Rainbow Electronics MAX5971А User Manual

Page 15

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MAX5971A

Single-Port, 40W, IEEE 802.3af/802.3at PSE

Controller with Integrated MOSFET

Powered Device Classification

(PD Classification)

During the PD classification mode, the MAX5971A forces
a probe voltage (-18V typ) at DET and measures the
current into DET. The measured current determines the
class of the PD. If the ILIM1 and ILIM2 pins are both left
unconnected, the MAX5971A classifies the PD based on
Table 33.9 of the IEEE 802.3at standard (see Table 2).
If the measured current exceeds 51mA, the MAX5971A
does not power the PD, but returns to the idle state
before attempting a new detection cycle.

Class 5 PD Classification

The MAX5971A supports high power beyond the IEEE
802.3at standard by providing an additional classifica-
tion (Class 5) if needed. To enable Class 5 detection
and select the corresponding current-limit/overcurrent

thresholds, ILIM1 and ILIM2 must be set based on
the combinations detailed in Table 3. Once Class 5 is
enabled, during classification, if the MAX5971A detects
currents in excess of the Class 4 upper limit threshold,
the PD is classified as a Class 5 powered device. The PD
is guaranteed to be classified as a Class 5 device for any
classification current from 51mA up to the classification
current-limit threshold.
The Class 5 overcurrent threshold and current limit is
set with ILIM1 and ILIM2. ILIM1 and ILIM2 are both
referenced to V

and are internally pulled up to the

digital supply. Leave ILIM1 and ILIM2 unconnected to
disable Class 5 detection and to be fully compliant to
IEEE 802.3at standard classification. Class 5 detection
is enabled, and the corresponding overcurrent threshold
and current limit is adjusted, by connecting one or both
to V

(see Table 3).

2-Event PD Classification

If the result of the first classification event is Class 0
through Class 3, then only a single classification event
occurs as shown in Figure 1. However, if the result is
Class 4 or Class 5 (when enabled), the device performs
a second classification event as shown in Figure 2.
Between the classification cycles, the MAX5971A per-
forms a first and second mark event as required by the
IEEE 802.3at standard, forcing a -9.0V probing voltage
at DET.

Powered State

When the MAX5971A enters a powered state, the t

FAULT

and t

DISC

timers are reset. When the startup timer has

timed out, the device enters a normal powered condition,
allowing power delivery to the PD .

Table 2. PSE Classification of a PD (Table
33.9 of the IEEE 802.3at Standard)

Table 3. Class 5 Overcurrent Threshold and Current-Limit Settings

MEASURED I

CLASS

(mA)

CLASSIFICATION

0 to 5

Class 0

> 5 and < 8

Can be Class 0 or 1

8 to 13

Class 1

> 13 and < 16

Either Class 1 or 2

16 to 21

Class 2

> 21 and < 25

Either Class 2 or 3

25 to 31

Class 3

> 31 and <35

Either Class 3 or 4

35 to 45

Class 4

> 45 and < 51

Either Class 4 or Invalid

ILIM1

CONFIGURATION

ILIM2

CONFIGURATION

OVERCURRENT

THRESHOLD (mA)

CURRENT

LIMIT (mA)

Unconnected

Class 5 disabled

Unconnected

748

850

Unconnected

792

900

836

950