Parasitic power on QTL1789 Injection Fixture

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#1 28/01/2016 - 12:04
anorrie

Parasitic power on QTL1789 Injection Fixture

A customer noted that, with no DUT attached, they saw a very small power draw on the Quarch XLC power module, when connected to the QTL1789 PCIe Power Injection Fixture

The PCIe Injection fixture has switches on-board that synchronize the PPM power with Host power, so that 12V is only applied when host 12v is present, and 3.3v is applied when host 3.3v is present. 

The switch circuitry has two parallel 10K pull up resistors to the PPM 3.3v supply (there is also a single 10K resistor on 12V), when host power is applied these resistors dissipate P = V2/R = (3.32)/5K = 2.178mW from the PPM 3.3V output. This equates exactly to the measurement that the customer reported.

The Injection fixture was designed for our original power module, where due to its resolution this level of dissipation was not such an issue. The sync circuit is needed to ensure that the DUT enumerates correctly on the host system, as PCIe slot devices will often fail to come up if the power is not switched on at exactly the right time.

Quarch are currently developing a new Power Module, which will have the ability to power active interposer. This will fix this issue, whilst preserving the synchronisation ability.

The easiest solution to this issue for now, is to remove the 2.178mW from the returned power results, (or if the voltage is being varied, use the equation above to remove the correct amount).