LM5070

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SNVS308G – OCTOBER 2004 – REVISED APRIL 2013

Classification

To classify the PD, the PSE will present a voltage between 14.5V and 20.5V to the PD. The LM5070 enables

classification mode at a nominal input voltage of 11.5V. An internal 1.5V linear regulator and an external resistor

connected to the RCLASS pin provide classification programming current. Table 2 shows the external

classification resistor required for a particular class.

The classification current flows through the IC into the classification resistor. The suggested resistor values take

into account the bias current flowing into the IC. A different desired RCLASS can be calculated by dividing 1.5V

by the desired classification current.

Per the IEEE 802.3af specification, classification is optional, and the PSE will default to class 0 if a valid

classification current is not detected. If PD classification is not desired (i.e., Class 0), simply leave the RCLASS

pin open. The classification time period may not last longer then 75ms as per IEEE 802.3af. The LM5070 will

Table 2. Classification Levels and Required External Resistors

Class

PMIN

PMAX

ICLASS

ICLASS

RCLASS

(MIN)

(MAX)

0

0.44W

12.95W

0mA

4mA

Open

1

0.44W

3.84W

9mA

12mA

150

Ω

2

3.84W

6.49W

17mA

20mA

82.5

Ω

3

6.49W

12.95W

26mA

30mA

53.6

Ω

4

Reserved

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 LM5070 contains a programmable line Under Voltage Lock Out (UVLO) circuit. The first resistor

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 LM5070 UVLO thresholds cannot be

programmed lower than 23V, otherwise the device would operate in classification mode with both the

classification current source and the SMPS enabled. The combined power dissipation of these two functions

could exceed the maximum power dissipation of the package.

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.

Power Supply Operation / Current Limit Programming

Once the UVLO threshold has been satisfied, the interface controller of the LM5070 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 375mA. The IEEE 802.3af specification requires that

the load capacitance be charged within 75ms.

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