PM8803 STMicroelectronics, PM8803 Datasheet - Page 22

• Manufacturer Part Number: PM8803
• Description: High efficiency integrated IEEE 802.3at PoE-PD interface and PWM controller type 2 PSE indicator, plus support for forward active clamp topology
• Manufacturer: STMicroelectronics

PD interface
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This feature is active only when working from an input voltage with a "frontal" connection,
that must use the hot-swap MOSFET; this voltage could be from the PoE interface or from
an external auxiliary adapter connected before the internal hot-swap MOSFET.
If the auxiliary source is connected after the hot-swap MOSFET, it will be opened and this
feature is disabled, allowing the converter to work with a low-voltage auxiliary source. The
PGOOD comparator includes hysteresis to allow the PM8803 to operate near the current
limit point without inadvertently disabling it. The MOSFET voltage must increase to 12 V
before PGOOD is deasserted.
This feature will also allow withstanding positive line transients of up to 12 V without
stopping DC/DC normal operations as shown in
the PWM section, adjusting its operating parameters accordingly without shutting down the
output voltage. The input current during the transient is controlled by the hot-swap MOSFET
at the DC current limit.
After PGOOD assertion, a comparator on the gate of the hot-swap MOSFET controls the
transition between the 140 mA to the programmed DC current limit, with a 2 V threshold.
The comparator is needed to ensure that the charge of the DC/DC input capacitor is
completed, avoiding current spikes on the last portion of the charge.
The PM8803 provides a default continuous current limitation of 640 mA. This is achieved by
leaving the pin DCCL floating. A different DC current limit can be set by connecting a
resistor R
equation:
Figure 10. DC current vs. R
R
DC
[
kΩ
I
DC
]
[mA]
DC
=
---------------------- -
I
DC
between DCCL and VSS whose value can be obtained by the following
22400
1120
1020
920
820
720
620
520
420
320
220
120
20
[
mA
20
]
40
Doc ID 018559 Rev 1
DC
60
R
80
DC
[kOhm]
Figure
100
9. The line transient is managed by
120
140
160
AM045100v1
PM8803