TPS2375 [TI]
IEEE 802.3af PoE POWERED DEVICE CONTROLLERS; IEEE 802.3af标准的PoE用电设备控制器
| 型号： | TPS2375 |
| 厂家： | 
TEXAS INSTRUMENTS |
| 描述： | IEEE 802.3af PoE POWERED DEVICE CONTROLLERS |
| 文件： | 总21页 (文件大小：423K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
TPS2375
TPS2376
D−8
PW−8
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
IEEE 802.3af PoE POWERED DEVICE CONTROLLERS
FEATURES
APPLICATIONS
•
•
•
•
•
VoIP Phones
•
•
•
•
•
•
•
•
•
•
•
Fully Supports IEEE 802.3af Specification
Integrated 0.58-Ω, 100-V, Low-Side Switch
15-kV System Level ESD Capable
Supports Use of Low-Cost Silicon Rectifiers
Programmable Inrush Current Control
Fixed 450-mA Current Limit
WLAN Access Points
Security Cameras
Internet Appliances
POS Terminals
TPS2376
TPS2375/77
(TOP VIEW)
(TOP VIEW)
Fixed and Adjustable UVLO Options
Open-Drain, Power-Good Reporting
Overtemperature Protection
1
2
8
7
6
5
1
2
8
7
6
5
VDD
VDD
N/C
ILIM
ILIM
CLASS
DET
CLASS
DET
UVLO
3
4
3
4
PG
PG
RTN
RTN
VSS
VSS
Industrial Temperature Range: -40°C to 85°C
8-Pin SOIC and TSSOP Packages
DESCRIPTION
These easy-to-use 8-pin integrated circuits contain all of the features needed to develop an IEEE 802.3af
compliant powered device (PD). The TPS2375 family is a second generation PDC (PD Controller) featuring
100-V ratings and a true open-drain, power-good function.
In addition to the basic functions of detection, classification and undervoltage lockout (UVLO), these controllers
include an adjustable inrush limiting feature. The TPS2375 has 802.3af compliant UVLO limits, the TPS2377 has
legacy UVLO limits, and the TPS2376 has a programmable UVLO with a dedicated input pin.
The TPS2375 family specifications incorporate a voltage offset of 1.5 V between its limits and the IEEE 802.3af
specifications to accommodate the required input diode bridges used to make the PD polarity insensitive.
Additional resources can be found on the TI Web site www.ti.com.
RJ−45
1
TX
Pair
Detect
Classify
Power Up & Inrush
Data to
Ethernet
PHY
2
Class 3
Current
Input
Current
DF01S
2 Places
R
100
kW
(DET)
24.9 kW,
V
(PG-RTN)
1 %
PG
DET
ILIM
R
(ILIM)
4
5
0.1mF,
100 V
10 %
178 kW,
1 %
V
100 mF,
100 V
DD
Spare
Pair
CLASS
V
RTN
7
8
RTN
R
(ICLASS)
357 W,
1 %
Spare
Pair
3
Note: Class 3 PD Depicted.
PG Pullup Resistor Is Optional.
Note: All Voltages With Respect to VSS.
RX
Pair
Data to
Ethernet PHY
6
Figure 1. Typical Application Circuit and Startup Waveforms
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas
Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2004, Texas Instruments Incorporated
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated
circuits be handled with appropriate precautions. Failure to observe proper handling and installation
procedures can cause damage.
ESD damage can range from subtle performance degradation to complete device failure. Precision
integrated circuits may be more susceptible to damage because very small parametric changes could
cause the device not to meet its published specifications.
AVAILABLE OPTIONS
UVLO THRESHOLDS (NOMINAL)
LOW
PACKAGE(1)
TA
MARKING
TYPE
HIGH
SO-8
TSSOP-8
802.3af
30.5 V
1.93 V
30.5 V
39.3 V
2.49 V
35.1 V
TPS2375D
TPS2376D
TPS2377D
TPS2375PW
TPS2376PW
TPS2377PW
2375
2376
2377
-40°C to 85°C
Adjustable
Legacy
(1) Add an R suffix to thedevice type for tape and reel.
