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RE: LM5175 48V Output

Prodigy 40 points

Replies: 5

Views: 248

Hello,

I have more or less the same problem.

I purchased the LM5175EVM evaluation board and changed some of the componentes to adequate to my requirements:

Vin=9,6-15V. Vout=48V. Iout=1A.

Q3,Q4: CSD19533Q5A 100V-13A

D1: substitute by D2 and none in the D2.

L=47uH.

Cout: 2*68uF +3.3uF.

R2=0.05Ohms

And some other changes in FB, COMP and SLOPE components.

After one of many start-ups, the LM5175 broke. It did not have Vcc voltage and the resistance between SW1-BOOT1 and SW-BOOT2 is very different (2MegaOhms to 140Ohms). Also the resistance between Vcc to ground was almost infinite.

The D4 and D5 diodes are not broken, also, the resistor divider for the EN/UVLO is OK and the voltage in VIN is OK.

5 Replies

  • Hi Asier,

    I think the issue you are seeing is described in this document:

    Controlling switch-node ringing in DC/DC converters 

    I think a zener clamp (~8.2V) across CBOOT1 and across CBOOT2 should help with this.

    Regards,

    -Vijay

  • In reply to Vijay86929:

    Hi Vijay,

    I added theese zeners between BOOT1-SW1 and BOOT2-SW2, a 2k resistor in R14 and R27, and it started working. But when I switched on the board 3 times it stopped working. Now, there is a short between CBOOT2 and GND.

    Thanks

  • In reply to Asier Urcelay:

    Hi Asier,

    I think I will need to see the schematic. Also did you change the LM5175 IC after placing the zeners.

    Regards,

    -Vijay

  • In reply to Vijay86929:

    Hello Vijay

    The schematic is the LM5175EVM evaluation board with some changes, and as the IC was broken, I changed it. I also added 2 zeners between BOOT1-2 and SW1-2.

    Vin=9,6-15V. Vout=48V. Iout=1A.

    Q3,Q4: CSD19533Q5A 100V-13A

    D1: substitute by D2 and none in the D2.

    L=47uH.

    Cout: 2*68uF +3.3uF.

    R2=0.05Ohms

    And some other changes in FB, COMP and SLOPE components.

    Many thanks

  • In reply to Asier Urcelay:

    Hi Asier,

    I used the quickstart calculator excel sheet on the LM5175 product webpage. I suggest you use it to modify the component values. In particular it is pointing to a 10uH inductor, 180/220 pF slope capacitor etc.
    Also it 9V the minimum VIN or 6V.

    Other than these I suggest using a 2k(ohm) resistor in series with VOSNS.

    In general lower QRR FETs result in lower ringing on SW node. It may help to use a lower QRR FET for Q3/Q4.

    Hope this helps.

    Regards

    -Vijay

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