NCP1080

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10

Description of Operation

Powered Device Interface

The PD interface portion of the NCP1080 supports the

IEEE802.3af defined operating modes: detection signature,

current source classification, inrush and operating current

limits. In order to give more flexibility to the user and also

to keep control of the power dissipation in the NCP1080,

both current limits are configurable. The device enters

operation once its programmable Vuvlo_on threshold is

reached, and operation ceases when the supplied voltage

falls below the Vuvlo_off threshold. Sufficient hysteresis

and Uvlo filter time are provided to avoid false power on/off

cycles due to transient voltage drops on the cable.

Detection

During the detection phase, the incremental equivalent

resistance seen by the PSE through the cable must be in the

IEEE802.3af standard specification range (23.75 k

W to

26.25 k

W) for a PSE voltage from 2.7 V to 10.1 V. In order

to compensate for the non−linear effect of the diode bridge

and satisfy the specification at low PSE voltage, the

NCP1080 presents a suitable impedance in parallel with the

25.5 k

W Rdet external resistor. For some types of diodes

(especially Schottky diodes), it may be necessary to adjust

this external resistor.

When the Detection_Off level is detected (typically

11.5 V) on VPORTP, the NCP1080 turns on its internal

3.3 V regulator and biasing circuitry in anticipation of the

classification phase as the next step.

Classification

Once the PSE device has detected the PD device, the

classification process begins. In classification, the PD

regulates a constant current source that is set by the external

resistor RCLASS value on the CLASS pin. Figure 6 shows

the schematic overview of the classification block. The

current source is defined as:

CLASS

VDDA1

1.2 V

VPORTP

VPORTN1,2

NCP1080

Rclass

Figure 6. Classification Block Diagram

Power Mode

When the classification hand−shake is completed, the

PSE and PD devices move into the operating mode.

Under Voltage Lock Out (UVLO)

The NCP1080 incorporates an under voltage lock out

(UVLO) circuit which monitors the input voltage and

determines when to apply power to the DC−DC controller.

To use the default settings for UVLO (see Table 3), the pin

signature resistor has to be placed directly between

Figure 7. Default UVLO Settings

UVLO

VPORTP

VPORTN1,2

NCP1080

VPORT

Rdet

To define the UVLO threshold externally, the UVLO pin

must be connected to the center of an external resistor

Figure 8. The series resistance value of the external

resistors must add to 25.5 k

W and replaces the internal

signature resistor.

Figure 8. External UVLO Configuration

UVLO

VPORTN1,2

NCP1080

VPORT

R2

R1

VPORTP

For a Vuvlo_on desired turn−on voltage threshold, R1 and

R2 can be calculated using the following equations:

R2 + 1.2