Teledyne LeCroy QPHY-USB - Manual (Page 57)

QPHY-USB Software Option
QPHY-USB Operator’s Manual Rev F
57
Inrush current
Inrush current is generated by devices when they are connected to a hub or host. Self-powered devices should
have negligible inrush current. So, this test normally does not indicate any inrush current for such devices. Inrush
is measured using the Inrush section of the test fixture.
The arrow on the current probe must point in the same direction as the arrow on the Test Fixture. The current
probe (a LeCroy CP030, or equivalent) is attached to channel 4 of the oscilloscope. The oscilloscope is set to
trigger on the rising edge of the current pulse measured on channel 4. Problems associated with switch bounce
are avoided when initiating the inrush measurement by plugging the device under test into the Inrush section of
the Test Fixture.
The sequence of operations is:
1. Move the switch in the Inrush section from the ON position to the Discharge position.
2. Unplug the device under test.
3. Move the switch back to the ON position. The oscilloscope may trigger and capture a waveform.
4. Plug the device under test into the Inrush port. The oscilloscope triggers again and captures a current
waveform as follows:
Figure 59. Inrush Current Pulse
5. If the waveform’s vertical amplitude is too small or too large (off the screen), adjust the vertical scale of
channel 4 and repeat steps 1 through 4 (previous). The inrush current is measured for a minimum of
100ms after attach. Repeat the steps until a good current trace is captured.
The inrush test measures regions where the current exceeds 100mA for at least 100µs. The region
containing the most charge is used to determine pass or fail. The inrush test integrates the current in each
region to obtain the total charge for each. The USB specification requires the total charge in each region
be less than 50µC.
6. The software then generates an HTML report on the inrush.