LM5071MT-50 NSC [National Semiconductor], LM5071MT-50 Datasheet - Page 12

• Manufacturer Part Number: LM5071MT-50
• Description: Power Over Ethernet PD Controller with Auxiliary Power Power Over Ethernet PD Controller with Auxiliary Power
• Manufacturer: NSC [National Semiconductor]

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Power Supply Operation
Once the UVLO threshold has been satisfied, the interface
controller of the LM5071 will charge up the SMPS input
capacitor through the internal power MOSFET. This load
capacitance provides input filtering for the power converter
section and must be at least 5uF per the IEEE 802.3af
specification. To accomplish the charging in a controlled
manner, the power MOSFET is current limited to 100mA.
The SMPS controller will not initiate operation until the load
capacitor is completely charged. The power sequencing be-
tween the interface circuitry and the SMPS controller occurs
automatically within the LM5071. Detection circuitry monitors
the RTN pin to detect interface startup completion. When the
RTN pin potential drops below 1.5V with respect to V
V
start function is enabled once the V
minimum operating voltage. The inrush current limit only
applies to the initial charging phase. The interface power
MOSFET current limit will revert to the default protection
current limit of 390mA once the SMPS is powered up and the
soft-start pin sequence begins.
High Voltage Start-up Regulator
The LM5071 contains an internal high voltage startup regu-
lator that allows the input pin (V
line voltages as high as 60V. The regulator output is inter-
nally current limited to 15mA. The recommended capaci-
CC
regulator of the SMPS controller is enabled. The soft-
FIGURE 5. Simplified Schematic Showing Low Voltage Auxiliary Supply
IN
) to be connected directly to
CC
regulator achieves
EE
, the
12
tance range for the V
When the voltage on the V
point of 7.8V, the controller output is enabled. The controller
will remain enabled until V
In typical applications, a transformer auxiliary winding is
diode connected to the V
the V
regulator. Though not required, powering V
iliary winding improves conversion efficiency while reducing
the power dissipated in the controller. The external V
capacitor must be selected such that the capacitor maintains
the V
(6.25V) during the initial start-up. During a fault condition
when the converter auxiliary winding is inactive, external
current draw on the V
power dissipated in the start-up regulator does not exceed
the maximum power dissipation capability of the LM5071
package.
If the V
iliary supply (wall transformer), the VCC pin could back feed
through the LM5071 to the VIN pin. A diode from VCC to VIN
should be used to clamp the VCC pin and prevent this
internal back feed. The winding voltage will remain the same
and extra power will be dissipated in the series resistor. Also,
note that when using a very low voltage auxiliary supply, a
diode from the AUX supply to the VCC pin should be used to
ensure VCC startup.
CC
CC
CC
voltage greater than the V
voltage above 8.1V to shut off the internal startup
auxiliary winding is used with a low voltage aux-
CC
CC
line should be limited such that the
regulator output is 0.1uF to 10uF.
CC
CC
CC
pin. This winding should raise
falls below 6.25V.
pin reaches the regulation
CC
UVLO falling threshold
CC
from an aux-
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CC