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Original question:

LM5171: Bidirectional current flow change during operation

LM5171-Q1: LM5171: Handling reverse current from inductive load (DIR switching vs ISET control)

Hyungmin Kim
Prodigy 30 points

Part Number: LM5171-Q1
Other Parts Discussed in Thread: LM5171, LM5171EVM-BIDIR

Hello 

I have a question regarding bidirectional operation under transient conditions, especially when handling inductive load turn-off.

In my system, both a resistive load and an inductive load are connected to VOUT.
When the inductive load is turned OFF, a back-EMF is generated, which causes:
- a temporary increase in VOUT (above nominal 14V)
- and a reversal of inductor current

To handle this, I am considering two methods:

1) Dynamically changing the DIR pin from HIGH (>2V, buck mode) to LOW (<1V, boost mode)
2) Keeping DIR fixed (buck mode) and allowing current direction to change by controlling ISET

My questions are:

1) Is it recommended to toggle DIR during such transient events, considering the internal soft-start/reset behavior?
2) Alternatively, is it better to keep DIR fixed and rely on ISET to handle current reversal during bidirectional operation?
3) In the ISET-based approach, can the device naturally absorb reverse current (from inductive kickback) without explicitly switching DIR?

Additionally, during this event, the resistive load remains connected to VOUT.
Should I be concerned about output voltage interruption or instability affecting the resistive load?

I would appreciate your guidance on the preferred control strategy for this situation.

Best regards,

  • 0
    TI__Mastermind 29200 points

    Hi Hyungmin,
    Please refer to Seamless Bidirectional Operation of LM5171.

    You are right, there are two methods to reverse the current. The "ISET-based approach" is simpler in your application. The circuit works automatically; you don't need to detect when back-EMF comes.

    As shown in Fig 4 in Seamless Bidirectional Operation of LM5171, you also need to select FPWM mode. The drawback is that the light load efficiency is not good for FPWM.

    Best Regards,
    Feng

  • 0 in reply to Feng Ji
    Prodigy 30 points

    Hello Feng,

    Thank you for your previous explanation regarding seamless bidirectional operation using the ISET-based approach.

    We are currently planning to design a 48V–14V converter based on the LM5171 EVM (LM5171EVM-BIDIR), and I would like to clarify the following points regarding practical implementation.

    1) EVM Setup for ISET-based Seamless Control

    For achieving seamless bidirectional control on the LM5171EVM-BIDIR:

    • What configuration is required to control the ISETx pin properly on the EVM?
    • Is additional external circuitry required for ISET control?
    • If so, is it recommended to implement this using MCU/DSP GPIO (DAC or PWM + filter), or is there a preferred analog method?

    2) Availability of Reference Schematic (Seamless Control)

    • Do you have a reference schematic (SCH) based on the LM5171EVM-BIDIR that implements seamless bidirectional control using ISET (without DIR toggling)?
    • If available, I would appreciate if you could share it for reference.

    Our goal is to implement seamless bidirectional current transition using ISET-based control on the EVM and apply the same concept to our hardware design.

    Best regards,
    Hyungmin Kim

  • 0 in reply to Hyungmin Kim
    TI__Mastermind 29200 points

    Hi Hyungmin,

    1. To get a buck converter with Vout=14V, you may follow section 2.4.1 Buck Mode with Voltage Regulation in the LM5171EVM user's guide. Based on this, remove J4 and J5 to operate in FPWM mode. In this configuration, the error amplifier regulates the ISET voltage to maintain a regulated output voltage.

    2. I don't have a reference schematic for the moment. You may refer to Figure 7. Bidirectional Operation with Single Voltage Loop in Seamless Bidirectional Operation of LM5171.

    Best Regards,
    Feng

  • 0 in reply to Feng Ji
    Prodigy 30 points

    Hello Feng,

    Thank you for your response.

    I have two additional questions regarding the seamless operation:

    1. Fault behavior during seamless operation
      Is it possible to execute protection sequences such as overcurrent protection (OCP) or overvoltage protection (OVP) during seamless operation?
      In other words, can fault shutdown or protection actions be properly triggered and completed while the converter is operating in seamless mode?

    2. VISET pin requirement for seamless operation
      To enable seamless operation, is it required to externally apply 1V to the VISET pin at initial startup?
      Or, in addition to the circuit configuration shown in the figure below, is it necessary to provide external control of the VISET pin (e.g., via an MCU or external circuitry)?

                      

    Best regards,

    Hyungmin Kim

  • 0 in reply to Hyungmin Kim
    TI__Mastermind 29200 points

    Hi Hyungmin,

    1. The OCP/OVP circuit also works in FPWM mode.

    2. The circuit attached is complete, it does not need external signals. But as mentioned in the app note, the HV-port voltage must be lower than the target regulation voltage to start switching.

    Best Regards,
    Feng

  • 0 in reply to Feng Ji
    Prodigy 30 points

    Hello Feng,

    Thank you for your prompt reply.

    Can we identify the R and C values used in the circuit setup shown in the image in your report?

    I am having difficulty implementing the seamless circuit using only the lm5171 quick start excel tool.

    If there is a way to implement it using only the quick start tool, could you please explain the method?


    Best regards,

    Hyungmin Kim

  • 0 in reply to Hyungmin Kim
    TI__Mastermind 29200 points

    Hi Hyungmin,

    These values can be calculated with the LM5171 quick start excel tool. The R and C are shown in the image below.

    Best Regards,
    Feng