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LM2587 typical boost regulator application

Hi, 

Recently I bought several LM2587-ADJ/NOPB, and now I meet some problems with application provided by datasheet.

Input 6V, 1A, DC source. The output side we connect 100ohms power resistor.

Actually, the circuit behaves well with no load, the output voltage is exactly like what we set 12V. However, as 100ohms resistor connect to the output side, the output voltage drop significantly, like 6-8V, and sometimes the chip gets hot and burns out.

Ask for help! How to set the output voltage when we get a load.

  • Hi Yusong,

    When you configure LM2587-ADJ in a boost topology, you have a traight path from input to output. So, when  load the output with 100Ohm, basically you are shorting your supply and bypassing the switch.

    In order to run this application you have to startup with a supply higher than 1A because this can easily reach 1A starting with load deoending on the inductor you are using. Or you slowly increase your load to 100Ohm. (0.12A). An external current limit circuit lower than 5A (current limit of the part) will be a protection not to burn your IC.

    This part is mean for higher output current, you can use lower current parts on the same family or you enter your specs on our Webench tools to suggest optimized solutions for your application.

     

    Regards,

    Ismail Jorio 

  • In reply to Ismail Jorio:

    You mean the inductor and the load we used is to small? And the load is short the chip.  However, our input is DC regulator which voltage is 6V and current is 1A, that means if the load short the circuit, our output should achieve the outpout voltage we want at startup moment. According to 5 times experiment, our output voltage never exceeded 7V. And that brings that if the load short the circuit, our diode has maximum reverse voltage which is 40V, and it could be impaired. However, what we burned is just IC.

    And with the input current requirement you mentioned, our DC-DC converter burned out with input current exceeding 1.5A. So, I guess that it is the duty cycle problem or the DC-DC internal structure problem?

  • In reply to Yusong Di:

    Hi,

    Can you post some more details. If possible it would help if I could see a waveform showing

    Channel 1: Vin.

    Channel 2: Vout

    Channel 3: PIn four ot the 2857, the Switch node.

    The time scale should be such that you can see several pulses on the switch node. One scope shot for no load, and one with the 100ohm load.

    Can you give me a bill of materials for the design you are testing?

    That should help me to diagnose the problem.

     

    I also have a few questions for clarification.

    1) I understood that with the 100ohm resistor tied to the output, the output voltage was 7V.  Is this correct?

     

    Regards,

    Marc

     

     

     

  • In reply to Marc Davis-Marsh:

    I have got a similar issue as well. Could you let me know the solution in this post please?
  • In reply to Zhongwen Jin:

    Hi,

    The other E2E forum member never responded to my question for more information.

    Can you post some more details about your issue. If possible it would help if I could see a waveform showing

    Channel 1: Vin.

    Channel 2: Vout

    Channel 3: PIn four ot the 2857, the Switch node.

    The time scale should be such that you can see several pulses on the switch node. One scope shot for no load, and one with the 100ohm load.

    Can you give me a bill of materials for the design you are testing?

    That should help me to diagnose the problem.

     

    Regards,

    Marc

  • In reply to Marc Davis-Marsh:

    My circuit is a bit different from the post. But basically the are the same circuits. Please check the schematic and PCB layout. I will post my test result as well.

  • In reply to Marc Davis-Marsh:

    This picture shows the testing setup. The load is 51 ohms. At beginning the load is not connected.

    Vin = 26V DC

    Vout = 35V DC

  • In reply to Marc Davis-Marsh:

    At the beginning, the load two terminals are open circuit, nothing is connected. Please see the waveform.

    The output voltage is working ok. The input is drawing 140mA.

  • In reply to Marc Davis-Marsh:

    While the circuit is working fine without a load, then I connected a 51ohm load. The circuit is still working okish. The input voltage drops to around 24V DC, some buzzing noise from the circuit.

    Please see the waveform.

  • In reply to Marc Davis-Marsh:

    1 Switch OFF the circuit,

    2 Connect the load

    3 Switch ON the circuit, then the input voltage drops to around 3V, current becomes the limit 3A of the power supply. The waveform is shown below.

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