Application Notes

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Using the Output Inhibit Control with the

PTB48540 Power-over-Ethernet Modules

The PTB48540 Power-over-Ethernet (PoE) modules

incorporate all the necessary interface requirements to

provide 10 W of regulated DC voltage from a recognized

Power-over-Ethernet source.

One of the operating features of these modules allows

the output voltage to be turned off, thereby placing the

module in an idle state. This may be useful for applica-

tions that have an alternative voltage source available,

such as a wall adapter.

The “Output Inihibit” control is provided by pin 13. The

module functions normally with this pin open-circuit,

providing a regulated output voltage whenever a valid

source voltage is supplied from the Ethernet connection.

When a low voltage is applied to pin 13, with respect to

the “Input Ref” terminal (pin 4), the output is turned off.

Even though the Ethernet source is still present.

Figure 3-1 is an application schematic, which shows how

the “Output Inhibit” control may be used. Either a discrete

“Output Inhibit” control pin has its own internal pull-up

(See notes 2 & 3). Table 3-1 gives the threshold require-

ments.

When placed in the “Off” state, the standby current

Parameter

Min

Typ

Max

Enable (VIH)

4.5 V

—

—

Disable (VIL)

—

—

0.8 V

Vo/c [Open-Circuit]

5 V

Iin [pins 13 & 4 connnected]

—

–1 mA

—

Notes:

1. The Output Inhibit control uses Input Ref (pin 4) as its

0-V reference. All voltages specified are with respect to

the Input Ref pin.

2. Use an open-collector device (preferably a discrete

transistor or switch) for the Output Inhibit input. A

pull-up resistor is not necessary. To turn the output

off, the control pin should be pulled low to less than

0.8 VDC.

3. The module’s idle current is typically less than 1 mA.

This is below the minimum power signature (MPS)

current, required for the PoE source to continue providing

the PD with a source voltage. This will inevitably result

in the removal of the module’s input source. However,

the PSE will endevour to periodically re-apply input

power after detecting that the PD is still a valid device.

This cycle may be prevented by also modifying the

module’s ‘Detect’ signature resistance so that it reflects

an invalid device to the PSE.

Figure 3-1

Turn-On Time:

applies a low-voltage to pin 13 and disables the module

be pulled high by an internal pull-up resistor. The module

produces a regulated output voltage within 60 ms. Figure

3-2 shows shows the output response of a PTB48540A

of the output voltage is short (<5 ms), the indicated delay

time will vary depending upon the input voltage and the

module’s internal timing. The waveform was measured

with a 48 Vdc input voltage, and a 1.4 A resistive load.

Figure 3-2

Vo (2V/Div)

Iin (0.2A/Div)

Q1Vgs (10V/Div)

HORIZ SCALE: 10ms/Div

Delay Time

+48 V

0 V

1 =Output Off

Q

1

BSS138

+5 V

0 V

Alternate

On/Off

Contact/Switch

PTB48540A

10

Data Line B

9

Spare Line A

4

Spare Line B

8

Input Ref

7

A- Detect -B

A- Class -B

Output Inhibit

Data Line A

+V

OUT

Adjust

–V

OUT

3

2

1

5

6

12

11

13

PTB48540 Series

Not Recommended for New Designs