Connect-Tek USB-2 Manual do Utilizador descarregar pdf (Página 4)

Application Note

www.tektronix.com/usb4

In-Rush Current Check

Because USB 2.0 is a hot-pluggable technology, extreme

care is required to ensure that the current drawn by a device

does not exceed a speciﬁed limit. If the current drawn exceeds

a speciﬁed value, the operation of other USB 2.0 devices

connected to the bus may be hampered. The in-rush current

check is performed for both self-powered and bus-powered

devices to verify that the device-under-test (DUT) does not

draw too much current when plugged into the port of a hub.

Typically, one expects a sharp intake of current when a device

is plugged in. One may observe small humps or perturbations

in the current trace depending on when the device is reset.

Theoretically, an in-rush current check involves the calculation

of the integral of current over a certain period of time (bounded

by the location of two vertical cursors on the oscilloscope).

The USB 2.0 speciﬁcation dictates that the total charge drawn

by the device should be less than or equal to 51.5 uC for a

BUS

value of 5.15 V. (The waiver limit for this test is less than

150 uC).

Test Equipment

The in-rush current check requires a real-time oscilloscope,

such as a DPO7254, and current probes, like the TCP0030.

This test also requires test software and a test ﬁxture, such as

the option USB compliance test package. The TDSUSB2 test

package can be used to automatically set up the oscilloscope

for the in-rush current check. This test package provides direct

readout of Charge (uC), Capacitance (uF) and an automatic

indication of pass or fail.

Drop Test

The USB 2.0 speciﬁcation requires powered USB ports to

provide a V

BUS

between 4.75 and 5.25 V while bus-powered

hubs maintain a V

BUS

at 4.4 V or greater. Drop testing

evaluates V

BUS

under both no-load and full-load (100 mA or

500 mA, as appropriate) conditions.

drop

= V

upstream

— V

downstream

upstream

= V

BUS

at the hub's upstream connection

downstream

= V

BUS

at one of the hub's downstream ports

Bus-powered hubs must have a V

drop

<=100 mV between

their downstream and upstream ports when 100 mA loads are

present on their downstream ports. This requirement ensures

that the hubs will supply 4.4 V to a downstream device. Bus-

powered devices with Captive cables must have V

drop

350 mV between the upstream connector and downstream

port, including the drop through the cable.

Figure 4. Illustration of a sharp intake of current using a DPO7254C.

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