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1137J–USB–01/06
AT43301
The on-chip phase locked loop generates the high frequency clock for the clock/data separation
circuit.
2.5 Power Management
A hub is allowed to draw up to 500 mA of power from the host or upstream hub. The AT43301’s
itself and its external circuitry typically consume about 24 mA. Therefore, in the bus-powered
mode, 100 mA is available for each of the hub’s downstream devices. In the self-powered mode,
an external power supply is required which must be capable of supplying 500 mA per port. The
power supplied to the ports is monitored and controlled by the AT43301.
The AT43301 reports overcurrent on a global basis. The overcurrent signal, which needs to be
detected by an external device, is read through the OVC
pin. A logic low at OVC is interpreted as
an overcurrent condition. This could be caused by an overload, or a short circuit, and causes the
AT43301 to set the Over-Current Indicator bit of the Hub Status Field, wHubStatus, as well as
the Over-Current Indicator Change bit of the Hub Change Field, wHubChange. At the same
time, power to the ports is switched off by de-asserting PWR
.
An external device is needed to perform the actual switching of the ports’ power under control of
the AT43301. Any type of suitable switch or device is acceptable. However, the switch should
have a low-voltage drop across it even when the port absorbs full power. In its simplest form, this
switch can be a high side MOSFET switch. The advantage of using a MOSFET switch is its very
low-voltage drop.
The power control pin, PWR
, is asserted only when a SetPortFeature[PORT-POWER] request is
received from the host. PWR
is de-asserted under the following conditions:
1. Power up
2. Reset and initialization
3. Overcurrent condition
4. Requested by the host though a ClearPortFeature[PORT_POWER] for ALL the ports
2.5.1 Self-powered Mode
In the self-powered mode, power to the downstream ports must be supplied by an external
power supply. This power supply must be capable of supplying 500 mA per port or 2A total with
good voltage tolerance and regulation. At full hub operating power, that is all downstream ports
drawing 500 mA each, the minimum voltage at the downstream port connector must be 4.75V.
The USB specification requires that the voltage drop at the power switch and board traces be no
more than 100 mV. A good conservative maximum drop at the power switch itself should be no
more than 75 mV. Careful design and selection of the power switch and PC board layout is
required to meet the specifications. When using a MOSFET switch, its resistance must be 40
mΩ or less under worst case conditions. A suitable MOSFET switch for an AT43301 based hub
is an integrated highside MOSFET switch such as the Micrel MIC2505.
2.5.2 Bus-Powered Mode
In the bus-powered mode all the power for the hub itself as well as the downstream ports is sup-
plied by the root hub or upstream hub through the USB. Only 100 mA is available for each of the
hub’s downstream devices and therefore only low-power devices are supported.
The power switch works exactly like the self-powered mode, except that the allowable switch
resistance is higher: 140 mΩ or less under the worst case condition. An example of a suitable
high side switch for a bus-powered hub is the Micrel MIC2525.