ABSOLUTE MAXIMUM RATINGS
over operating free-air temperature range (unless otherwise noted) (1), voltages are referenced to V(VSS)
TPS237x
VDD, RTN, DET, PG(2)
-0.3 V to 100 V
-0.3 V to 10 V
-0.3 V to 12 V
0 to 515 mA
0 to 5 mA
0 to 1 mA
0 to 50 mA
0 to 1 mA
2 kV
Voltage
ILIM, UVLO
CLASS
RTN(3)
Current, sinking
Current, sourcing
ESD
PG
DET
CLASS
ILIM
Human body model
Charged device model
System level (contact/air) at RJ-45(4)
500 V
8/15 kV
TJ
Maximum junction temperature range
Storage temperature range
Internally limited
-65°C to 150°C
260°C
Tstg
Lead temperature 1,6 mm (1/16 inch) from case for 10 seconds - Green Packages
Lead temperature 1,6 mm (1/16 inch) from case for 10 seconds - Nongreen Packages
235°C
(1) Stresses beyond those listedunder “absolute maximum ratings” may cause permanent damage to thedevice. These are stress ratings
only, and functional operation of the deviceat these or any other conditions beyond those indicated under“recommended operating
conditions” is not implied. Exposure toabsolute–maximum–rated conditions for extended periods may affectdevice reliability.
(2) I(RTN) = 0
(3) SOA limited to V(RTN) = 80 Vand I(RTN) = 515 mA.
(4) Surges applied to RJ-45 ofFigure 1 between pins of RJ-45,and between pins and output voltage rails per EN61000-4-2,1999.
DISSIPATION RATING TABLE(1)
POWER RATING
θJA (LOW-K)
°C/W
θJA (HIGH-K)
°C/W
(HIGH-K)
TA = 85°C
mW
PACKAGE
D (SO-8)
238
150
159
266
251
PW (TSSOP-8)
258.5
(1) Tested per JEDEC JESD51.High-K is a (2 signal – 2 plane) test board and low-K is a double
sidedboard with minimum pad area and natural convection.
2
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
RECOMMENDED OPERATING CONDITIONS
MIN
0
MAX
57
UNIT
V
VDD, PG, RTN
Input voltage range
UVLO
0
5
V
Operating current range (sinking)
Classification resistor(1)
RTN
0
350
4420
500
2
mA
Ω
CLASS
255
62.5
0
(1)
R(ILIM) Inrush limit program resistor
kΩ
mA
°C
°C
Sinking current
PG
TJ
Operating junction temperature
Operating free–air temperature
-40
-40
125
85
TA
(1) Voltage should not beexternally applied to CLASS and ILIM.
ELECTRICAL CHARACTERISTICS
V(VDD) = 48 V, R(DET) = 24.9 kΩ, R(CLASS) = 255 Ω, R(ILIM) = 178 kΩ, and –40°C ≤ TJ≤ 125°C, unless otherwise noted. Positive
currents are into pins. V(UVLO) = 0 V for classification and V(UVLO) = 5 V otherwise for the TPS2376. Typical values are at 25°C.
