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LM2733: LM2733 + TPS7A4701 problerms with load over 10mA

Oriol Sanchez
Intellectual 360 points
Part Number: LM2733

Hello,

I'm experiencing a problem using the LM2733 + TPS7A4701. The LM2733 provides aprox 26V to the TPS7A4701 that gives a low noise power rail of 24V.

Till here, no problem, With no load I get 24.2V as expected. The problem appears when the load is about 5-10mA and upper, voltage goes down drastically. At 50mA it's 20V and the coil clearly sounds/vibrate.

Also at 50mA the 5V input current goes up to 500mA.

Could you please help me to understand what is happening here and how to solve it?

Thanks!

Oriol.

  • 0
    TI__Intellectual 2645 points
    Hi Oriol
    You could separate LM2733 and TPS7A4701 circuit and test them separately to find which part is wrong. And then we could find the reason.
  • 0 in reply to Jing Ji
    Intellectual 360 points

    Hello Jing,

    Not splitted yet bust just connected the electronic load onto "VIN" rail to see how the LM2733 reacts under an smalll load, and I think here relies the problem:

    - No load Output = 28.5V

    - 20mA load Output = 27.8V

    - 40mA load Output = 27.8V

    - 50mA load Output = 30.5V

    The only condition where the coil does not "sound" is under 50mA load, but it's 450mA on 5V that I can't handle on the device. At 100mA load it goes crazy.

    As my load conditions will be under 20mA it should work without any problems but that's not the case. Inductor used: www.mouser.es/.../963-NRH3012T100MN

    Now same conditions on TPS -> 28V input from an Agilent E3632A:

    - No load Output = 24.2V

    - 20mA load Output = 24.18V

    - 40mA load Output = 24.18V

    - 60mA load Output = 24.19V

    - 100mA load Output = 24.21V

    At 60mA the VIN Rail Current is 63mA. At 100mA the VIN rail current is 103mA.

    Measured with an Agilent 34401A and Kikusui PLZ72W load.

    So looks like that the problem does not relay here. Under 20mA that will be the circuit load is almos the same current on VIN rail.

    The problem is that the +24V rail is connected to a tube anode, so it introduces a lot of noise onto it and obviously it's audible, it's a high-pitch humming sound like a siren. I've been using the SC4503 before and your team suggested me to change to this design and then use an LDO to minimize the noise introduced to the circuit, but in fact now I have even more!

    Looking to your response.

    Oriol.

  • 0 in reply to Oriol Sanchez
    TI__Intellectual 2645 points

    Hi Qriol

      My replay is the blue word.

    Not splitted yet bust just connected the electronic load onto "VIN" rail to see how the LM2733 reacts under an smalll load, and I think here relies the problem:

    - No load Output = 28.5V

    - 20mA load Output = 27.8V

    - 40mA load Output = 27.8V

    - 50mA load Output = 30.5V 

     (But in the first post, you said "The problem appears when the load is about 5-10mA and upper, voltage goes down drastically. At 50mA it's 20V and the coil clearly sounds/vibrate."  I am not sure what is the output voltage at Iout=50mA)

    The only condition where the coil does not "sound" is under 50mA load, but it's 450mA on 5V that I can't handle on the device. At 100mA load it goes crazy.

    (According to   Iin * Vin * efficiency = Iout * Vout, Iin= 50mA*30.5V/5V/0.8= 380mA. Can you handle this?)

    As my load conditions will be under 20mA it should work without any problems but that's not the case. Inductor used: www.mouser.es/.../963-NRH3012T100MN

    (This inductor's saturation current is 730mA. I suggest you to choose a inductor with 1A saturation current.)

    Now same conditions on TPS -> 28V input from an Agilent E3632A:

    - No load Output = 24.2V

    - 20mA load Output = 24.18V

    - 40mA load Output = 24.18V

    - 60mA load Output = 24.19V

    - 100mA load Output = 24.21V

    At 60mA the VIN Rail Current is 63mA. At 100mA the VIN rail current is 103mA.

    Measured with an Agilent 34401A and Kikusui PLZ72W load.

    So looks like that the problem does not relay here. Under 20mA that will be the circuit load is almos the same current on VIN rail.

    The problem is that the +24V rail is connected to a tube anode, so it introduces a lot of noise onto it and obviously it's audible, it's a high-pitch humming sound like a siren. I've been using the SC4503 before and your team suggested me to change to this design and then use an LDO to minimize the noise introduced to the circuit, but in fact now I have even more!

    In a word, you found LM2733 can't give your a big current (up to 50mA) and inductor have some noise. Is it right?

    Could you test some waveform for me?

    Vin=5V Iout=50mA and Iout=20mA

    SW, inductor current, output voltage