MIC2529 Datasheet, PDF (10/11 Pages) Micrel Semiconductor – Single-Channel USB Power Controller Advance Information

MIC2529
Suspend Current
Universal Serial Bus Specification places a maximum sus-
pend current requirement of 500µA on devices. For hubs,
Universal Serial Bus Specification Revision 1.1 clarifies this
issue. Revision 1.1, section 7.2.3, stipulates that the maxi-
mum suspend current for a configured hub is 2.5mA.
This number is derived by allocating 500µA for up to four
downstream ports plus 500µA for the hub’s internal functions.
A nonconfigured hub is considered a low-power device and
cannot consume more than 500µA. In a nonconfigured state
all downstream devices will be switched off. In most cases, a
nonconfigured hub is not a practical state for the system.
Therefore, the 2.5mA specification is the applicable target
specification for the suspend state. In a bus-powered hub
with less than 4 ports, the hub may use the additional current
for internal functions.
Micrel
The 500µA worst-case suspend current must be further
divided among the data port termination resistors and internal
functions. The termination resistors will consume
3.6V ÷ (16.5kΩ – 5%) = 230µA worst case. This leaves only
270µA for internal functions. Assuming 100µA as the maxi-
mum USB controller suspend current, 170µA remains for the
rest of the system. The MIC2529 will consume 150µA maxi-
mum, leaving a margin of 20µA.
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. For a 4-port
ganged hub, this resistance reduces to 250mΩ. The MIC2529
easily satisfies this requirement.
MIC2529
10
June 1999