Rainbow Electronics MAX5971B User Manual

Page 17

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MAX5971B

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

PSE Controller with I

high, enabling high-capacitance detection. Unless high-
capacitance detection is needed, connect LEGACY to
V

to disable this function. If high-capacitance detec-

tion is enabled, PD signature capacitances up to 47FF
(typ) are accepted.

Powered Device Classification

(PD Classification)

During PD classification, the MAX5971B forces a probe
voltage (-18V, typ) at DET and measures the current into
DET. The measured current determines the class of the PD.
After each classification cycle, the MAX5971B sets
CL_END (R04h[4] and R05h[4]) to 1 and reports the
classification results in the classification status bits,
CLASS[2:0] (R0Ch[6:4], see Table 13). The CL_END reg-
isters are reset to 0 when read through the CoR (clear-
on-read) register, R05h, or after a reset event.
If ILIM1 and ILIM2 are both left unconnected, the
MAX5971B classifies the PD based on Table 33.9 of the
IEEE 802.3at standard (see Table 2). If the measured

current exceeds 51mA, the MAX5971B does not power
the PD, but returns to idle state before attempting a new
detection cycle.

Class 5 PD Classification

The MAX5971B 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 MAX5971B 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 MAX5971B per-
forms a first and second mark event as required by the
IEEE 802.3at standard, forcing a -9.3V probing voltage
at DET.

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