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UCC15240-Q1: Starting-up problem

Mamoru Sasaki
Prodigy 20 points
Part Number: UCC15240-Q1

Hi TI engineers,

I am using UCC15240-Q1 on my hardware but it doesn't output desired voltage. The designed outputs are VDD-VEE = 25 V and COM-VEE = 5 V.

The below picture shows waveforms of VDD-VEE(yellow) and COM-VEE(cian) at starting-up.

COM-VEE does not stay around 5 V and goes upto 11 V, then decreases. VDD-VEE stays around 25 V for a while, then decreases.

I read "11.3.2 Output Voltage Soft Start" in the data sheet, but didn't understand what causes this problem. 

Could you help me figuring out what is going on here? Thank you.

  • 0
    TI__Mastermind 32775 points

    Mamoru,

    The cyan waveform is COM-VEE and you set the FBVEE to regulate at 5V? If the resistor divider is correct, then at startup, your waveform is showing that COM-VEE is trying to regulate higher than 5V. If you are to show VDD-COM on your scope plot, you would likely see it being less than the target set value. If the COM-VEE (or VDD-VEE) is more then ±10% above/below the set target value, output UVLO overvoltage protection is enabled and I believe this is what you are seeing. The most common reason for this is mismatched charge balance between VDD-COM and COM-VEE. This mismatch can either happen because there is an imbalanced load or more common is that the capacitor ratios are not correct. Be sure to use the Excel design tool and follow the correct capacitor results as close as possible. Also, the UCC14240EVM-052 has the output set for VDD-VEE=25V and COM-VEE=5V so you can look follow this as another example.

    Steve