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Part Number: LM7800
We are using the following TI components on our PCB: LM7812 and LM7912
The LM7812 provides +12V and LM7912 for -12V which are supplied by +15V / -15V.
!! With respect to the used electrical circuit the output of the LM7912 (-15V) is connected to the output of the LM7812 (+12V) by using a diode.
The anode of the diode is connected to -12V. The cathode is connected to +12V.
Test procedure:Step 1: Switch on -15V input voltage first.Step 2: Switch on +15V input voltage.
Test result:Unfortunately our test engineer found out that the +12V power supply drops to 0,8V.If the diode is removed from the voltage regulators the power supply is stable.If the test procedure is inverted the +12V power supply is stable, too.QUESTION TO ST:Can you explain this behavior ?Why does the output voltage of the LM7812 depends on the power on sequence ?Is there any protection mechanism of the device which is responsible for this behavior ?
Thank you !
In applications that have positive and negative rails many positive regulators can be prevented from starting up properly if the negative rail is supplied first. This is due to the current limit of the positive regulator.
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