PM8800 STMicroelectronics, PM8800 Datasheet - Page 29

• Manufacturer Part Number: PM8800
• Description: Integrated IEEE 802.3af compliant PoE-PD interface and PWM controller with support of external source
• Manufacturer: STMicroelectronics

PM8800A
7
Auxiliary sources
The majority of powered devices is designed to work with power from either PoE network or
auxiliary sources. Even though having both sources simultaneously connected is not the
normal operating case, the presence of an auxiliary supply allows PDs to be used also when
the PoE is not available or not sufficient. This is particularly true for appliances requiring
more than 12.95 W connected to 802.3af compliant PSE.
High degree of flexibility is granted by the PM8800A for designs based on external sources.
As depicted in the following figure there are two available options for connecting auxiliary
sources to the PM8800A. One possibility consists in diode O-ring the auxiliary source into
the PM8800A PoE pins (VIN and VSS) referred to "Option I ". In the second alternative,
referred to "Option II" the external supply voltage is diode ORed into the PWM section of the
device.
Figure 19. Alternatives for auxiliary source connection
The two options differ for the in-rush and DC current protection, for the allowed auxiliary
source voltage range, power sequence and prevalence of the external source with respect
to the PoE one.
Option I configuration has been designed to take advantage of the inherent in-rush and DC
current protection also when working with voltages from external power supplies.
When the auxiliary source is connected prior to PD detection by the PSE and has a voltage
above the V
on, starting the inrush phase. Inrush and DC current limit can be programmed by respective
external resistors see
Internal under voltage lock out threshold can be lowered by connecting the auxiliary source
to the AUXI_IRL pin. Wall adapter with supply voltage lower than 38 V can be used, still
profiting from the inrush and DC current limit provided by the hot-swap MOSFET.
UVLO_R
Option I
(38-57 V), the internal UVLO is released and the MOSFET is switched
Section 5.4
AUXI_IRL
VIN
DCCL
RCLASS
VSS
PM8800
and
A
COMP
AGND
nPGD
AUXII
GND
VCC
VFB
GD
CS
SS
RT
Section
Option II
5.5.
Auxiliary sources
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