PM8805 datasheet(39/48 Pages) STMICROELECTRONICS

No. de pieza	PM8805

Descripción Electrónicos	IEEE 802.3bt PoE-PD interface with embedded dual active bridge

PDF	48 Pages

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Fabricante Electrónico	STMICROELECTRONICS [STMicroelectronics]

Página de inicio	http://www.st.com

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39 / 48 page

DS12813 Rev 2

39/48

PM8805

Thermal aspects

load of about 150 mW, considering that the size of those exposed pads is smaller than the

central one.

However, the hot swap pad is internally electrically and physically connected to the central

pad, so the most critical pads are the 4 pads at device corner. Figure 12 shows two possible

2-pair configurations depending on the polarity of the PSE.

Figure 12. 4-pair power dissipation configurations

The max. total power dissipated on each of the 4-pair configurations is:

Equation 5

In this case the central exposed pad and the hot swap MOSFET dissipate much more power

than the 2-pair cases: 1.01 W, while two corner pads dissipate 0.15 W each as in the 2P

cases.

In the assumption to require Tj=125°C max. at Ta=85°C, the package soldered on a PCB is

needed to show a global Rja of:

Equation 6

This value is comparable with the Rja of a standard VFQFPN 8x8mm with a single exposed

pad.

See Section 6 for a detailed description on the minimum layout that must be implemented

on the PCB in order to reach such value of global thermal resistance.

Ptot_4P = 4 x ((1A)

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

= 0.6W + 0.6W + 0.11W = 1.31W

Rja = 40

C/1.3W = 30.5C/W

PM8805 Datasheet(PDF) 39 Page - STMicroelectronics