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LM3429-Q1: Analog Dimming/CSH Resistor Not Recognized. Cannot set/adjust current without a scope probe attached to the CSH pin.

Nick Winston
Prodigy 60 points
Part Number: LM3429-Q1
Other Parts Discussed in Thread: LM3429

Hello,

I've designed a boost driver with the LM3429 that's to drive 4 strings of 16 LEDs/string (VF ranging 55V to 64V) from on 48VDC. The designed current range is 1.1A/string down to 300mA/string. The PCB for this is intended to have nine of these drivers on it but as of now I've only populated one driver for testing purposes. Problem is I'm able to power it on and the LEDs are turned on but the driver doesn't seem to recognize the CSH resistor/trimpot (I can't set current) and thus it continually ramps up the current until it hits the MOSFET's current limit set by the IS pin.

However when I take my scope probe and probe the CSH pin I'm fully able to set the current and the chip responds swiftly. It is then no longer hitting the FET current limit. Is there any explanation to this? I'd like to fix this problem with just a few BOM changes instead of refabbing more PCBs.

I can email the schematic, PCBDOC & design guide that I've used.

Thanks,

Nick

  • 0
    TI__Mastermind 28655 points

    Hi Nick,

    CSH pin is fairly noise susceptible so the circuitry connected to it should be routed carefully.  What circuitry is connected to the CSH pin?

    What values do you have for Rhsn, Rhsp, Rsns, Rcsh, Rlim (MOSFET current sense resistor), Ccmp (compensation capacitor)?  Are you using Rfs and Cfs?

    You may be able to check if the circuit is working correct by just having Rcsh installed (nothing else tied to CSH) and vary its value to see if the circuit behaves as it should.  Also, is there a solid ground for the IC and associated components around it?

    Regards,