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LMZ12002: Can't get 6-7.5V if Vin = 11V

Aleksei Varkentin
Prodigy 20 points
Part Number: LMZ12002

Hello, i want to get from LMZ12002 VOUT=6-7v IOUT>1.5A with VIN=11v and i have problem.
I have only 4.5 VOUT ... I can't get VOUT>4.5V, my schematics is attached.
I think this is my mistake and LMZ 12002 needs more drop voltage. 
If i change VIN to 15V i have calculated VOUT...
Big quantity of PCB's is ready, i can't change pcb... :(
I can change any components (include LMZ12002) to other pin to pin components if it will be help.
if you have any solution or minds, write it please...

  • 0
    TI__Intellectual 1155 points

    Hi Aleksei,

    What is your desired switching frequency? I had a look on the Ron resistor value, which is 4.7kOhm. Is that correct?

    The datasheet says that the On-resistor has to be in the range between 25kOhm and 124kOhm. Because of this I did some calculations.

    Probably you hit the minimum ON time limit or the minimum OFF time limit.

    The  typical datasheet values for these are:

    - tmin ON: 150ns (typical) ; 200ns (+30% headroom for my calculations)

    - tmin OFF 260ns (typical) ; 350ns (+30% headroom for my calculations)

    Refering to page 17 of the datsheet (Ron resistor selection), I checked some values.

    - Based on the resistor divider on the FB pin, your desired output voltage is 6.8V. With an input voltage of 11V, his gives me a Duty cycle of 62%.

    - Assuming you hit the minimum ON time: 0.62=200ns/(200ns+tmin OFF) --> tmin OFF = 122ns  

    This value is lower than the value in the datasheet. So the device is not able to approach the desired Duty cycle of 62%.

    - Assuming, you reach the minimum OFF time: 0.62=ton/(ton+350ns) --> tmin ON = 571ns

    571ns + 350ns = 921ns which gives you switching frequency of 1.08MHz (this is the max frequency to reach with this device)

    Now you can calcite the ON time resistor: Ron = Vout/(1.3*10^-10 * fsw) = 6.8V/(1.3*10^-10*1.08MHz) = 48.2kOhm

    Hope this can help you in fixing your issue. Please don't hesitate to contact me again, in case there are questions left.

    Best regards,

    Michael

  • 0 in reply to Michael Daimer
    Prodigy 20 points

    You are right. RON is not correct, it is my mistake! Thank you very much for answer and for really fast support. 
    Now it is work correctly!