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linear voltage regulator lock up

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terdol

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Hello,
Some linear voltage regulators do not work if the output voltage drops below 0V (-0.5V for example) before input voltage is applied. The problem can be solved by placing a schottky diode from the output to ground in order to prevent the output voltage from falling much below the 0V.
Does anyone know the reason for this behaviour of linear voltage regulators? Thanks.
 

saro_k_82

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I guess the reason for this failure is not related particularly to regulators but true for any analog module. If any node goes ~0.6V below the most negative supply, the drain-bulk junction would start to conduct current and could render the feedback useless. To prevent against this, a schottky diode which has a cut-in voltage at-least 200mV lower than the p-n diode is used to limit the voltage. In a regulator, the external load could kick in huge transients while switching large currents and inductive loads, so it deserves this protection diode more than any other pin.
NMOS linear regulators that have their bulk connected to local IC ground would be highly prone to this type of failure.
 
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    terdol

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