendation is to change to LM5176, a pin-to-pin compatible device, and it can work better with TI FETs.

Note that when you change to the LM5176, you need to adjust the RT pin resistor to stay at the same switching frequency, and also add 2k resistor to the VISNS pin and remove the VISIN pin capacitor. The rest of your design can stay the same.

Thanks,
Youhao Xi, Applications Engineering

0 Ali Tatari38 over 4 years ago in reply to Youhao Xi

Intellectual 470 points

What would be recommended Qg spec for FETs - can you suggest a better FET from some other manufacturer since we have 100s of boards already.

0 Ali Tatari38 over 4 years ago in reply to Ali Tatari38

Intellectual 470 points

I added a 100uF cap at output which has solved the problem.

0 Ali Tatari38 over 4 years ago in reply to Ali Tatari38

Intellectual 470 points

so total capacitance is 300 uf

0 Youhao Xi over 4 years ago in reply to Ali Tatari38

TI__Guru 68495 points

Hi Ali,

Thank you for the update. It sounds like your original 180uF capacitor did not perform well and your output voltage hit output OVP (10% above Vout setting point). Congratulations to you for solving the issue.

Best Regards,
Youhao

0 Ali Tatari38 over 4 years ago in reply to Youhao Xi

Intellectual 470 points

Can you please explain " their Qg is too small, and the LM5175 has a strong driver, this may results in a very fast di/dt or dv/dt,"

0 Ali Tatari38 over 4 years ago in reply to Youhao Xi

Intellectual 470 points

To me it seems like LM5175 is a weak driver since it needs more charge

0 Youhao Xi over 4 years ago in reply to Ali Tatari38

TI__Guru 68495 points

Hi Ali,

What do you man it is a weak driver? The TI FET that you selects just has 4nC Qg. The LM5175 driver appears to be too strong. If you check the standard EVM, it employs the FET of Qg roughly 30~40nC. To turn-on or off a FET, the driver needs to supply the total gate charge. Smaller Qg will take short time to complete the turn-on and off transition, and thus faster di/dt and dv/dt are resulted.

Thanks,
Youhao

0 Ali Tatari38 over 4 years ago in reply to Youhao Xi

Intellectual 470 points

Hi Youhao,

Thanks for your help! Would adding Rg in series with gate help with meeting turnon and turnoff time requirements for LM5175 gate drivers. Can I size Rg for TI Fets (4nC Qg) to get the approximate turnoff and turnon transition time that I would get for a MOSFET with 30`40 nC Qg.

Regards,

Ali

0 Garrett Roecker over 4 years ago in reply to Ali Tatari38

TI__Mastermind 21720 points

Ali,

Adding gate resistance will slow the turn on of the MOSFET and thus help to reduce the overshoot and undershoot of the switch nodes. If gate resistors are added the dead time of the high-side and low-side FET should be check to ensure there is not shoot through during switching.

-Garrett

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LM5175: LM5175 Issues with MOSFETs