[FAQ] Why is soft-start so important with PoE devices?

During startup, the DC-DC converter tends to overcompensate for the low output voltage by attempting to operate at maximum duty cycle. This is prevented by the soft-start circuit that slowly increases the peak current of the current mode DC-DC converter. However, if the soft-start capacitor is inadequate and significant current overshoot occurs, the input current to the DC-DC converter may exceed the current limit threshold of the hot swap MOSFET. If this occurs, the power good condition can be violated, which will subsequently shut off the DC-DC Converter, preventing startup. Load conditions during start-up should also be considered when selecting the soft-start capacitor; the regulator should start cleanly over all loading conditions. More information is discussed on page 19 of TPS23753A datasheet.

Additional Resources

• Check out TI's PoE landing page: www.TI.com/PoE

• Learn about the IEEE802.3bt standard: Power over Ethernet: Getting Started with the New Standard