Teledyne LeCroy QPHY-USB - Manual (Page 49)

QPHY-USB Software Option
QPHY-USB Operator’s Manual Rev F
49
Bus-powered Hubs
1. Switch the loads to the 100 mA position one at a time and verify that the voltage is above 4.4 V.
2. Repeat as necessary for all ports of the hub.
V
DROP
= V
UPSTREAM
– V
DOWNSTREAM
Where
V
UPSTREAM
= V
BUS
at a hub’s upstream connection
And
V
DOWNSTREAM
= V
BUS
at one of the hub’s downstream ports
Test Criteria
Section 7.2.2 of the USB 2.0 specification requires self-powered downstream USB ports to provide a V
BUS
between 4.75 and 5.25 V while bus-powered hubs must maintain V
BUS
at 4.40 V or greater. Drop testing evaluates
V
BUS
under both no-load and full-load (100 or 500 mA, as appropriate) conditions. Self-powered hubs, systems,
and laptops must provide a voltage between 4.75 and 5.25 V under all load conditions. Bus-powered hubs must
have a V
DROP
<= 100 mV between their upstream and downstream ports when 100 mA loads are present on all
downstream ports. This ensures they supply 4.4 V to a downstream device, given a 4.75 V upstream supply,
minus 100 mV drop through the hub and 250 mV drop through the upstream cable. If the hub does not use a
captive cable (the USB cable has a B plug), the voltage drop is the difference between the measured upstream
voltage level and the lowest measured downstream value. Bus-powered hubs with captive cables (the USB cable
does not have a B plug) must have V
DROP
less than or equal to 350 mV between the upstream connector and their
downstream ports. This includes the drop through the cable. Special consideration should be made for laptops
unable to provide compliant voltages with 500 mA loads while running on battery power, provided they can meet
the required voltages with one or more of the loads reduced to 100 mA. However, the end user may experience
confusion and difficulty in this situation, unless the operating system or laptop vendor provides a warning
message window alerting the user that a high-power device cannot be used under battery power.