SLVAEJ0 April   2020 TPS25830-Q1 , TPS25830A-Q1 , TPS25831-Q1 , TPS25832-Q1 , TPS25833-Q1 , TPS25840-Q1 , TPS25842-Q1 , TPS25846-Q1

 

  1.   Ground Shift Phenomenon Recommendation of TPS2583x
    1.     Trademarks
    2. 1 Introduction
    3. 2 Data Communication Issue Caused by Ground Shift
    4. 3 Charging Current Issue Caused by Ground Shift
    5. 4 Suggestions for Ground Shift Issue
    6. 5 References

1 Introduction

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.

fig1.pngFigure 1. Simplified Block Diagram for Ground Shift of TPS2583/4x-Q1

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.

Table 1. Common Mode Test – High Speed [2]

Test Setup Test Setup 4. UUT is a PD. PET simulates a SDP
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.

Table 2. CDP Ground Offset Test – High Speed [2]

Test Setup Test Setup 5. UUT is an CDP, PET simulates a PD.
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.