All voltages are with respect to VSS unless otherwise noted.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
DETECTION
DET open, V(VDD) = V(RTN) = 1.9 V, measure
I(VDD) + I(RTN)
Offset current
0.3
4
3
µA
µA
DET open, V(VDD) = V(RTN) = 10.1 V, measure
I(VDD) + I(RTN)
Sleep current
12
DET leakage current
V(DET) = V(VDD) = 57 V, measure I(DET)
0.1
56
5
µA
µA
V(RTN) = V(VDD)
,
V(VDD) = 1.4 V
53.7
395
58.3
R(DET) = 24.9 kΩ,
measure I(VDD) + I(RTN)
I(DET)
Detection current
+
V(VDD) = 10.1 V
410
417
µA
CLASSIFICATION
R(CLASS) = 4420 Ω, 13 ≤ V(VDD)≤ 21 V
R(CLASS) = 953 Ω, 13 ≤ V(VDD)≤ 21 V
R(CLASS) = 549 Ω, 13 ≤ V(VDD)≤ 21 V
R(CLASS) = 357 Ω, 13 ≤ V(VDD)≤ 21 V
R(CLASS) = 255 Ω, 13 ≤ V(VDD)≤ 21 V
Regulator turns on, V(VDD) rising
Regulator turns off, V(VDD) rising
Hysteresis(2)
2.2
10.3
17.7
27.1
38.0
10.2
21
2.4
10.6
18.3
28.0
39.4
11.3
21.9
0.78
2.8
11.3
19.5
29.5
41.2
13.0
23
I(CLASS)
Classification current(1)
mA
V(CL_ON)
V(CU_OFF)
V(CU_H)
Classification lower threshold
Classification upper threshold
Leakage current
V
V
0.5
1
V
V(CLASS) = 0 V, V(VDD) = 57 V
1
µA
PASS DEVICE
rDS(on)
On resistance
I(RTN) = 300 mA
0.58
1.0
15
Ω
V(VDD) = V(RTN) = 30 V,
V(UVLO) = 0 V (TPS2376)
Leakage current
µA
Current limit
Inrush limit
V(RTN) = 1 V
405
100
461
130
515
180
mA
mA
I(LIM)
V(RTN) = 2 V, R(ILIM) = 178 kΩ
V(RTN) falling, R(ILIM) = 178 kΩ, inrush
state→normal operation
85%
91%
100%
(3)
Inrush current termination
R(ILIM) = 69.8 kΩ, V(RTN-VSS) = 5 V,
I(RTN) = 30 mA→300 mA, V(VDD) increasing
past upper UVLO
15
25
2
(2)
Current rise time into inrush
µs
Apply load ∞Ω→20 Ω, time measured to
I(RTN) = 45 mA
2.5
1
Current limit response time(2)
Leakage current, ILIM
µs
V(VDD) = 15 V, V(UVLO) = 0 V
µA
(1) Classification is tested withexact resistor values. A 1% tolerance classification resistor assurescompliance with IEEE 802.3af limits.
(2) Not tested inproduction.
(3) This parameter specifies theRTN current value, as a percentage of the steady state inrush current, belowwhich it must fall to make PG
assert (open-drain).
3
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
ELECTRICAL CHARACTERISTICS (continued)
V(VDD) = 48 V, R(DET) = 24.9 kΩ, R(CLASS) = 255 Ω, R(ILIM) = 178 kΩ, and –40°C ≤ TJ≤ 125°C, unless otherwise noted. Positive
currents are into pins. V(UVLO) = 0 V for classification and V(UVLO) = 5 V otherwise for the TPS2376. Typical values are at 25°C.
All voltages are with respect to VSS unless otherwise noted.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
PG
(4)
Latchoff voltage threshold rising
PG deglitch(5)
V(RTN) rising
9.5
75
10.0
150
10.5
225
0.4
V
Delay rising and falling PG
µs
I(PG) = 2 mA, V(RTN) = 34 V,
V(VDD) = 38 V, V(RTN) falling
0.12
V
Output low voltage
Leakage current
I(PG) = 2 mA, V(RTN) = 0 V, V(VDD) = 25 V, for
TPS2376 V(UVLO) = 0 V
0.12
0.1
0.4
1
V
V(PG) = 57 V, V(RTN) = 0 V
µA
UVLO
V(UVLO_R)
V(UVLO_F)
V(VDD) rising
V(VDD) falling
Hysteresis(5)
V(VDD) rising
V(VDD) falling
Hysteresis(5)
V(VDD) rising
V(VDD) falling
Hysteresis(5)
V(UVLO) = 0 V to 5 V
38.4
29.6
8.3
39.3
30.5
8.8
40.4
31.5
9.1
TPS2375 Voltage at VDD
TPS2376 Voltage at UVLO
V
V
2.43
1.87
0.53
34.1
29.7
4.3
2.49
1.93
0.56
35.1
30.5
4.5
2.57
1.98
0.58
36.0
31.4
4.8
TPS2377 Voltage at VDD
V
TPS2376 Input leakage, UVLO
-1
1
µA
THERMAL SHUTDOWN
Shutdown temperature(5)
Temperature rising
135
°C
°C
Hysteresis(5)
20
BIAS CURRENT
Operating current
I(VDD)
240
450
µA
(4) Start with V(RTN) = 0 V, then increase V(RTN) until I(RTN) ceases.
