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LM5023: Issue with power cycle under load

Asmir Gogic
Prodigy 110 points
Part Number: LM5023

Hello,

we are working on a product actually two of them which utilize LM5023 a quasi-resonant PWM controller. Schematic (attached) is modified version of EVM-LM5023. There are two switching regulators connected to 19.2V output. We have built small batch of boards, and they all behaved very similar (few of them are working just fine). Scenario is as follows

- If we start the device (110VAC provided same behavior with 220VAC) with no load or load at 12V switching regulator (13W or 30W or 46W) its working fine. However, if we remove power while LM5023 is under load and try start it again (AC power cycle) it will not start until we leave it for 15 seconds (this means that there is overvoltage protection). OVP is set to 22V but we have tried also 24V by setting R59 to 15k.

- same scenario connect and leave for some time load and remove it then do the power cycle everything is fine.

- for analysis we have used information from LM5023 datasheet and created a Matlab script to perform calculation which differ from what is seen in the datasheet. If needed I will attach the script as well.

Please advise as we are in final phase of product design. 

  • 0
    TI__Expert 8210 points

    Asmir

    If this condition is causing a OVP fault to trigger, VCC needs to discharge below VRST (5V typ) before it can restart.  If the overload condition is exceeded, VCC needs to discharge below VCCOFF (7.5V typ) before it can restart.  Did you measure the voltage on the LM5023 IC to verify what fault is preventing LM5023 from restarting?

    If the OVP latch fault response is in fact causing the shutdown during this input cycle, you may need to discharge VCC faster so it can reduce below VRST (5V typ) to restart.  This could either be done by reducing the VCC capacitance or adding a circuit that pulls VCC down faster.

    Best Regards,

    Eric