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LM2700 output voltage dropping under expected Loads

Simon Horneber
Prodigy 40 points
Other Parts Discussed in Thread: LM2700

Dear employees,

I recently ordered two LM2700 Step-Up PWM DC/DC Converter. One of them should power a Microcontroller-Board with 8V and 140 mA. The other one was considered to power a cooling device with 5V and 1.3A. Both set-ups (8V and 5V Output-Voltage) were given in the Application Information section of the data sheet at page 11 (8V) and page 12 (5V).The range of the input voltage that I adjust on the Vin-pin of each Step-Up-Converter lies between the recommended values of 2.5V and 4.2V. Also my power sources are able to provide enough current to power my devices.

And now to my problem: As long as I don't connect any load (Micorcontroller-Board or cooling device) to those settings described above, the Output-Voltage is stable at 8V and 5V. But as soon as I connect those loads the Output Voltage drops. Even with the highest Input Voltage recommended at 4.2V, I only receive 7.4V at the 8V Output setting and 2.95V at the 5V Output setting.

I also tried to replace the devices by using resistors that were scaled to pull the same amount of amps as the devices. But this set-up still gave me the same results.

My question now is if the voltage drop is considered to be the usual behavior of the LM2700 or did I make any mistakes in my set-up?

Thank you very much for your efforts.

Best regards

Simon Horneber

  • 0
    TI__Intellectual 2010 points
    Hi Simon,
    If you set up your output to be 8V and use the schematic diagram from the datasheet, it should remain ~8V as long as your load doesn't exceed 3.6A. IN this case the device will lose regulation because of over current and drop the voltage. What is your switching frequency setting? Are you using output cap? and can you monitor your switch pin and share some screenshots. It sounds like a short circuit happening at the output when you connect your uC board. Please share schematic if possible.
    Thanks
  • 0 in reply to Kaisar Ali
    Prodigy 40 points

    Hi Kaisar,

    thank you so much for answering me. My switching frequenzy is 600 kHz. Therefore I grounded the FSLCT-Pin. At the output I am using a 22µF capacitor as recommended in the typical application section in the data sheet. But I am using an electrolyte capacitor instead of a ceramic capacitor as recommended. Does that have that much of an impact on the output voltage? 

    I monitored my switch pin and shot some pictures (sorry for the bad quality). My oszilloscope shows an average frequency of 588 kHz and a mean voltage of 2.95V on the SW-Pin at different scale settings (picture 1 at 5µs, picture 2 at 1µs and picture 3 at 1ms). As soon as I connect my load the voltage drops (picture 4) and the frequency at the SW-pin drops as well at 331.43 kHz (picture 5)

    I hope I could give you the informations that you need. If there is anything I forgot please tell me. I loaded te pictures up in google drive and shared the link down below. If you have any trouble to open it pls tell me and I will send the pictures in another way.

    drive.google.com/open

    Thank you so much for your help

  • 0
    TI__Mastermind 28660 points

    Hello Simon,

     

    Can you please detail what your input voltage is at?  Also, please verify that your input supply is not current limiting.  it would also be helpful if you could share the schematic for the 8V and 5V supplies.

     

    Thanks.

  • 0 in reply to David Baba
    Prodigy 40 points
    Hello David,

    thank you for responding. My input voltage is between 2.5V and 4.2V which is in the range of the data sheet recommendations. I tried every input voltage in this range in 0.1V steps. My input supply can provide a current up to 3A. The problem according to the input current is that I cant find any hints in the data sheet that can tell me how much current is necessary to output a certain current at the output voltages of 8V or 5V.
    I attached you the link to the folder where I placed the schematics. If you have any problems to open the folder please tell me so I can send you the pictures in another way.
    Thank you so much for your help.

    drive.google.com/folderview
  • 0 in reply to Simon Horneber
    TI__Mastermind 28660 points
    How much is your load current?

    The average input current for a boost is simply the following.

    Vout x Iout = Pout

    Pin = Pout/Eff (estimated)

    Say Eff = 0.9

    Pin/Vin min = Iin max.

    Can you email me the schematic please?

    David.baba@ti.com.

    Thanks.
  • 0 in reply to David Baba
    Prodigy 40 points
    Hi David,

    I send you an email with the informations you wanted. I am posting those infos here again just for the completeness of the discussion in the forum:

    attached you will find my schematics. I created them in EAGLE 7.2.0. Additionally I send you the pictures of those schematics if you should have any problems to open them.
    I also calculated the input current as you showed me and I came up with Iin=2.7A at a efficency rate of 85% an output voltage of 5V and an input voltage of 2.5V. So I should not have any problems to provide enough current at the input. My load current should be arround 1.3A.
    In addition I isolated my microcontroller from the evaluationboard so it can be powered with 5V as well and even less load current (less than 50mA). But the output voltage still drops for about 200mV when I connect my microcontroller to the output of the step up.