PM8805 datasheet(36/45 Pages) STMICROELECTRONICS

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Thermal aspects

PM8805

36/45

DS12813 Rev 1

Thermal aspects

5.1

Power dissipation

In this paragraph some worst case calculations are performed in order to show the thermal

behavior of the PM8805 in 2-pair and 4-pair PoE operation modes.

Assumptions:

– Worst case current on the 4 MOSFETs at the 4 corner of package: 1A

– MOSFET Ron at 25C 90 mΩ, 1.6x at 125C = 150mΩ

– Worst case current on the hot swap MOSFET: 1A per each 2-pairs

– PoE input voltage= 54 V

– Controller quiescent current consumption= 2 mA

Figure 11 shows four possible 2-pair configurations depending on the polarity and the

Alternative by the PSE.

The max. total power dissipated on each of the 2-pair cases is:

Equation 4

Figure 11. 2-pair power dissipation configurations

The wide central pad has a total thermal load of 0.11+0.15=0.26 W, but it is connected to a

wide copper plane of the PCB (VOUT signal) and not considered critical.

The hot swap is estimated to dissipate about 0.15 W, and can be considered the most

critical point of the device together with the one powered corner pad, also having a thermal

Ptot_2P = 2x ((1A)

2 x 0.15Ohm) + ((1A)2 x 0.15Ohm) + (54V x 2mA)

= 0.3W + 0.15W + 0.11W = 0.56W

PM8805 Datasheet(PDF) 36 Page - STMicroelectronics