PM8800 STMicroelectronics, PM8800 Datasheet - Page 30

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

Auxiliary sources
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The minimum external voltage to be used in this configuration is about 16 V.
Option I connection is activated when the voltage at the relevant pin is higher than 2 V and a
current I
The AUXI_IRL pin can be connected to the auxiliary source through a diode being the
current internally limited to about 300 µA.
It is important to highlight that UVLO override and inrush current limit are mutually exclusive,
and when UVLO is overridden, default inrush current limit is provided. The PSE will not
detect the presence of a valid PD, since the PoE diode bridges are reversed biased.
When the auxiliary source is connected after the inrush phase, two possibilities arise
depending on the voltage of the external power supply. If the voltage is lower than the one
from PoE, the diode is reverse biased and PSE will keep on supplying the PD. If the auxiliary
source has a voltage greater than the PoE, the PoE diode bridge will be reverse biased and
auxiliary source will supply the PD. In case the voltages are comparable, current sharing
occurs between the sources according to their respective source resistance.
The Option II configuration bypasses the PD interface and connects directly the external
source to the PWM section. This type of connection is not limited by the internal current limit
circuits and therefore is more suitable for lower auxiliary voltages.
Internal under voltage lock out threshold will be lowered by connecting the auxiliary source
to the AUXII pin. The minimum external voltage to be used in this configuration is about
12 V.
Note that inrush current in this case is not limited and an external solution must be found:
the simplest solution is to put a low value resistor in series, but this lower the converter
efficiency. More efficient solution is the use of a MOSFET as power switch, able to limit the
current during the charging phase, and to add only few milliohm in series during normal
operation.
Main advantage of this connection is the "prevalence" over PoE voltage, that can be
obtained pulling up AUXII pin with a R
than 100 µA (see following equation). In this case the PD interface is switched off regardless
the presence of PoE voltage.
AUXII can be connected directly to the auxiliary voltage through a diode. The current in this
case is internally limited to about 250 µA.
If a prevalent AUXII is detected the PD interface is switched off, the hot swap MOSFET is
opened regardless the PoE voltage level.
I
AUXI
I
AUXII
=
AUXI
=
V
-------------------------------------- -
AUXI
V
------------------------------------------ -
higher than 70 µA is forced into the pin.
AUXII
R
AUXI
–
R
V
AUXII
–
D
V
D
–
–
2
1.1
≥
·
70µA
≥
100µA
AUXII
resistor and forcing a current into the pin greater
PM8800A