SNLA434 November   2023 DP83TC812R-Q1 , DP83TD510E , DP83TG720R-Q1 , LMK1C1103 , LMK1C1104 , LMK5B12204 , LMK6C

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Power Supply
    1. 2.1 Internal Supply Rails
    2. 2.2 External Supply Lines
    3. 2.3 Requirements for PoDL
    4. 2.4 Clocking
      1. 2.4.1 Topology 1
      2. 2.4.2 Topology 2
  6. 3Summary
  7. 4References

2.3 Requirements for PoDL

Some systems not only have one power source, but can be powered either from a local power supply or from a combination of data and power line, such power over data lines. Here are some additional rules that can apply to prevent interference from the power supply into the data signal.

The IEEE standards 802.3bu and 802.3cg specify the requirements in terms of ripple voltage at certain frequencies for the different types for PD and PSE. Looking at a type E powered device, there is specified, in IEEE 802.3bu table 104-7 item 3a, a allowed ripple voltage of 0.1 Vpp in the range of 1 kHz to 10 MHz. Item 3b specifies for the same range 0.01 Vpp.

Item 3a and 3b specify different methods for measuring the voltage at the PD. Both have in common to power the device through a DC bias network and measure the ripple voltage through a differential probe. This differential probe is specified to have a specific impedance and transfer function. The two plots H1(f) and Z(f) Figure 2-7 and Figure 2-8 show the impedance of this differential probe over frequency, as well as the transfer function in the given frequency range for 803.3cg, type E.

GUID-20231108-SS0I-MHMX-VCVC-WVZTTDKCN64J-low.svgFigure 2-7 H1(f) as Given by IEEE802.3cg
GUID-20231108-SS0I-MX7K-KLWP-T2PLWFCRWQQ8-low.svgFigure 2-8 Z(f) as given by IEEE802.3cg

The voltage measured with this probe must not exceed 0.1 Vpp in 1 kHz to 10 MHz range, otherwise the voltage can impact the data transmission.

Moreover, also a post processing transfer function H2(f) is given. The measurement must be scaled with this and must not exceed 0.01 Vpp in the given frequency range.

GUID-20231108-SS0I-QDDQ-KN1G-GLWKF06HNBJF-low.svg Figure 2-9 H2(f)

This results in a maximum ripple voltage as shown in Figure 2-10.

GUID-20231108-SS0I-27WP-M6R8-ZDJZXH1LZ3RV-low.svg Figure 2-10 Maximum Ripple Voltage

The system must not exceed these voltage limits and the filtering has to be done properly.