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-

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

CC

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.

20168423

FIGURE 5. Simplified Schematic Showing Low Voltage Auxiliary Supply

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