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LMG3410-HB-EVM: Using a 3.3Vdc supply

Part Number: LMG3410-HB-EVM

I would like to use the LMG3410-HB-EVM with a +3.3Vdc supply.

In the reference design, the isolation transformers (T1 and T2) have a 1.3:1 turns ratio (P/N 760390014), such that with a +5Vdc supply the output voltage is ~14.8Vdc (including the diode drops). With a +3.3Vdc supply, the output voltage would be ~9.6Vdc, which is probably rather marginal.

If the transformers are swapped for units with a 1:1.1 turns ratio (P/N 760390012), then the output voltage would be ~13.9Vdc, which seems to be a better match. The new transformers have the same footprint as the reference design, but would need to be rotated 180deg with respect to the screen print in order to achieve the correct turns ratio.

Is this viable?

Regards,

Greg

  • Hi Greg,

    This isolated power supply is aimed to provide a supply power to the LMG3410 on the EVM. LMG3410 typically requires 9.5 to 18 V of supply voltage. In this case, 9.6V is a little marginal. But if you are able to get 13.9V out of the circuit, you should be fine with a 3.3V input voltage on the primary side. The transformer also looks fine.


    Thanks and regards,
    Lixing

  • Hi Lixing,

    The isolation transformers have center-taps on both the primary and secondary. On the primary side, current is supplied to the center-tap and passes through only half of the windings. The secondary side uses all of the windings. So the output voltage is 2x turns ratio x input voltage. But the rectifier is configured as a voltage doubler, so the DC output is 4x turns ratio x input voltage. And then you have a few diode drops as well.

    For the reference design with 5V, the turns ratio is 1:0.769 (transformer installed 'backwards') giving an output of 15.4V - 0.6V = 14.8V.
    For my proposed design with 3.3V, the turns ratio would be 1:1.1 (transformer installed 'forwards') giving the 13.9V output.

    My main concern is whether the 3.3V supply will be able to deliver enough current. My switching frequency will be ~165kHz.

    Best regards,

    Greg
  • Hi Greg,

    Thanks for the explanation.

    Our datasheet indicates about 43mA of supply current for 1MHz switching. While you have 165kHz in your design, you should get less than 10mA of supply current. For your case, it is less than 0.14W of power consumption. The transformer driver SN6505 that you are using has an output current capability of 1A, and with 3.3V voltage supply, it should not be a problem to deliver up to 0.14W. Also I think the transformer should be fine.

    Thanks and regards,
    Lixing
  • Lixing,

    I did a similar analysis and came to the same conclusion. But it is always good to hear it from someone else. And hopefully this will be of use to someone else.

    Best regards,

    Greg