SLVAEJ0 April 2020 TPS25830-Q1 , TPS25830A-Q1 , TPS25831-Q1 , TPS25832-Q1 , TPS25833-Q1 , TPS25840-Q1 , TPS25842-Q1 , TPS25846-Q1
The TPS2583x-Q1 and TPS2584x-Q1 is the USB Type-C or Type-A charging controller, which is widely used in the automotive USB charging application. In the application report, the customer sometimes meets the problem that data is disconnected or the charging current is small. Ground shift issues may be one a cause for this phenomenon.
This application report has an introduction of ground shift phenomenon. And introduce how ground shift disrupts data communication and also reduce the charging current. Some suggestions are given to avoid or reduce ground shift phenomenon.
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Ground shift problems involve the current through a trace or wire causing voltage shift due to resistance of trace wire or USB cable, which appears on the ground according to Ohm’s Law. The ground voltage shift not only increases the VBUS voltage drop that leads to the lower current charging and boosts the data signal voltages at the downstream port terminal, which causes the data communication disruption.
As indicated in Figure 1, due to the existence of parasitic resistor on ground line, TPS2583x/4x-Q1 side data line(DP,DM) voltage will be higher than Phone side DP DM voltage. According to USB2.0 specification, the maximum voltage drop for all cables between upstream and downstream on GND is 125 mV [1].
Table 7-7 in the USB2.0 spec indicates that high speed disconnect detection threshold is from 525 mV to 625 mV. Table 7-1 also provides the disconnect envelop detector. Disconnection must be indicated when the amplitude of the differential signal at the downstream facing driver’s connector ≥ 625 mV, and it must not be indicated when the signal amplitude is ≤ 525 mV. So for ground shift voltage, it’s better to be controlled below 125 mV.
Based on USB battery charging 1.2 compliance plan [2], test item 7.14, the common mode test for high speed data communication will apply a maximum voltage of 375 mV offset to UUT ground. Then, check whether the enumeration is succeeded or not.
Test Setup | Test Setup 4. UUT is a PD. PET simulates a SDP |
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Pre-conditions | This test should be performed with a good battery in order to minimize extra current flowing in the cable ground, as the PET provides the ground offset itself |
Purpose | This test verifies that a UUT is able to communicate with a charging downstream port at high-speed when cable ground is dropping Imax_bc x 0.25 Ω + 5 mV |
Description | This test will apply a maximum of 375 mV offset to UUT ground, and then verify UUT enumerates and works successfully. PET simulates an SDP to reduce the risk that the UUT itself draws a significant current. |
Parameters | |
Checklist | PD33 |
Pass Criteria | Step 5: UUT enumerates successfully
Step 6: UUT functions as expected |
The USB battery charging 1.2 compliance plan also includes test item 9.6, CDP ground offset test for the high speed signal, which also validates the ground shift influence for charging and data communication.
Test Setup | Test Setup 5. UUT is an CDP, PET simulates a PD. |
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Pre-conditions | When running this test, ensure that an isolated computer is used for the tester host (for example, laptop). |
Purpose | To verify that charging downstream port fulfills USB2.0 specification common mode voltage requirements at high speed. |
Description | Insert specified command voltage offset and check UUT can enumerate reference device and operate well. |
Parameters | |
Checklist | CDP11 |
Pass Criteria | Step 9: Enumeration is successful. |
In order to validate the ground shift influence for USB data communication, the bench shown in Figure 2 was built.
Figure 3 shows the data communication and charging waveforms without additional resistor added.
In CDP mode, without cable compensation, pixel2 phone, and parasitic resistor not added, only USB cable and connector wire.
With secondary detection finished (CDP mode), enumeration starts and also the phone draws current. With a process of recognition from full speed to the high speed signal, data communication starts successful. The ground voltage shift is about 129 mV. Voltage at the terminal of IC is about 5 V . Current draw by the phone is about 3A.
Per the USB2.0 spec, the maximum voltage drop for all cables between upstream and downstream on GND is 125 mV. Although the data communication is normal, after you plug and unplug the phone several times, the phone data communication fails., see Figure 3.
Figure 4 shows related waveforms with parasitic resistor added.
In CDP mode, without cable compensation, pixel2 phone, and 20 mΩ parasitic resistor added on ground line.
The ground voltage shift is about 347 mV. The data signal voltage is about 400 mV + 347 mV = 747 mV, which is far out of the range of 625 mV. Current drawing is about 2.6A due to VBUS voltage drop. Data communication also fails. As shown in Figure 4, too high parasitic resistor on the ground line leads to too high ground voltage shift, which will boost the data signal voltage out of threshold causing data communication failure and VBUS drops with low charging current.