LM3150: Instable - not working (ESR problem?)

Ruben Figueiredo

Part Number: LM3150
Other Parts Discussed in Thread: CSD18563Q5A, , LM61460, LM76005

Hello there,

Need help ASAP please!

I designed in the power designer tool a circuit to convert 14-40V DC to 12V DC with a maximum output current of 5A.

I connected a 12V led to the output. When i turn on the input supply it blinks a few times but most of the times it doesn t work at all.

I had this same layout and circuit that worked before but when i reproduced it in a new pcb it doesn t work and i don t know why.

Some quick measurements with 15.2V at the input:

Cvcc=6.2V (fix value)

SS = instable -> 0 or 300mV

Cbst (between SW and BST) = instable -> 0 or 3V

SW = instable -> 0 or 2V

Output = instable

The gate signals are instable too.

I can t share all the circuit but you can see the relevant part.

UPDATE:
Removed the parallel capacitors at the output and I left only one and started working with 11.3V - 11.6V of output (it fails after 1 minute)
I use ceramic capacitors with low ESR. How can i solve this?

over 4 years ago

0 Timothy Hegarty over 4 years ago

TI__Guru 72992 points

Hi Ruben,

No need for 100V MOSFETs here if Vin-max is 40V. Try 60V devices instead, e.g. CSD18563Q5A, for better efficiency.

Note the VCC of the LM3150 is 5.95V, so the FETs should preferably be rated for Rdson at Vgs = 4.5V. The lower gate threshold voltage of a 60V device helps with switching based on the LM3150's gate drive amplitude.

You might also try increasing the SS cap (to avoid hitting current limit at startup) and lowering the impedance of the ripple generation circuit.

If you need further help, send on the waveforms for VOUT, SW and FB. Btw, we have converters (with integrated FETs) that can do 5A, e.g. LM61460.

Regards,

Tim

0 Ruben Figueiredo over 4 years ago in reply to Timothy Hegarty

Prodigy 10 points

I used this mosfet model because it was the only one I had at the moment but thank you for the note. Will try to buy that model in the future. The LM6146 can t reach 40V but thank you for the recommendation.

I will try to increase the SS cap , it s currently 15nF... Any recommended value? hope it helps to solve this.

Can t understand how removing the capacitors at the output helped to at least start the regulator. I mean this device is able to use low ESR capacitors right?

Thank you for the help. Will get in touch soon. Hopefully with good news.

0 Timothy Hegarty over 4 years ago in reply to Ruben Figueiredo

TI__Guru 72992 points

Thanks, Ruben.

The LM61460 has a max operating of 36V and abs max of 42V, so it's a bit tight. The LM76005 60V/5A converter is a better fit.

If the problem occurs at startup, it may be current limit (especially is the load is applied at startup). This would align with reduction of Cout improving the behavior. Try Css of 47nF.

Low ESR ceramic caps shouldn't be a problem here. The type-3 ripple network generates the appropriate ripple amplitude at FB for this controller. However, lower impedance will help to reduce possible noise coupling. See app note SNVA874 for more detail: https://www.ti.com/lit/an/snva874/snva874.pdf

Also, you can use the Excel quickstart calculator available in the LM3150 product folder: https://www.ti.com/product/LM3150#design-development##design-tools-simulation

Regards,

Tim

0 Ruben Figueiredo over 4 years ago in reply to Timothy Hegarty

Prodigy 10 points

Thank you for the fast response and the explanations.

Makes sense now. I believe that increasing the Css will solve the issue since tss is only 1ms with 15nF.

Regarding the 3 type ripple network I just used the values that Texas power designer tool gave me. Hope they are good enough.

Thank you again and will get in touch soon.

Best regards

0 Timothy Hegarty over 4 years ago in reply to Ruben Figueiredo

TI__Guru 72992 points

No problem, Ruben.

You can check how close your current limit setpoint is to your full-load current. We usually allow ~50% margin (so a 7.5A current limit is normal for a 5A load).

Regards,

Tim

0 Ruben Figueiredo over 4 years ago in reply to Timothy Hegarty

Prodigy 10 points

Hello,

Changed the Css value to 39nF and 100nF and did not solve!
Now the output never work even if i remove the output capacitor as before.

The feedback is 300mV and can t go higher, SW and Vout goes to 1-2V a few times.
BST pin is 5V and sometimes the gate of M2 is fixed at 6V.

Don t know what to do. It s worse and not working at all!

0 Timothy Hegarty over 4 years ago in reply to Ruben Figueiredo

TI__Guru 72992 points

Have you got an LM3150 EVM where you can build and verify your circuit?

0 Ruben Figueiredo over 4 years ago in reply to Timothy Hegarty

Prodigy 10 points

No, but i used the same layout and circuit in a previous PCB and worked good.

0 Timothy Hegarty over 4 years ago in reply to Ruben Figueiredo

TI__Guru 72992 points

Must be a component issue then - maybe replace the controller and FETs.

0 Ruben Figueiredo over 4 years ago in reply to Timothy Hegarty

Prodigy 10 points

ok, i will se the components

0 Timothy Hegarty over 4 years ago in reply to Ruben Figueiredo

TI__Guru 72992 points

Thanks, Ruben.

Regards,

Tim

+1 Ruben Figueiredo over 4 years ago in reply to Timothy Hegarty

Prodigy 10 points

Hello,
The circuit is working now after cleaning and removing some flux...don t know why.
The output is 11.6V with 15V of input and 12.4V with 25V of input...need to improve this values.. Any recomendation?

0 Timothy Hegarty over 4 years ago in reply to Ruben Figueiredo

TI__Guru 72992 points

Ruben, sorry for the delay as I missed your reply. Glad your circuit is operating. In terms of the output voltage variation, check the injected ripple amplitude is not too big. The ripple increases with Vin and the controller regulates at the valley of the ripple, hence the higher Vout as Vin increases.

Regards,

Tim

0 Ruben Figueiredo over 4 years ago in reply to Timothy Hegarty

Prodigy 10 points

How can i fix that ripple and how know it is high or low? I just used the recommended values

The output is 12.06 without load but with load increases to 12.3 or 12.4. What can i do to prevent this

0 Timothy Hegarty over 4 years ago in reply to Ruben Figueiredo

TI__Guru 72992 points

The triangular injected ripple at FB should be about 50mV pk-pk. Please adjust the ripple injection RC accordingly. See app note snva874 for more detail.

0 Ruben Figueiredo over 4 years ago in reply to Timothy Hegarty

Prodigy 10 points

ok, thank you.

0 Timothy Hegarty over 4 years ago in reply to Ruben Figueiredo

TI__Guru 72992 points

Thanks, Ruben.

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LM3150: Instable - not working (ESR problem?)