LMZ14203H: LMZ14203: LMZ14203H Startup Problem

Emanuel Marinescu

Intellectual 270 points

Part Number: LMZ14203H

Tool/software:

I have a very strange problem with the circuit in the attached drawing.

The Vin is 18-30VDC, nominal 24VDC and the required output Vo 14.2V.

The UVLO is set at 16.2V. Output capacitors are all ceramic. The load is linear at 350mA.

What happens is that when I apply +24VDC the output voltage is only about 6V with a high ripple.

Also, it apparently overloads the input protection circuit (not shown in the drawing).

If I quickly disconnect the power and connect again it works normally.

At 28V it does the same but it won't start no matter how many times I switch the power on/off.

But at 20V it works normally, starts every time at the working parameters.

I am working with the LMZ a lot in many products, same circuit and never had such a phenomena.

The only thing changed in this circuit are the output capacitors, I used so far tantalum and here I changed to ceramic, same value.

I will appreciate any help.

5 months ago

0 Ryan 5 months ago

TI__Expert 3400 points

Hi Emanuel,

I will get back to you shortly while I look into this issue.

Thank you,

Ryan

0 Ridge Lahti 5 months ago in reply to Ryan

TI__Mastermind 18155 points

Hello,

Is your load a resistive load or electronic load?

Are you current limiting the input supply?

The 20nF feedforward capacitor is tool large. Generally, Cff is in the pico-farad range.

Best regards,

Ridge

0 Emanuel Marinescu 5 months ago

Intellectual 270 points

The load is 3 LED in series with a linear constant current source.

Cff according to the data sheet is correct, supposed to be 0.022uF.
I can say for sure that this is not the problem, I used the LMZ1420x series in more than 13000 projects so far with the same capacitor.

0 Ridge Lahti 5 months ago in reply to Emanuel Marinescu

TI__Mastermind 18155 points

Hello,

Do you have oscilloscope waveforms for Vout, SS, and Vin which you can share?

Best regards,

Ridge

0 Emanuel Marinescu 5 months ago in reply to Ridge Lahti

Intellectual 270 points

No, but I can make some screenshots.
I need to update - while checking if other changes were made to the present circuit except the output capacitors, I found something.

As part of the input EMI filter we use also a couple of ferrite beads. Usually they are at the input, before th surge protection circuit but here they are after, followed by a common mode choke and inductor. If I cancel them with jumpers the LMZ works normally. I have no explanation.

0 Ridge Lahti 5 months ago in reply to Emanuel Marinescu

TI__Mastermind 18155 points

Hello,

Please also share the schematic for the EMI filter and other components related to the input of the buck converter which are not shown in the schematic above.

Best regards,

Ridge

0 Ridge Lahti 4 months ago in reply to Ridge Lahti

TI__Mastermind 18155 points

Hello,

Since we have not heard from you in a while, I am going to close this thread. You can re-open this thread by replying or create a new thread.

Best regards,

Ridge

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LMZ14203H: LMZ14203: LMZ14203H Startup Problem