The USB Specification requires that the D- pin of low-speed devices be pulled up to a 3.0V to 3.6V voltage source through a 1.5K-ohm resistor. An alternative implementation is to pull the D- pin to Vcc (~5V) with a 7.5K. Voltage divider (7.5K pull-up and 15K pull down on the hub side) gives around 3.3V at the D- pin which is within the level required by the Spec. Although many designs use this approach with no problem, one of the drawbacks is that the voltage on D- will be dependent upon Vcc. In order to stay above the 3.0V lower limit, Vcc must be at least 4.5V even though the IC may be guaranteed to function at lower voltages.