It is used like this, the 5th foot of the UC3840 is the ground.
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First test the fifth and eighth pins, the eighth pin output reference voltage 5V, testing this foot will soon be able to judge the UC3840 good or bad
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3, if the 8th pin voltage is normal, under normal circumstances, there is no problem with UC3840, and the others do not need to be measured.
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4, if the 8th pin voltage is not, test the 7th pin to see whether the power supply voltage is correct. However, the 7-pin voltage value depends on the specific circuit design.