(5) Not tested inproduction.
4
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
DEVICE INFORMATION
FUNCTIONAL BLOCK DIAGRAM
DET
3
Detection
Comparator
12 V
+
−
VDD
8
CLASS
2
10 V
Regulator
Classification
Comparator
22 V
+
PG
6
PG Comparator
Delay
150 uS
−
1.5 V
& 10 V
−
+
R
S
R
Q
0 = Fault
0 = Inrush
S
Q
UVLO
RTN
5
Comp.
2.5 V
Thermal Shutdown,
Counter, and Latch
−
UVLO
+
1 = Limiting
1
’76 Only
See
Note
Current
Mirror
7
45 mV
+
−
EN
1:1
2.5 V
Current
Limit Amp.
+
0
ILIM
1
−
1 kOhms
0.08
Ohms
VSS
4
Note: For The TPS2376, The UVLO Comparator
Connects To The UVLO Pin And Not The UVLO
Divider.
5
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
DEVICE INFORMATION (continued)
TERMINAL FUNCTIONS
PIN NUMBER
PIN NAME
I/O
DESCRIPTION
TPS2375/77
TPS2376
Connect a resistor from ILIM to VSS to set the start-up inrush current limit.
The equation for calculating the resistor is shown in the detailed pin
description section for ILIM.
ILIM
1
1
O
Connect a resistor from CLASS to VSS to set the classification of the
powered device (PD). The IEEE classification levels and corresponding
resistor values are shown in Table 1.
CLASS
2
2
O
Connect a 24.9-kΩ detection resistor from DET to VDD for a valid PD
detection.
DET
VSS
RTN
PG
3
4
5
6
-
3
4
5
6
7
O
I
Return line on the source side of the TPS2375 from the PSE.
Switched output side return line used as the low-side reference for the
TPS2375 load.
O
O
I
Open-drain, power-good output; active high.
Used only on the TPS2376. Connect a resistor divider from VDD to VSS to
implement the adjustable UVLO feature of the TPS2376.
UVLO
NC
7
8
-
No connection
VDD
8
I
Positive line from the rectified PSE provided input.
Detailed Pin Description
The following descriptions refer to the schematic of Figure 1 and the functional block diagram.
ILIM: A resistor from this pin to VSS sets the inrush current limit per Equation 1:
25000
I
+
(LIM)
R
(ILIM)
(1)
where ILIM is the desired inrush current value, in amperes, and R(ILIM) is the value of the programming resistor
from ILIM to VSS, in ohms. The practical limits on R(ILIM) are 62.5 kΩ to 500 kΩ. A value of 178 kΩ is
recommended for compatibility with legacy PSEs.
Inrush current limiting prevents current drawn by the bulk capacitor from causing the line voltage to sag below
the lower UVLO threshold. Adjustable inrush current limiting allows the use of arbitrarily large capacitors and also
accommodates legacy systems that require low inrush currents.
The ILIM pin must not be left open or shorted to VSS.
CLASS: Classification is implemented by means of an external resistor, R(CLASS), connected between CLASS
and VSS. The controller draws current from the input line through R(CLASS) when the input voltage lies between
13 V and 21 V. The classification currents specified in the electrical characteristics table include the bias current
flowing into VDD and any RTN leakage current.
