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Original question:

LM3488: LM3488 isn't working correctly

LM3488: LM3488 isn't working correctly

Romain Pichonneau
Prodigy 70 points
Part Number: LM3488


Hi,

My thread is locked so I post a new thread. I have some trouble on several boards. I don't understand three points :

- why COMP signal (blue curve) increases to a 1.9V level,

- why LM3488 begins in continuous conduction mode even without load,

- why it stay in continuous conduction mode with the load (solenoid valve).

Schematic :

Curve 1 (COMP signal in blue + DR signal in pink) [board KO]

Curve 2 (COMP signal in blue + DR signal in pink) [board OK]

Curve 3 (DR signal in pink + ISEN signal in blue [peaks in discontinuous mode is just a ground cable length too long])

Curve 4 (DR with load [board KO])

Curve 5 : (DR with load [board OK])

  • 0
    TI__Mastermind 31315 points

    Hi Romain,

    Thanks for the images. Can you specify your test conditions; for example- in your no load conditions, did you disconnect the load from the circuit? 

    It looks like COMP is behaving properly here, when COMP is high, you will want switching.

    Can you provide startup waveform with and without load (VIN, VOUT, SW, COMP) zoomed-out to see full start-up cycle and zoomed-in (~5 cycles?). 

    Thanks,

    Richard 

  • 0 in reply to Richard Fu22
    Prodigy 70 points

    Hi Richard,

    Thanks for your help. Yes, in no load conditions I disconnected the load from the circuit.

    Vin (blue) & Vout (pink) during startup

    SW (pink) & COMP (blue) during startup

    I think LM3488 works. But when Vin is near Vout (above 11V), Ton is lower than Ton min.

    In this configuration, I have excessive oscillations (COMP, VOUT...)

  • 0 in reply to Romain Pichonneau
    TI__Mastermind 31315 points

    Hi Romain,

    If VIN close to VOUT, then the part is pulse skipping, which means part will skip pulses as necessary in order to achieve the desired on-time. This may mean that VOUT will rise for a bit, and then stop switching as energy delivered to output cap is more than required to achieve the desired output voltage, which is shown in the picture.

    Thanks,

    Richard