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LM5121: LM5121 EVM adjustment

Mike Lee88
Prodigy 80 points
Part Number: LM5121

Hi

   I have an EVM circuit as follows. 

How to adjust the output voltage from 12V to 48V?

I change the R26 to 2K, Output voltage clamps at 20V.

Thanks.

  • 0
    TI__Guru 68495 points
    What is your load? You need to adjust R2 to allow your new load current at 48V, otherwise it may high the current limit. Then, please note the following:

    1. You should not increase the output voltage setting by just changing the R26. The output capacitors are not rated for 48V output. All these capacitors must be changed to the ones rated >48V. Y
    2. Choose R26 = ~1.28k in order to set the voltage to 48V.
    3. Change the SS capacitor to 0.47uF.
  • 0 in reply to Youhao Xi
    Prodigy 80 points
    Hi Youhao

    What is your load? ==> 48V/0.5A
    1. You should not increase the output voltage setting by just changing the R26. The output capacitors are not rated for 48V output. All these capacitors must be changed to the ones rated >48V. ==> MOS and capacitors rating were changed.
    2. Choose R26 = ~1.28k in order to set the voltage to 48V.==> change to 1.27K ,but output voltage still clamps at 20V
    3. Change the SS capacitor to 0.47uF.==> changed.
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Finally,I deleted R10. The output voltage boost to 48V.
    Please explain this phenomenon,thanks.

    Mike
  • 0 in reply to Mike Lee88
    TI__Guru 68495 points
    Please note that R10 and/or D2 should not be populated for Vout>15V, or the VCC pin can be damaged. Use of R10 and D20 directly placed the VCC pin's ESD structure as an load, which clamps the output by the ESD structure at about 20V.
  • 0 in reply to Youhao Xi
    Prodigy 80 points
    Hi Youhao
    Base on my application. Are there any other parameters need to be adjusted?
    thanks.

    Vin = 10.2V~13.8V
    Vout = 48V 0.5A
  • 0 in reply to Mike Lee88
    TI__Prodigy 690 points

    Hi Youhao,

    Please help to feedback customer question as below, thanks.

    1. What's the fuction for this diode(VCC to Vin)?

    2. What's the fuction for D6?

    3. Which mode we recommend for customer spec as below,  DCM, forced PWM? And do they need to pop 100pf?

    Vin = 10.2V~13.8V
    Vout = 48V,  Iout=0.5A

    4, They are drawing schematic now, will follow our EVM first, any parameter need to change by their spec?

  • 0 in reply to user4979999
    TI__Guru 68495 points
    Answers to your questions:

    1. The diode is the body diode of the VCC regulator FET. It is specially shown to remind the user the current path from VCC to VIN. Therefore, if VIN can fall below VCC during operation, the VCC can be discharged. To prevent this from occurring, a blocking diode should be add to the VIN pin.

    2. D6 provide the VCC bias power directly from the output voltage, thus blocking the internal VCC regulator. This is more efficient but note that this can only be used when the output voltage is between 8 to 12V. Exceeding 14V will damage the VCC pin.

    3. Mode should be decided by the customer, and the LM5121 offers both options. If the customer is not certain, they can test with both modes then decide which is their preferred mode. I don't think the capacitor is needed.

    4. The EVM is 12V output ant customer's use case is 48V. Surely they need to change a lot of parts. Please use the Webench as the start point to help them select the design.