LTC3872 question

Hi, I have a question about LTC3872: The carrying load ability for the output of the LTC3872 circuit is not good, I do not know what is wrong with my LTC3872 circuit configuration? can ADI engineer help me? And I will be very grateful!!!

1circuit configuration

Mosfet: Infineon BSZ018N04LS6 key performance parameters as follows

2Test result

1no load model, the circuit of LTC3785 working normally, and the output voltage is 15.5V

2the circuit of LTC3785 carrying 100mA load and it is working normally, the output voltage is 14.5V

3when the circuit of LTC3785 carrying 500mA load, it is operate abnormally. The output voltage is about 4.2V. the abnormal condition as follows:

(3.1)the voltage of run pin is about 880mvoscilloscope measurement error, the voltage of run pin maybe below 0.85v.according to LTC3872 datasheet, With the RUN/SS pin

below 0.85V, the chip is off and the input supply current is typically only 8μA. But my gate  pin still have drive waveform? What is wrong with the LTC3872 circuit?

(3.2) the voltage of run pin depends on the input voltage of LTC3872, the input voltage is still about 4V,but the voltage of run pin is about 880mvoscilloscope measurement error, the voltage of run pin maybe below 0.85v)?What is wrong with the LTC3872 circuit?

(3.3) according to LTC3872 datasheet, with the Iprg pin is gnd, the PK limit current of input is 70mv@LTC3872I. the rds(on) of mosfet is 1.8mΩ. so the PK limit current is about 38A. therefore, the circuit of LTC3872 operated abnormally not because of current limit.

(3.4) according to LTC3872 datasheet, I configured the RCR network of Ith PINbut the circuit of LTC3872 still operated abnormally.

Parents
  • 0
    •  Analog Employees 
    on Jan 11, 2021 7:40 AM

    Hi, 

    Can you please specify the input and output specs of your circuit? It would also be helpful if you can post your schematic here. 

    For 3.1 and 3.2

    - The 0.85V is just a typical voltage level of the RUN pin. The RUN pin level is different for every unit of LTC3872. The minimum and maximum level that you would expect on the RUN pin is indicated at the electrical characteristics table on the datasheet.

    For 3.3

    - It looks like the controller was not able to drive your MOSFET and the voltage you see on the output is just the input voltage plus the voltage drop of the diode. To verify this, try to probe the voltage across the drain to source of the MOSFET. The issue that you are seeing could be related to layout issue. Make sure that you have properly followed the PCB layout checklist on the datasheet. 

    For 3.4

    - You can verify if you have the proper RCR network by checking the loop compensation through bode plot. You must have enough gain at the low frequency to prevent DC regulation error. Moreover, you must have enough phase and gain margin to achieve a desired transient response and noise rejection at higher frequencies. 

    Regards, 

    Fritz

Reply
  • 0
    •  Analog Employees 
    on Jan 11, 2021 7:40 AM

    Hi, 

    Can you please specify the input and output specs of your circuit? It would also be helpful if you can post your schematic here. 

    For 3.1 and 3.2

    - The 0.85V is just a typical voltage level of the RUN pin. The RUN pin level is different for every unit of LTC3872. The minimum and maximum level that you would expect on the RUN pin is indicated at the electrical characteristics table on the datasheet.

    For 3.3

    - It looks like the controller was not able to drive your MOSFET and the voltage you see on the output is just the input voltage plus the voltage drop of the diode. To verify this, try to probe the voltage across the drain to source of the MOSFET. The issue that you are seeing could be related to layout issue. Make sure that you have properly followed the PCB layout checklist on the datasheet. 

    For 3.4

    - You can verify if you have the proper RCR network by checking the loop compensation through bode plot. You must have enough gain at the low frequency to prevent DC regulation error. Moreover, you must have enough phase and gain margin to achieve a desired transient response and noise rejection at higher frequencies. 

    Regards, 

    Fritz

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