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LM5170EVM-BIDIR: Buck Mode Operation at low current setpoint

Lelin Cai
Prodigy 20 points
Part Number: LM5170EVM-BIDIR
Other Parts Discussed in Thread: LM5170-Q1

Hello

Our lab want to establish a small scale DC grid demostration platfrom for illustrating some gird control algorithm. We want to use LM5170-Q1 to represent each grid node. The board will receive ISETD signal from external MCU to control its behavior.

Currently I am testing the buck mode functionality. The HV side is connected to a 30V 3A power supply and the LV side is connected to two 25ohm resistors in paralle. All jumper settings are as default. A 100KHz pwm signal is provided to the ISETD pin of J17. The question is that the board is not able to output the correct value corresponding to the given pwm.

I programmed the ISETD pwm to gradually increase from 1% to 3%. When the dutycycle is below 1.5%, the LV output is not responding to the pwm change. The LV current reading is clamped at around 350mA and the LV voltage is around 3.8V. For pwm in the range of 1.6% to 2.5%, the LV output is responding to the pwm change but the output value is not correct. I am comparing the output with the calculation result of equation (10) on datasheet page 20. For example, 1.6% ISETD should be able to provide 1A output on the LV side, but our current reading is around 550mA. After pwm reached 2.5%, the LV voltage is clampped at 20V. I understand this is because of the over voltage protection.

I have tried using ISETA and it has the same problem. I also tried to further decrease the LV load. The responding pwm range increased when I do so but it was not providing the correct value as calculated. Is there any setting that I am doing wrong? Or is it a problem caused by improper load? Because of the limitation of lab equipment the largest HV power supply I have now is this one. On the LV side, I have some Lithium-ion battery cells and I am planning to connecting them to form a 12V battery. Will this be helpful?

Best Regards

Lelin Cai

  • 0
    TI__Mastermind 46905 points
    Hi Lelin
    I wanted to let you know that the expert on this question is out of office. He will be back to office next week.
  • 0
    TI__Mastermind 21720 points
    Hi Lelin,

    Can you please share the schematic and the connections that you have present on the EVM? I suggest increasing the current sense resistor to match the maximum load of your application.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 20 points

    Hi Garrett

    The connections are shown in the following pictures. EVM receives EN, ISETD and CH1 signal from external MCU. Other jumpers are set as default. HV port is connected to 30V,3A power supply. LV port is connected to two 25ohm resistors in parallel. The oscilloscope current sensor is sensing one branch of current. 

    The two tables show the test results when changing the duty cycle of ISETD with different load connected (12.5ohm and 8.33ohm).  Note that the I_L here is the branch current of the parallel 25ohm resistors. The ripple indicates the start and end point where significant ripple can be observed. I include an output figure when LV voltage is at 3.62V to show this. In between the ripple start and end range, EVM output is changing corresponding to ISETD  duty cycle but the value does not match the calculation. 

    I hope this information can help. Thank you very much for your kindly reply.

    Best Regards

    Lelin Cai

  • 0 in reply to Lelin Cai
    TI__Guru* 78240 points
    Hi Lelin,

    The EVM is designed to deliver >60A and uses 1mOmh sense resistor and 4.7uH inductor. Please follow the datasheet's design guideline to re-select the inductor and current sense resistor for your few ampere use case.

    Thanks,
    Youhao