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The PSE determines the class of a PD by applying a volt-
age at the PD input and measures the current sourced
out of the PSE. When the PSE applies a voltage between
12.6V and 20V, the MAX5940A/MAX5940B exhibit a cur-
rent characteristic with values indicated in Table 2. The
PSE uses the classification current information to classify
the power requirement of the PD. The classification cur-
rent includes the current drawn by the 25.5k
detection
signature resistor and the supply current of the
MAX5940A/MAX5940B so the total current drawn by the
PD is within the IEEE 802.3af standard figures. The classi-
fication current is turned off whenever the device is in
power mode.
Power Mode
During power mode, when V
IN
rises above the undervolt-
age lockout threshold (V
UVLO,ON
), the MAX5940A/
MAX5940B gradually turn on the internal N-channel MOS-
FET Q1 (see Figure 2). The MAX5940A/MAX5940B
charge the gate of Q1 with a constant current source
(10μA, typ). The drain-to-gate capacitance of Q1 limits
the voltage rise rate at the drain of the MOSFET, thereby
limiting the inrush current. To reduce the inrush current,
add external drain-to-gate capacitance (see the
Inrush
Current
section). When the drain of Q1 is within 1.2V of
its source voltage and its gate-to-source voltage is
above 5V, the MAX5940A/MAX5940B asserts the
PGOOD/
PGOOD
outputs. The MAX5940A/MAX5940B
have a wide UVLO hysteresis and turn-off deglitch time
to compensate for the high impedance of the twisted-
pair cable.
Undervoltage Lockout
The MAX5940A/MAX5940B operate up to a 67V supply
voltage with a default UVLO turn-on (V
UVLO,ON
) set at
35V (MAX5940A) or 39V (MAX5940B) and a UVLO turn-
off (V
UVLO,OFF
) set at 30V. The MAX5940B has an
adjustable UVLO threshold using a resistor-divider con-
nected to UVLO (see Figure 3). When the input voltage
is above the UVLO threshold, the IC is in power mode
and the MOSFET is on. When the input voltage goes
below the UVLO threshold for more than t
OFF_DLY
, the
MOSFET turns off.
M
IEEE 802.3af PD Interface Controller
For Power-Over-Ethernet
_______________________________________________________________________________________
7
CLASS
0
1
2
3
4
USAGE
Default
Optional
Optional
Optional
Not Allowed
R
CL
(
)
10k
732
392
255
178
MAXIMUM POWER USED BY PD (W)
0.44 to 12.95
0.44 to 3.84
3.84 to 6.49
6.49 to 12.95
Reserved*
*
Class 4 reserved for future use.
Table 1. PD Power Classification/R
CL
Selection
CLASS CURRENT SEEN AT V
IN
(mA)
IEEE 802.3af PD CLASSIFICATION
CURRENT SPECIFICATION (mA)
CLASS
R
CL
(
)
V
IN
* (V)
MIN
0
9.17
17.29
26.45
36.6
MAX
2
11.83
19.71
29.55
41.4
MIIN
0
9
17
26
36
MAX
4
12
20
30
44
0
1
2
3
4
10k
732
392
255
178
12.6 to 20
12.6 to 20
12.6 to 20
12.6 to 20
12.6 to 20
*
VINis measured across the MAX5940 input pins, which does not include the diode bridge voltage drop.
Table 2. Setting Classification Current
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| MAX5940BESA | IEEE 802.3af PD Interface Controller For Power-Over-Ethernet |
| MAX5941B | IEEE 802.3af-Compliant Power-Over-Ethernet Interface/PWM Controller for Power Devices |
| MAX5941A | IEEE 802.3af-Compliant Power-Over-Ethernet Interface/PWM Controller for Power Devices |
| MAX5941 | IEEE 802.3af-Compliant Power-Over-Ethernet Interface/PWM Controller for Power Devices |
| MAX5941ACSE | ECONOLINE: RBM - New Micro Size SIP 6 Package- Industry Standard Pinout- 3kVDC Isolation- UL94V-0 Package Material- Efficiency to 85% |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| MAX5940AESA+ | 功能描述:輸入/輸出控制器接口集成電路 IEEE 802.3af PD Int Controller RoHS:否 制造商:Silicon Labs 產(chǎn)品: 輸入/輸出端數(shù)量: 工作電源電壓: 最大工作溫度:+ 85 C 最小工作溫度:- 40 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-64 封裝:Tray |
| MAX5940AESA+T | 功能描述:輸入/輸出控制器接口集成電路 IEEE 802.3af PD Int Controller RoHS:否 制造商:Silicon Labs 產(chǎn)品: 輸入/輸出端數(shù)量: 工作電源電壓: 最大工作溫度:+ 85 C 最小工作溫度:- 40 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-64 封裝:Tray |
| MAX5940AESA-T | 功能描述:輸入/輸出控制器接口集成電路 RoHS:否 制造商:Silicon Labs 產(chǎn)品: 輸入/輸出端數(shù)量: 工作電源電壓: 最大工作溫度:+ 85 C 最小工作溫度:- 40 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-64 封裝:Tray |
| MAX5940BCSA+ | 制造商:Maxim Integrated Products 功能描述:- Rail/Tube |
| MAX5940BCSA+T | 制造商:Maxim Integrated Products 功能描述:IEEE 802.3AF PD INTERFACE CONTROLLER FOR POWER-OVER-ETHERNET - Tape and Reel |
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