Table 1. CLASSIFICATION
CLASS
PD POWER (W)
0.44 – 12.95
0.44 – 3.84
3.84 – 6.49
6.49 – 12.95
-
R(CLASS) (Ω)
4420 ±1%
953 ±1%
549 ±1%
357 ±1%
255 ±1%
802.3af LIMITS (mA)
0 - 4
NOTE
0
1
2
3
4
Default class
9 - 12
17 - 20
26 - 30
36 - 44
Reserved for future use
6
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
The CLASS pin must not be shorted to ground.
DET: Connect a resistor, R(DET), between DET and VDD. This resistor should equal 24.9 kΩ, ±1% for most
applications. R(DET) is connected across the input line when V(VDD) lies between 1.4 V and 11.3 V, and is
disconnected when the line voltage exceeds this range to conserve power. This voltage range has been chosen
to allow detection with two silicon rectifiers between the controller and the RJ-45 connector.
VSS: This is the input supply negative rail that serves as a local ground to the TPS2375.
RTN: This pin provides the switched negative power rail used by the downstream circuits. The operational and
inrush current limit control current into the pin. The PG circuit monitors the RTN voltage and also uses it as the
return for the PG pin pulldown transistor. The internal MOSFET body diode clamps VSS to RTN when voltage is
present between VDD and RTN and the PoE input is not present.
PG: This pin goes to a high resistance state when the internal MOSFET that feeds the RTN pin is enabled, and
the device is not in inrush current limiting. In all other states except detection, the PG output is pulled to RTN by
the internal open-drain transistor. Performance is assured with at least 4 V between VDD and RTN.
PG is an open-drain output; therefore, it may require a pullup resistor or other interface.
UVLO: This pin is specific to the TPS2376; it is not internally connected on the TPS2375 and TPS2377. The
UVLO pin is used with an external resistor divider between VDD and VSS to set the upper and lower UVLO
thresholds. The hysteresis, as measured as a percentage of the upper UVLO, is the same as the TPS2375.
The TPS2376 enables the output when V(UVLO) exceeds the upper UVLO threshold. When current begins to flow,
VDD sags due to cable resistance and the dynamic resistance of the input diodes. The lower UVLO threshold
must be below the lowest voltage that the input reaches.
The TPS2376 implements adjustable UVLO thresholds, but is otherwise functionally equivalent to the TPS2375.
The TPS2375 offers fixed UVLO thresholds designed to maximize hysteresis while maintaining compatibility with
the IEEE 802.3af standard. The TPS2377 offers fixed UVLO thresholds optimized for use with legacy PoE
systems.
VDD: This is the positive input supply to the TPS2375, which is also common to downstream load circuits. This
pin provides operating power and allows the controller to monitor the line voltage to determine the mode of
operation.
7
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
TYPICAL CHARACTERISTICS
Graphs over temperature are interpolations between the marked data points.
CLASSIFICATION TURNON
PD DETECTION RESISTANCE
VOLTAGE
vs
vs
I(VDD) + I(RTN) IN DETECTION
V(PI)
TEMPERATURE
6
5
4
3
2
35
30
25
20
15
10
11.3
11.2
11.1
11.0
T
= 125°C
A
T
= 25°C
A
Specification Limits
1
0
T
A
= −40°C
0
1
2
3
4
5
6
7
8
9
10 11
−40 −20
0
20 40 60 80 100 120
1
3
5
7
9
11
V
− V
T
A
− Free-Air Temperature − °C
V
− V
(VDD)
(PI)
Figure 2.
Figure 3.
Figure 4.
CLASSIFICATION TURNOFF
PASS DEVICE
RESISTANCE
vs
VOLTAGE
vs
I(VDD) CURRENT
vs
TEMPERATURE
VDD
TEMPERATURE
21.94
21.93
21.92
21.91
21.90
0.350
0.300
0.250
0.200
0.9
0.8
0.7
0.6
0.5
0.4
T
= 125°C
A
T
A
= 25°C
T
= −40°C
A
0.150
0.100
−40 −20
0
20 40 60 80 100 120
22
27
32
37
42
47
52
57
−40 −20
0
20 40 60 80 100 120
T
A
− Free-Air Temperature − °C
T
A
− Free-Air Temperature − °C
VDD − V
Figure 5.
