AS1138 datasheet(19/30 Pages) ETC2

19 / 30 page

AS1138

Akros Silicon, Inc.

6399 San Ignacio Avenue, Suite 250, San Jose, CA 95119 USA

408.746.9000

www.AkrosSilicon.com

High, indicating connection to a Type 2 PSE. If the PD

controller

detects

only

single-event

classification,

identifies the PSE to be Type 1 and the ATDET pin is

asserted low.

In real applications, noise or transient ringing on the line

during classification phase can lead to false 2-event

classification or Type 2 PSE detection. To prevent such

false positives, the AS1138 integrates a proprietary digital

filter to filter out noise events as long as 100uS during the

classification phase, ensuring a very reliable AT Detection.

If a PSE is Type 1, to classify the AS1138, the Type 1 PSE

presents a voltage between 14.5V and 20.5V to the PD and

determines its class by measuring the resulting PD load

current.

If a PSE is Type 2, in order to classify the AS1138 as the

Type 2 PD, AS1138 has to be set as Class 4 (classification

setting shown in Table 3), so that AS1138 returns a Class 4

signature.

The AS1138 allows the user to program the classification

current via an external resistor connected to the RCLASS

pin. Current, power levels and programming resistor values

for each class are shown in Table 3. Note that for Class 0,

Rclass pin needs to be Pulled Up to VDD5 pin. This can be

a direct short to VDD5 or using a resistor up to 100k .

Use the following equation to determine the typical

classification current:

]

[

2360

]

[

Ω

CLASS

Tolerance = Maximum of ±1.8mA or ±9%

RCLASS > 63.4k

Once the classification process is done, the PD removes

the classification current to conserve power.

Note: During the classification of a Type 2 PD, the PSE

outputs a sequence of voltage signals (Class 1, Mark 1,

Class 2, Mark 2, and Class 3) within the specific timing

requirement. The PD has to respond within the limit range

of voltage/current/impedance for each event according to

section 33.3.5 PD Classifications of 802.3at standard.

Table 7 - Classification Settings

Class

Power (W)

ICLASS (mA)

RCLASS (k )

0.44-12.95

0-4 mA

Pull-up (0-100k ) to

VDD5 pin

0.44-3.84

9-12 mA

280k , 1%

3.84-6.49

17-20 mA

143k , 1%

6.49-12.95

26-30 mA

90.9k , 1%

12.96-25.5

36-44 mA

63.4k , 1%

After the classification process, the PD enters Idle mode

while it waits for On-state power delivery from the PSE. PD

Current usage is limited to monitoring circuitry to detect the

On-state voltage threshold.

In the On state, the AS1138 is supplying power across the

Ethernet line(s) to the PD. At a voltage at or below 42V, the

PD turns on and full power is available via the AS1138 DC-

DC Controller.

Local Power Mode (LVMODE)

The LVMODE pin can be used in applications where the

PD appliance is designed to draw power from either the

Ethernet cable or an external DC local power adapter. The

LVMODE pin is a current-mode input pin, with low and high

thresholds as defined in the parametric tables (see Error!

Reference source not found.

). The LVMODE is asserted

when the input current exceeds the IIH threshold, and is de-

asserted when the input current is below the IIL threshold. If

LVMODE operation is not desired, the LVMODE pin should

be connected to GND.

Figure 30 below shows a simplified internal implementation

and external application circuit required to use the

LVMODE feature. When power is applied at the local

adapter input, the AS1138 enters Local Voltage Mode. This

opens the internal Hot-Swap FET switch while the DC-DC

converter is in operation.

In this configuration, local power always takes priority, even

in presence of PoE power, irrespective of their relative

voltages. If local power is removed, the device will exit

Local Voltage Mode operation and PoE power will be used,

if available.

Figure 29 - LVMODE Implementation

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