LM5071 datasheet(11/18 Pages) NSC

11 / 18 page

Classification (Continued)

TABLE 2. Classification Levels and Required External

Resistors

Class

PMIN

PMAX

ICLASS

(MIN)

ICLASS

(MAX)

RCLASS

0.44W

12.95W

0mA

4mA

Open

0.44W

3.84W

9mA

12mA

150

Ω

3.84W

6.49W

17mA

20mA

82.5

Ω

6.49W

12.95W

26mA

30mA

54.9

Ω

Reserved

36mA

44mA

38.3

Ω

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 should be

connected between the V

IN to UVLO pins; the bottom resis-

tor in the divider should be connected between the UVLO

and UVLORTN pins.

The divider must be designed such that the voltage at the

UVLO pin equals 2.0V when V

IN reaches the desired mini-

mum 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 4 for an example of a

simple yet robust auxiliary configuration.

20168422

FIGURE 4. Simplified Schematic Showing Auxiliary Implementation

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