SL2.1A is a single 5V power supply chip. The Integrated LDO(Include one 5v->3.3v LDO and one 3.3v->1.8v LDO) make the solution so simple. Only some capacitances needed for PCB. The chip also integrates Power-On-Reset circuit.
 Fully compliant with USB spec. Rev 2.0(USB 2.0)
12M crystal oscillator must be matched when using SL2.1A
 Upstream facing port supports HS (480MHz) and FS(12MHz) modes
4 downstream facing ports support HS(480MHz), FS(12MHz) and LS(1.5MHz) modes.
Integrated USB 2.0 Transceivers.
 Connected external 12MHz Crystal Oscillator is recommended for this chip.