Figure 6.
Figure 7.
8
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
TYPICAL CHARACTERISTICS (continued)
Graphs over temperature are interpolations between the marked data points.
TPS2375
UVLO RISING
vs
TPS2375
UVLO FALLING
vs
TPS2376
UVLO RISING
vs
TEMPERATURE
TEMPERATURE
TEMPERATURE
39.5
39.4
2.489
2.488
2.487
2.486
2.485
2.484
30.60
30.56
30.52
30.48
30.44
30.40
39.3
39.2
−40 −20
0
20 40 60 80 100 120
−40 −20
0
20 40 60 80 100 120
−40 −20
0
20 40 60 80 100 120
T
A
− Free-Air Temperature − °C
T
A
− Free-Air Temperature − °C
T
A
− Free-Air Temperature − °C
Figure 8.
Figure 9.
Figure 10.
TPS2376
UVLO FALLING
vs
TPS2377
UVLO RISING
vs
TPS2377
UVLO FALLING
vs
TEMPERATURE
TEMPERATURE
TEMPERATURE
35.20
35.15
35.10
35.05
35.00
34.95
30.65
30.60
30.55
30.50
30.45
1.929
1.928
1.927
1.926
1.925
1.924
1.923
−40 −20
0
20 40 60 80 100 120
−40 −20
0
20 40 60 80 100 120
−40 −20
0
20 40 60 80 100 120
T
A
− Free-Air Temperature − °C
T − Free-Air Temperature − °C
A
T
A
− Free-Air Temperature − °C
Figure 11.
Figure 12.
Figure 13.
9
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
TYPICAL CHARACTERISTICS (continued)
Graphs over temperature are interpolations between the marked data points.
INRUSH STATE TERMINATION
THRESHOLD
vs
INRUSH CURRENT
vs
TEMPERATURE
CURRENT LIMIT
vs
TEMPERATURE
TEMPERATURE
440
435
430
425
350
325
300
275
250
225
200
175
150
125
100
94.0
93.5
93.0
92.5
92.0
91.5
75 kΩ
125 kΩ
178 kΩ
91.0
90.5
−40 −20
0
20 40 60 80 100 120
−40 −20
0
20 40 60 80 100 120
−40 −20
0
20 40 60 80 100 120
T − Free-Air Temperature − °C
A
T
A
− Free-Air Temperature − °C
T
A
− Free-Air Temperature − °C
Figure 14.
Figure 15.
Figure 16.
PG DEGLITCH PERIOD
vs
TEMPERATURE
180
160
140
120
−40 −20
0
20 40 60 80 100 120
T
A
− Free-Air Temperature − °C
Figure 17.
10
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
APPLICATION INFORMATION
OVERVIEW
The IEEE 802.3af specification defines a process for safely powering a PD over a cable, and then removing
power if a PD is disconnected. The process proceeds through three operational states: detection, classification,
and operation. The intent behind the process is to leave an unterminated cable unpowered while the PSE
periodically checks for a plugged-in device; this is referred to as detection. The low power levels used during
detection are unlikely to cause damage to devices not designed for PoE. If a valid PD signature is present, then
the PSE may optionally inquire how much power the PD requires; this is referred to as classification. The PD
may return a default full-power signature, or one of four other choices. Knowing the power demand of each PD
allows the PSE to intelligently allocate power between PDs, and also to protect itself against overload. The PSE
powers up a valid PD, and then monitors its output for overloads. The maintain power signature (MPS) is
presented by the powered PD to assure the PSE that it is there. The PSE monitors its output for the MPS to see
if the PD is removed, and turns the port off, if it loses the MPS. Loss of MPS returns the PSE to the initial state of
detection. Figure 18 shows the operational states as a function of PD input voltage range.
