MIC2537

Micrel

December 1999

10

MIC2537

USB Voltage Regulation

USB specifications require a minimum downstream voltage

supply of 4.40V from a bus-powered hub port (See

Applica-

tion Note 17 for details). The USB specification allows for a

100mV drop across the hub, leaving 250mV for PCB, up-

stream cable, and connector resistance. Therefore, the on-

resistance of the switch for each port, not including PCB

resistance, must be about 100mV

÷ 100mA = 1Ω. The

MIC2537 has a maximum on-resistance of 700m

Ω, which

easily satisfies this requirement.

Overcurrent Indication

The USB Specification does not require bus-powered hubs to

report overcurrent conditions to the host since the hub is

already current-limited at the upstream port. However, if it is

desired to report overcurrent, the Hub Descriptor Status

Register must be programmed to indicate this. The MIC2537

provides a flag output for this application.