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LM5071: AN 1430 reference design

Steve Smith86
Prodigy 10 points
Part Number: LM5071

Hi

We have an existing design based on LM5071 and the DA2257 transformer to give a 3V3 output, based on the AN1430 app note, fig 12.

We now want to add a 5V5 rail by using Sec2 from T1 (pins 6 & 10).

We built it, and it does give a 5.6V output, but the output at J6 seems to be only regulated by zener Z4 - if we remove the zener the voltage at J6 is 11V, even though the voltage at J4 is 3V3.

This means the Z4 is constantly conducting, pulling around 18mA.

Is this expected, or have we done something wrong?

  • 0
    TI__Mastermind 41760 points
    Hi Steve,

    We have assigned this to one of our apps engineer who can look into this. Can you forward a schematic of your design? Thanks!
  • 0 in reply to Darwin Fernandez
    TI__Genius 9230 points
    Hi Steve,

    If this is occurring at light load/no load, then it is working as intended. The Zener is there to clamp at very light load to keep the voltage from running away. If this is occurring with loaded outputs:

    If your setpoint on the 3v3 is also a little high, I suggest adjusting the biasing of U3 to achieve the correct output setpoint.

    If just your 5.5V output is high, I suggest choosing a rectifying diode with a similar forward voltage drop as D8 used in the LM5071 dual output schematic (fig 12).

    Thanks,
    Tom
  • 0 in reply to Darwin Fernandez
    TI__Genius 9230 points
    Hi Steve,

    The zener is there to clamp the output voltage at no load/very light load to keep the voltage from running away. If this is occurring with LOADED OUTPUTS:

    If the 3.3V is also high, then adjust biasing of U3 to achieve the correct setpoint.

    Or, if just the 5.5V rail is not regulating, make sure you are using a diode with the same forward voltage drop as D8 in the schematic (fig 12).

    Thanks,
    Tom