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LDO Linear RegulatorsHow to use
If the output voltage becomes higher than the input voltage, a large current may flow from the output to the input, which may damage the IC.
This is caused by a parasitic diode in the internal MOSFET. The parasitic diode may turn on if the output voltage is higher than input voltage by its Vf. In the following cases, however, the reverse current might do no harm: the current path to the input is cut off during reverse voltage application; the output voltage source has no ability to let a large current flow.
In normal operation, Ricoh CMOS process LDO does not require any protection diode, for example, when the LDO is disabled, reverse current may flow from the output capacitors, but the energy is very low.
ICs with built-in reverse current protection can protect themselves from being damaged by a reverse current. ICs without built-in reverse current protection is also possible to prevent damage by bypassing the reverse current through an external diode.
Adding an external diode to the output decreases the output voltage by the Vf of the added diode.
As for the LDO whose output pin (Vout pin) and feedback pin (VFB) are separated, by inserting a diode at the position shown in the below figure, the output voltage can be maintained as the original set output voltage.
Adding an external diode to the input, the input voltage decreases by the Vf of the diode.
In order to make the LDO regulator conduct properly, dropout voltage specification must be considered.
Inserting an external diode between the input and the output makes it possible to bypass a reverse current.
Choose a Shottky barrier diode whose Vf is lower than that of the parasitic diode inside the IC.
Also, choose the enough current capability Shottky barrier diode to bear the reverse current.