The PD input is typically an RJ-45 (8-pin) connector, referred to as the power interface (PI). PD input
requirements differ from PSE output requirements to account for voltage drops in the cable and margin. The
specification uses a cable resistance of 20 Ω to derive the voltage limits at the PD from the PSE output
requirements. Although the standard specifies an output power of 15.4 W at the PSE output, there is only
12.95 W available at the input of the PD due to the worst case power loss in the cable.
The PSE can apply voltage either between the RX and TX pairs, or between the two spare pairs as shown in
Figure 1. The applied voltage can be of either polarity. The PSE cannot apply voltage to both paths at the same
time. The PD uses input diode bridges to accept power from any of the possible PSE configurations. The voltage
drops associated with the input bridges cause a difference between the IEEE 802.3af limits at the PI and the
TPS2375 specifications.
The PSE is required to current limit between 350 mA and 400 mA during normal operation, and it must
disconnect the PD if it draws this current for more than 75 ms. The PSE may set lower output current limits
based on the PD advertised power requirements, as discussed below.
The following discussion is intended as an aid in understanding the operation of the TPS2375, but not as a
substitute for the actual IEEE 802.3af standard. Standards change and should always be referenced when
making design decisions.
Shut -
down
Normal Operation
Classify
Detect
0
2.7
10.1
14.5
20.5
30
PI Voltage (V)
36
57
42
Figure 18. IEEE 802.3 PD Limits
INTERNAL THRESHOLDS
In order to implement the PoE functionality as shown in Figure 18, the TPS2375 has a number of internal
comparators with hysteresis for stable switching between the various states. Figure 19 relates the parameters in
the Electrical Characteristics section to the PoE states. The mode labeled idle between classification and
detection implies that the DET, CLASS, PG, and RTN pins are all high impedance.
11
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
APPLICATION INFORMATION (continued)
PD Powered
Idle
Classification
Detection
1.4V
V
(VDD)
V
(CU_H)
V
V
(UVLO_R)
(UVLO_F)
V
V
(CU_OFF)
(CL_ON)
Figure 19. Threshold Voltages
DETECTION
This feature of IEEE 802.3af eliminates powering and potentially damaging Ethernet devices not intended for
application of 48 V. When a voltage in the range of 2.7 V to 10.1 V is applied to the PI, an incremental resistance
of 25 kΩ signals the PSE that the PD is capable of accepting power. A PD that is capable of accepting power,
but is not ready, may present an incorrect signature intentionally. The incremental resistance is measured by
applying two different voltages to the PI and measuring the current it draws. These two test voltages must be
within the specified range and be at least 1 V apart. The incremental resistance equals the difference between
the voltages divided by the difference between the currents. The allowed range of resistance is 23.75 kΩ to
26.25 kΩ.
The TPS2375 is in detection mode whenever the supply voltage is below the lower classification threshold. The
TPS2375 draws a minimum of bias power in this condition, while PG and RTN are high impedance and the
circuits associated with ILIM and CLASS are disabled. The DET pin is pulled to ground during detection. Current
flowing through R(DET) to VSS (Figure 1) produces the detection signature. For most applications, a 24.9-kΩ, 1%,
resistor is recommended. R(DET) can be a small, low-power resistor because it only sees a stress of about 5 mW.
When the input voltage rises above the 11.3 V lower classification comparator threshold, the DET pin goes to an
open-drain condition to conserve power.
The input diode bridge incremental resistance can be hundreds of ohms at the low currents seen at 2.7 V on the
PI. The bridge resistance is in series with R(DET) and increases the total resistance seen by the PSE. This varys
with the type of diode selected by the designer, and it is not usually specified on the diode data sheet. The value
of R(DET) may be adjusted downwards to accommodate a particular diode type.
CLASSIFICATION
Once the PSE has detected a PD, it may optionally classify the PD. This process allows a PSE to determine the
PD power requirements in order to allot only as much power as necessary from its fixed input power source. This
allows the PSE to power the maximum number of PDs from a particular power budget. This step is optional
because some PSEs can afford to allot the full power to every powered port.
