Detailed description, Table 1. setting classification current – Rainbow Electronics MAX5969B User Manual
Page 10
IEEE 802.3af/at-Compliant, Powered Device Interface
Controllers with
Integrated Power MOSFET
MAX5969A/MAX5969B
10 _____________________________________________________________________________________
Detailed Description
Operating Modes
Depending on the input voltage (V
IN
= V
DD
- V
SS
), the
MAX5969A/MAX5969B operate in four different modes:
PD detection, PD classification, mark event, and PD
power. The devices enter PD detection mode when the
input voltage is between 1.4V and 10.1V. The device
enters PD classification mode when the input voltage is
between 12.6V and 20V. The device enters PD power
mode once the input voltage exceeds V
ON
.
Detection Mode (1.4V
≤ V
IN
≤ 10.1V)
In detection mode, the PSE applies two voltages on V
IN
in the range of 1.4V to 10.1V (1V step minimum) and
then records the current measurements at the two points.
The PSE then computes DV/DI to ensure the presence
of the 24.9kω signature resistor. Connect the signature
resistor (R
DET
) from V
DD
to DET for proper signature
detection. The MAX5969A/MAX5969B pull DET low in
detection mode. DET goes high impedance when the
input voltage exceeds 12.5V. In detection mode, most of
the MAX5969A/MAX5969B internal circuitry is off and the
offset current is less than 10µA.
If the voltage applied to the PD is reversed, install pro-
tection diodes at the input terminal to prevent internal
damage to the MAX5969A/MAX5969B (see the Typical
Application Circuit). Since the PSE uses a slope tech-
nique (DV/DI) to calculate the signature resistance, the
DC offset due to the protection diodes is subtracted and
does not affect the detection process.
Classification Mode (12.6V
≤ V
IN
≤ 20V)
In the classification mode, the PSE classifies the PD
based on the power consumption required by the PD. This
allows the PSE to efficiently manage power distribution.
Class 0 to 5 is defined as shown in Table 1. (The IEEE
802.3af/at standard defines only Class 0 to 4 and Class 5
for any special requirement.) An external resistor (R
CLS
)
connected from CLS to V
SS
sets the classification current.
The PSE determines the class of a PD by applying a volt-
age at the PD input and measuring the current sourced
out of the PSE. When the PSE applies a voltage between
12.6V and 20V, the MAX5969A/MAX5969B exhibit a cur-
rent characteristic with a value shown in Table 1. The
PSE uses the classification current information to classify
the power requirement of the PD. The classification cur-
rent includes the current drawn by R
CLS
and the supply
current of the MAX5969A/MAX5969B so the total current
drawn by the PD is within the IEEE 802.3af/at standard
figures. The classification current is turned off whenever
the device is in power mode.
2-Event Classification and Detection
During 2-event classification, a Type 2 PSE probes PD
for classification twice. In the first classification event,
the PSE presents an input voltage between 12.6V and
20.5V and the MAX5969A/MAX5969B present the pro-
grammed load I
CLASS
. The PSE then drops the probing
voltage below the mark event threshold of 10.1V and
the MAX5969A/MAX5969B present the mark current
(I
MARK
). This sequence is repeated one more time.
Table 1. Setting Classification Current
*V
IN
is measured across the MAX5969A/MAX5969B input V
DD
to V
SS
.
CLASS
MAXIMUM
POWER USED
BY PD
(W)
R
CLS
(I)
V
IN
*
(V)
CLASS CURRENT SEEN AT
V
IN
(mA)
IEEE 802.3at PSE
CLASSIFICATION CURRENT
SPECIFICATION (mA)
MIN
MAX
MIN
MAX
0
0.44 to 12.95
615
12.6 to 20
0
4
0
5
1
0.44 to 3.94
117
12.6 to 20
9
12
8
13
2
3.84 to 6.49
66.5
12.6 to 20
17
20
16
21
3
6.49 to 12.95
43.7
12.6 to 20
26
30
25
31
4
12.95 to 25.5
30.9
12.6 to 20
36
44
35
45
5
> 25.5
21.3
12.6 to 20
54
64
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