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About transient response

Part Number: LM5141

Hi, I have two questions about the transient response of LM5141.When I change the load current from 0A to 4A,and then change it from 4A to 0A ,why the ripple of output voltage value is different in two different work mode(DEMB mode and FPWM mode)?And why the voltage ripple recover time is longer in DEMB mode than  in FPWM mode? The attachment is my test result .Thank you!Transient respond.docx

  • Hello Jessica,

    When in Demb most internal circuits are shutdown in order to obtain very low standby currents. Going from very light load to a much higher current will cause a very different Trans response because it takes time to wake up the internal circuit which results in a transient response that is not as good as when the device is set in FPWM.


    Hope this helps?

    David
  • Hello David,
    Thank you for your reply. But I found under the DEMB, the ripple voltage will need a longer time to recover compared with FPWM when the load current change from 4A to 0A.That is to say the internal circuit will shutdown under DEMB, if it is because the shutdown of internal circuit will affect the recover time ,how dose it? But when the load current change from 0A to 4A, the response time is the same under two work mode. That is different from what you described. Thank you and look forward to your reply.
  • Jessica, let me clarify,

    David's post is referring to 0A-4A. The IC needs time to wake up which is why this voltage undershoot is more intense in DEMB vs FPWM.

    Your second question refers to the overshoot recovery time. You are correct that this is not due to the IC having to take time to wake up. This is due to the diode-emulation operation of the IC. Diode emulation controls the low-side FET to emulate a diode meaning it will not sink current. Any sunk current is wasted current which will negatively affect efficiency and the goal of DEMB is to have the best efficiency possible.

    The load turns off and we see a VOUT overshoot due to the finite bandwidth of the feedback loop. FPWM will see this and sink current in order to bring that voltage back to its proper value. DEBM, however, will not sink current which means that voltage will stay there until something else (the load) sinks current to bring that voltage back down to proper regulation. But since there's no load, that voltage stays for a while.

    -Sam
  • Hi Sam,
    Thank you so much, you described very clear, I understand now ^-^.