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LMZ12008: Voltage setting not working

Marc Willem
Prodigy 20 points
Part Number: LMZ12008

Dear Ladies and Gentlemen,

I have a pcb according to your design, but found out that the LMZ12008 is not outputting the 5 Volts requested, but a lot less voltage.
I used tried the following resistor configurations without luck:
R_ffb=1k89; R_fbt=10 k (calculation based on datasheet)
R_ffb=1k; R_fbt=7 k (calculation based on webench (approximate values)

The EN-pin is driven to 3.3 V by a microcontroller.
I tried the setting both with a 470n on SS as well as without the cap.
The input and output caps are designed as per datasheet requirements.

Also I noticed the voltage on the feedback pin not rising sufficiently when trying to get the 5 volt setting.

I prepared two configurations along with measurements under the link below:
1. R_ffb=1k; R_fbt=7 k (should output 5v but only outputting about 2 V)
2. Feedback to output, without voltage divider to get the minimum voltage
(The output was loaded with a 220 Ohms resistor)

https://postimg.cc/gallery/DXV0ycx

Could you please give me some hints why this problem occurs to me?

Thanks advance.

Kind regards

  • 0
    TI__Genius 17905 points

    Hi Marc,

    I have some questions to help me understand the issue better:

    What is the input voltage?

    What is input and output cap?

    For case 1 with Rfbb=1kohm, Rfbt = 7kohm, what is the load?

    If I understand your images correctly, yellow is Vout and red is feedback pin, is that correct? It looks like you may have some stability issues as the output ripple frequency should be at the switching frequency of about 350kHz (or 2.85us period) but I do not see a stable ripple waveform with this frequency. Can you please retake the measurement with 20MHz bandwidth limit?

  • 0 in reply to Kris F
    Prodigy 20 points

    Hi Kris,

    thanks for your quick reply, Here is the information you requested:

    - Input Voltage: 13.3 V in the measurements. There is a dependency of the output voltage of the reg to the input voltage, e.g. when going down with the voltage to 12 V it outputs slightly less then if the I try 15 V.

    - Input Caps:
    2x 47n Ceramic
    3x 10 uF Ceramic
    1x 150 uF Aluminum Elko (EEE-FK1H151P)

    - Output Caps:
    2x 47n Ceramic
    1x 47u Ceramic
    1x 330uF Tantalum (T520D337M006)

    I also attached a 20 uF and 1mF Elko to the input and output directly at the chip, but without any obvious (to me) effect.

    The load in the measurements provided is a 220 Ohms resistor.

    In my old images yellow is Vout, red is feedback.

    Please find the measurement with 20MHz Bandwith here:
    postimg.cc/.../dtNKJTn
    (Yellow -> VOUT; Red -> Feedback, Green -> VIN (Note the different scaling)

    Hint: The current consumption is about 29.4 mA

    I also attached another measurement when switching on (without C_SS)
    (the voltage decreasing is an effect of the current shunt of the multimeter for current measurement)
    In another measurement without the shunt an influence on the VIN is visible.

    When sweeping the VIN voltage, there is a strange behaviour to be seen, also attached in the image files above.

  • 0 in reply to Marc Willem
    TI__Genius 17905 points

    Hi Marc,

    Thanks for the information and the additional measurement waveforms. By the way, you can directly upload images to your post. I am not able to go to your link through our corporate proxy on my computer. I had to view on my personal phone. :)

    Hmm. I was looking for signs of your input supply dropping to indicate it is current limited, but that doesn't seem to be the case.

    Would you be able to share schematic and layout? You can email directly to me at kristoffer.flores _at_ ti.com if you do not feel comfortable sharing on the forum.

    Are there any other circuits powered from the input 13.3V? Do you only have this one board, and have you tried replacing the device? If multiple boards, all show the same behavior?

    Kris