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Part Number: UCC28780
I am having an issue with a DCDC converter prototype using the UCC28780. I have chosen the RVS1 resistor for the appropriate brown-in/brown-out voltage desired, yet the actual brown-in threshold observed is much higher.
Additionally, there appears to be no difference between the brown-in and brown-out threshold, whereas the datasheet specifies the brown-out threshold to be 83% of the brown-in threshold.
RVS1: 27K0RVS2: 8K20Np: 13Na: 2Ns: 4Vout: 30V
Desired brown-in threshold: approximately 66VDC (with RVS1 = 27K0 I should have 64VDC)
I have set the input voltage to 125V and the converter operates as expected:
CH1 (yellow): QL gate voltageCH2 (green): QL drain voltageCH3 (blue): VbulkCH4 (pink): Vaux
With a Na/Np ratio of 2/13, a Vbulk of 125V should give approximately 19V for the negative swing of Vaux. The 20.5V measured is pretty close to this.
Measuring the VS pin:
CH1 (yellow): QL gate voltageCH2 (green): QL drain voltageCH3 (blue): VbulkCH4 (pink): VS voltage
The VS pin is clamping the voltage to -262.5mV, which matches the specified -250mV.
I am not sure if the VS voltage when QL is off should look like this, with the large spike towards the beginning. Perhaps the scope probe is interfering with the signal.
When the input voltage is reduced to below 121VDC, it enters what looks like brown-out detection:
CH1 (yellow): QL gate voltageCH2 (green): QL drain voltageCH3 (blue): VddCH4 (pink): Vaux
The switching for 56ms (close to the 60ms brown-out timer) followed by a UVLO cycle leads me to believe this is a genuine brown-out fault.
When the input voltage is raised above 122VDC, the converter operates as in the first screen capture. There is practically no hysteresis with the brown-in/brown-out.
I feel like I am missing something obvious.
I would appreciate any insight into this problem.
A colleague of mine has discovered that the small capacitance of the pad under pin 9 (VS) to a ground plane was interfering with the measurement. Lifting the pin off the pad and rewiring RVS1 and RVS2 solved the issue. Follow the layout guidelines in the datasheet to avoid this problem.
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In reply to Connor Duignan:
We're glad to hear that you have resolved this issue. If you have any further questions on UCC28780 or any other TI device please let us know.
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