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LM3444: Inductor Current Overshooting Setpoint

Sam Maxted
Prodigy 10 points
Part Number: LM3444

I have a design based on the LM3444 (using the typical application circuit) that is not behaving as specified in the datasheet. 

On initial turn on current ramps up in the inductor and overshoots the setpoint applied at the FILTER pin. This can be seen in the below scope trace:

In the this image the blue trace is the voltage applied to the FILTER pin and red is the voltage  at the ISNS pin (across the sense resistor) 

As can be seen the ISNS voltage overshoots the FILTER voltage and continues to ramp up to 770mV before switching off. 

I have purchased a reference implementation (LM3444-EDSNEV/NOPB) and performed the same measurements. A scope trace of this can be seen below:

In the this image the blue trace is the voltage applied to the FILTER pin and red is the voltage  at the ISNS pin (across the sense resistor) 

As can be seen this circuit behaves as expected; the ISNS voltage climbs until the setpoint is reached and the MOSFET switches off halting current. 

Attached is a copy of my schematic, what could be causing the LED Driver to not be switching off at the setpoint?

LM3444-prototype.pdf

  • 0
    TI__Genius 15335 points

    Hi Sam,

    Could you check if the +12V power supply (the U101 component in your schematic) is powered on as expected?

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 10 points

    Hi Steven,

    The +12V PSU is fine. The below scope trace shows a steady +12V with no droop on switching.

    Blue: Output of +12V PSU (TP105)

    Red: Gate (TP106)

    Thanks,

    Sam

  • 0 in reply to Sam Maxted
    TI__Genius 15335 points

    Hi Sam,

    May I understand how the circuit behaves after the overshoot? I need more information to help me find out what's happening here.

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 10 points

    Hi Steven,

    On the next switch on event the voltage across R111 continues to rise from where it was at when the gate switched off at the end of the last event. This continues for a number of cycles (I've seen this pattern occur in one, two, three or four switch on events) until some threshold is reached (this threshold seems to change). Once the threshold is hit the LM344 waits for a variable amount of time before starting the process again from zero current. An example of this can be seen below:

    In the this image the blue trace is the voltage applied to the FILTER pin and red is the voltage  at the ISNS pin (across the sense resistor) 

    Regards,

    Sam

  • 0 in reply to Sam Maxted
    TI__Genius 15335 points

    Hi Sam,

    So the output never comes to the target point. Right?

    May I know what is the signal you applied to the FILTER pin and how is the connection of the "I_SET" net in your schematics?

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 10 points

    Hi Steven,

    Correct, the design appears to spend most of it's time in over current lockout.

    The signal applied to the filter pin is a 200mV DC output from a benchtop PSU. The DC voltage is provided through a wire attached to TP109. Previously I_SET was the output of a potentiometer. When our team discovered that the potentiometer position did not alter output current we removed it and used the benchtop PSU instead. 

    Regards,

    Sam

  • 0 in reply to Sam Maxted
    TI__Genius 15335 points

    Hi Sam,

    Where do you probe the ISNS voltage? Do you probe it at a place which is close to the pin of the IC? If not, please do so and show me the oscilloscope capture.

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 10 points

    Hello Steven,

    The test point where I've been measuring ISNS (TP107) is very close to the pin of the LM3444 (see layout below)

    For completeness I've also probed the signal on the pin of the LM344 alongside TP107. As you can see in the below trace these signals are equal. The callouts in the layout image are the signals below were probed:

    Thanks,

    Sam

  • 0 in reply to Sam Maxted
    TI__Genius 15335 points

    Hi Sam,

    I will have a discussion with my colleagues and reply to you later. Thanks for your patience.

    Best Regards,

    Steven