Inquiry from customer wanting to better understand the ADP198 reverse polarity protection:
The ADP198 is used with 3.3V at the input and would like to apply a 5V at the output and activate the reverse polarity protection.
It does not seem to work appropriately.
Can you help?
In order for the reverse current circuitry to be activated, the Vout to Vin differential must at least 50mV-75mV. Normally, if the part is not enabled it is easy for the 50mV differential to be created across an off FET.
However, if the part is enabled, this means that the FET is on and the resistance is about 50 milli-ohms. When the 5V is applied to the output, there is only the 50 milli-ohm resistor between the output and input causing the 3.3V to potentially “float up” to 5V if the 3.3V rail cannot sink current.
For the reverse polarity to operate correctly the EN pin needs to be driven low when 5V is present at Vout and before the input is allowed to float high.
If the EN pin is left high when 5V is applied to the output, over 1A of reverse current will be needed to create this 50-75mV differential and for the ADP198 to engage the reverse polarity protection.
This also applied to other similar load switched like the ADP195.
Answered and Closed