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LM5121-Q1: Is it Possible to Use IC without Inrush Current Limit and Disconnection Switch Functions ?

Part Number: LM5121-Q1
Other Parts Discussed in Thread: LM5121

Hello Team,

My question is, in the "simplified application diagram" that is mentioned in the datasheet, what would happen, if i could choose not to use the MOSFET in input side (which gate is connected to DG) and the resistor between CSP and CSN pins. To summarize my question, is it possible to left floating pins that are DS, DG, CSN and CSP. In my application, i do not need current limit and disconnection switch functions. Do I still use the LM5121-Q1 boost converter properly depending on my application ? I am looking forward to hear more from yours. Thanks for yours advice.

Best Regards,

  • Hi Kaan,

    The CSN and CSP and the sensing resistor must be connected because that's not just for current limit but in the current control loop as well. Please let me know if there is any question.

    Thanks,

    Yinsong

  • Hello Yinsong,

    Thanks for your immediate response. In terms of disconnection load switch, is it possible to neglect the MOSFET which is connected to the DG pin with its gate. It was my second question also. I kindly would like to ask your help. Thanks for all the support that you have already did.

    Best Regards,

  • Hi Kaan,

    Are you talking about removing the disconnection switch? I'll need to confirm with our designer what to do with the DG and DS pin if the disconnection switch is removed.

    Thanks,

    Yinsong

  • Hello Yinsong,

    Thanks a lot for your reply. That's true, I exactly mean that I would like to remove disconnection switch function by removing mosfet in the design. After that, I am planning to left DG and DS pins as float. Is that possible ? 

    FYI; as far as I understand from the datasheet DG and DS are output pins and their basic functionality to reduce the risk of inrush current problem during the startup transient. And in datasheet it is in blue diagram to show differences. Please confirm me about the meaning of the configuration too.

    Best Regards,

  • Hi Kaan,

    You are right about the disconnection switch control in the block diagram. The inrush current limiter information can be found in datasheet 7.3.7 and the enable/disable comparator information can be found in 7.3.12. I'm still waiting for response from designer and I'll let you know.

    Thanks,

    Yinsong

  • Hi Yinsong,

    I really appreciate your kindest help for my questions. However, I still need your help me to cover my other question about LM5121-Q1. I will use this IC to charge my hold up capacitor bank which has around 4 milifarad high capacitance value. I am expecting my hold up capacitor bank will supply around 150 Watt power for at least 50 milisecond. The boost converter may boost my input voltage to 50 Volts as an output. Moreover, in that point i really need to analyse what will be the inrush current when the output Mosfet starts to conduct my hold up bank. How i could be sure that load transient will not fall down during the charging process. Please give me advices on this occasion as soon as possible. I look forward to hear more from yours.

    Best Regards,

  • Hi Kaan,

    No problem. I'll need more information regarding this question. Are you saying that it has some load while the boost is charging the hold up cap? If so, how much is the load?

    Thanks,

    Yinsong

  • Hello Yinsong, 

    Thanks a lot for your answer. I meant that this boost circuit will have only one specific mission which is to charge my 4 mF (milliFarads) hold up capacitors for one time. The hold up capacitor banks will be compromised of four parallel 1 millifarads caps and equivalent ESR value should be around 250 milliohm (when four of them are connected in parallel).

    I try to design LM5121 which will be active on 10V input voltage at least and it will boost this input to 50V output (the circuit must be active when the input voltage reaches 10V). Max allowable output voltage is 55 Volts desired. I try to use the design example solution in the datasheet as a reference to calculate the values of passive components in this case.

    However, I still doubt that if this circuit will supply my hold up capacitors properly at this range. Because, The output power must be 150Watt (the case is 50V out and 3 A output current)  which my hold up capacitors need it. Because this 150Watt power will be supplied from my hold up capacitors to other circuitries for a specific time when there is no power. I have already calculated the capacitance value of the hold up bank. Therefore it is rigid constraints. The load will be only hold up capacitor bank. Please help me on this occasion.

    Best Regards, 

  • Hi Kaan,

    Thank you for the detailed information. It looks like you just need to use the LM5121 to charge the 4mF hold capacitor to 50/55V without other load, is that correct?

    Thanks,

    Yinsong

  • Hi Yinsong,

    Thanks for your reply. That's true. The load of the LM5121 will be only hold capacitors. In this case, the point that we have to focus is LM5121 will only supply current to hold up capacitors in spesific amount of time. After the capacitors are fully loaded, there will be no needing to supply current. However, considering the fact that hold up capacitors are generally huge in terms of their capacitance values, how should I be encouraged to overcome inrush current problem ? Because when we set the input voltage at desired level, the LM5121 will start to charge boostrap capacitors to drive mosfets, and when the high side mosfet start to drive, there will be huge amount of current will be demanded from the capacitor bank. In this case, even the LM5121 will be burned or severely damaged. That's the main focus. I hope you see the point. I look forward to hear more from you.

    Best Regards,

  • Hi Kaan,

    Thank you for the explanation. Based on this condition and the concern of inrush current, LM5121 is definitely a fit with all the protection features it integrates. LM5121 has five types of protection to ensure its function safety during high transient. I'll give a brief introduction below and the details can be found in the datasheet.

    1. cycle-by-cycle current limit. This protection only triggers the pull down of LO and prevents the low side switch damage from conditions like inductor saturation. (75mV internal threshold)

    2. Soft start. By programming the soft start time through the SS capacitor, the startup current (and voltage) overshoot after the switching begins can be controlled.

    3. inrush current limit. Before the soft start and any switching, the natural power cycle also induce high current through the power loop especially in high capacitance case. This protection can clamp the GS voltage of the disconnection switch in the saturation region. The SS will only be triggered after the Vgs pass this and reach a threshold. (110mV internal threshold)

    4. circuit breaker. The inrush current limit may not be able to respond if the transient is too fast. In this condition, disconnection switch can serve as a circuit breaker to quickly turn off the inrush current. (160mV internal threshold)

    5. Hiccup mode. This protection is for abnormal short circuit and overload conditions. Whenever the current limit is detected for a period of time, the SS and the disconnection switch drive voltage will be pulled low. It will be latched until a programmed restart time has passed.

    We have a reference design of LM5121 which also charge a 4700uF capacitor (although it has additional resistive load). It should be a good reference for your case. https://www.ti.com/tool/PMP9372

    Thanks,

    Yinsong