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LM5176: controller issues operating at -40°C

Kevin Hua
Prodigy 25 points
Part Number: LM5176

When operating at room temperature at 22°C to our high temperature at 71°C, the converter operates as designed. When the chip is brought down to -40°C, the output voltage seems to be bucking down to 13.1V. I monitored the high-side driver and low-side drivers and can confirm that the chip is operating in the buck region, rather than the buck-boost region we see at room temp. The reference voltage for the feedback is set to 0.8V, and the feedback we are seeing is 0.67V. This is what is expected with our resistor divider. Measuring the soft-start pin we see the voltage at 1.2V as expected and the comp pin is railed high at 3.2V as expected. With these conditions, the converter should provide some boosting in order to get the voltage up. I can confirm that PGOOD is low. Since I have control on the input voltage, I see that lowering the input voltage causes the converter to stay in buck and lowers the output voltage. Increasing the output voltage I can get to my 15.75V output once the input voltage reaches 19V. As I raise my voltage above that, the converter regulates the output at 15.75V (showing that the error amplifier is operation and that the Vref is still set to 0.8V with tolerances).

 For some background:

Input Voltage: 15.8V

Output Voltage 15.75V

Output Current: 1A

Operating Temperatures: -40°C ambient to 71°C ambient

  • 0
    TI__Guru 65170 points

    Hello Kevin,

    Please share your schematic. Would you please check the voltage on the current sense resistor as well?

    Please check if the problem is gone when you reduce the sense resistor value to half the actual value.

  • 0 in reply to Brigitte
    Prodigy 25 points

    Hi Brigitte,

    The current sense resistor I am using between the CS and CSG is 20 milli-ohms. Since my operating frequency is at 300kHz, I know when the current sense protection is activating as the datasheet states the controller turns off for 4,000 cycles and I'd see about 13.3ms of cycle delay. I have seen this operation when my soft-start was set too low, causing the inrush upon turn on to trigger the unit into hiccup mode. Thus I want to rule out the current sense resistor as an issue. I did go ahead and half the current sense resistor to 10 milli-ohms to test and saw that the chip is still have issues running cold.

    Since this chip is being used for military application, there are ITAR regulations that will not allow me to share the schematic. Is there an application engineer I can exchange information with directly? My colleague directed me to Colin Gillmor since he is helping us with our other products, but his email seems to be inactive.

    Thanks,

    Kevin Hua

  • 0 in reply to Kevin Hua
    TI__Guru 65170 points

    Hello Kevin,

    I sent you a friendship request for private discussion.

    In the meantime, please check the input and output voltage as well as the switch nodes at the same time in a scope plot.