Pulldown resistors on EN pins are needed in some situations. When the UCD90xxx device is in reset or in initialization, GPIO pins are in high-impedance state (FPWM pins are in drive-low state). If the EN pin of the POL controller has internal pullup, the EN pins of the UCD90xxx (GPIO pins in Hi-Z state) are unable to keep the POL converters off. If this is the case, pulldown resistors are needed to keep EN pins low when UCD90xxx is in reset or initialization.
For more FAQ please check the following document.
UCD90xxx Family Frequently Asked Questions and Answers
UCD9090 UCD9090A UCD90120 UCD90120A UCD90124 UCD90124A UCD90160 UCD90160A UCD90240 UCD90320 UCD90320U