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Current control - LM5122

Ðorde Gluhovic
Prodigy 20 points
Other Parts Discussed in Thread: LM5155, LM5175, LM5176

Hi,

I was simulated LM5122 circuit in PSpice for TI earlier.I plan to use the LM5122 circuit to control the boost. In the datasheet of that circuit, there is a proposal where it is controlled using a voltage divider from the output that is fed to the FB pin.
 However, I need to operate the steering by electricity. I need to filter the current signal from the saw-shaped shunt (maximum 30 mV voltage) and amplify it 10 times (this would be done by the first part of the circuit), 
and then give the second part of the circuit, on which the value is set, the current value (shown also by the voltage, which will after amplification the maximum will be 300 mV), 
regulate according to the mean value of the current and that the voltage at the output of 1.2 V is fed to the FB pin of the circuit LM5122 (internal error amplifier) ​​which will start the circuit to do the regulation. 
In short, can you offer a circuit that will do regulation by the mean value of the choke current? I need to modify the LM5122 circuit, by removing the voltage control offered in your datasheet,
 and adding a current control that represents a feedback loop that leads from the shunt to the FB pin of the circuit.

Best regards,
Đorđe
  • 0
    TI__Mastermind 19295 points

    Hello Đorđe,

    Thanks for reaching out to us via e2e.

    I got a little bit confused by this sentence:
    > However, I need to operate the steering by electricity.
    First I thought you are looking for a device that can be controlled by an external voltage instead of the FB pin.
    In that case I would recommend the LM5123 which provides a Track pin that allows you to control the output voltage by a control voltage that you feed to this pin.

    But reading further, I believe you meant to say that you want to regulate on the load current instead of the voltage.
    When the average load current goes up, the booster shall turn down.

    Please have a look at the datasheet of the LM5155: www.ti.com/.../lm5155
    I know that this part is a controller for an asynchronous boost stage, but it includes some pictures which will help to understand the possible concepts.

    - For lower power, simply use a shunt and feed the signal (voltage drop) directly into the FB pin Figure 10-9. Low-Cost LED Driver
    - To avoid big losses, you can use a current sense amplifier to amplify that voltage drop
    - Use a current mirror, as shown here: Figure 10-13. LED Driver with High-Side Current Sensing

    These concepts will only regulate on current, but not on voltage.

    If you want to regulate on the voltage, and also limit the current as well, please have a look at the LM5175 / LM5176.
    These parts offer an additional built-in average current feedback loop.

    These parts have three control loops:
    The inner cycle-by-cycle peak current loop (CS, CSG pins), the voltage control loop (FB pin) and the outer average current control loop (ISNS pins).

    Best regards,
    Harry