RCLASS

AUX

OUT

RT

FB

CS

COMP

SS

RTN

ARTN

+PoE

+AUX

- AUX

UVLO

UVLORTN

LM 5071

0.1 PF

100 k:

-PoE

100 k:

LM5071

www.ti.com

SNVS409D – NOVEMBER 2005 – REVISED APRIL 2006

Undervoltage Lockout (UVLO)

The IEEE 802.3af specification states that the PSE will supply power to the PD within 400ms after completion of

detection. The LM5071 contains a programmable line Under Voltage Lock Out (UVLO) circuit. The first resistor

between the UVLO and UVLORTN pins.

minimum operating level. If the UVLO threshold is not met, the interface control and SMPS control will remain in

standby.

UVLO hysteresis is accomplished with an internal 10uA current source that is switched on and off into the

impedance of the UVLO set point divider. When the UVLO threshold is exceeded, the current source is activated

to instantly raise the voltage at the UVLO pin. When the UVLO pin voltage falls below the 2.00V threshold, the

current source is turned off, causing the voltage at the UVLO pin to fall. The LM5071 UVLO thresholds cannot be

programmed lower than 25V, the AUX pin should be used to force UVLO release below 25V.

There are many additional uses for the UVLO pin. The UVLO function can also be used to implement a remote

enable / disable function. Pulling the UVLO pin down below the UVLO threshold disables the interface and SMPS

controller unless forced on via AUX pin operation.

AUX Pin Operation

The AUX pin can be used to force operation (UVLO release) of the interface and switching regulator at any input

voltage above 9.5V. This is especially useful for auxiliary input (wall transformer) input voltages. The pin has a

2.5V threshold (0.5V hysteresis) and an input impedance of approximately 350k

Ω. The input resistor provides a

defined pull down impedance if the pin is left open by the user. An external pull down resistor should be used to

provide additional noise immunity. The resultant pin voltage from the external resistor divider should be well

above the 2.5V threshold to ensure proper auxiliary operation. See Figure 5 for an example of a simple yet

robust auxiliary configuration.

Figure 5. Simplified Schematic Showing Auxiliary Implementation

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.

Copyright © 2005–2006, Texas Instruments Incorporated

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