The classification process applies a voltage between 14.5 V and 20.5 V to the input of the PD, which in turn
draws a fixed current set by R(CLASS). The PSE measures the PD current to determine which of the five available
classes (Table 1) that the PD is signalling. The total current drawn from the PSE during classification is the sum
of bias currents and current through R(CLASS). The TPS2375 disconnects R(CLASS) at voltages above the
classification range to avoid excessive power dissipation (Figure 18 and Figure 19).
The value of R(CLASS) should be chosen from the values listed in Table 1 based on the average power
requirements of the PD. The power rating of this resistor should be chosen so that it is not overstressed for the
required 75-ms classification period, during which 10 V is applied. The PD could be in classification for extended
periods during bench test conditions, or if an auxiliary power source with voltage within the classification range is
connected to the PD front end. Thermal protection may activate and turn classification off if it continues for more
than 75 ms, but the design must not rely on this function to protect the resistor.
12
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
APPLICATION INFORMATION (continued)
UNDERVOLTAGE LOCKOUT (UVLO)
The TPS2375 incorporates an undervoltage lockout (UVLO) circuit that monitors line voltage to determine when
to apply power to the downstream load and allow the PD to power up. The IEEE 802.3af specification dictates a
maximum PD turnon voltage of 42 V and a minimum turnoff voltage of 30 V (Figure 19). The IEEE 802.3af
standard assumes an 8-V drop in the cabling based on a 20-Ω feed resistance and a 400-mA maximum inrush
limit. Because the minimum PSE output voltage is 44 V, the PD must continue to operate properly with input
voltages as low as 36 V. The TPS2375 UVLO limits are designed to meet the turnon, turnoff, and hysteresis
requirements.
Various legacy PSE systems in the field do not meet the minimum output voltage of 44 V. The TPS2377 UVLO
limits are designed to support these systems with a lower turnon voltage and smaller hysteresis. Although the
TPS2377 works with compliant PSEs, it could potentially exhibit startup instabilities if the PSE output voltage
rises slowly. The TPS2375 is recommended for applications with compliant PSEs.
In order to provide flexibility for noncompliant designs, the TPS2376 allows the designer to program the turnon
thresholds with a resistor divider. The hysteresis of the TPS2376, measured as a percentage of the turnon
voltage, is similar to that of the TPS2375. To use the TPS2376, connect a resistor divider between VDD and
VSS with the tap connected to the UVLO pin. The total divider resistance appears in parallel with the R(DET), and
the combination of the two should equal 24.9 kΩ. The divider ratio should be chosen to obtain 2.5 V at the UVLO
pin when V(VDD) is at the desired turnon voltage.
The TPS2375 uses the UVLO function to control the load through an onboard MOSFET switch. Figure 19
graphically shows the relationships of the UVLO thresholds defined in the Electrical Characteristics section to the
TPS2375 operational states.
PROGRAMMABLE INRUSH CURRENT LIMIT AND FIXED OPERATIONAL CURRENT LIMIT
Inrush limiting is beneficial for a number of reasons. First, it provides a mechanism to keep the inrush current
below the 400 mA, 50 ms, maximum inrush allowed by the standard. Second, by reducing the level of the current
limit below the PSE operational limit, which can be as low as the classification power divided by the PSE voltage,
it allows an arbitrarily large bulk capacitor to be charged. Third, some legacy PSEs may not tolerate large inrush
currents while powering their outputs up.
The TPS2375 operational current limit protects the internal power switch from instantaneous output faults and
current surges. The minimum operational current limit level of 405 mA is above the maximum PSE output current
limit of 400 mA. This current limit allows the PD to draw the maximum available power and also allows the PSE
to detect fault conditions.
The TPS2375 incorporates a state machine that controls the inrush and operational current limit states. When
V(VDD) is below the lower UVLO threshold, the current limit state machine is reset. In this condition, the RTN pin
is high impedance, and at V(VDD) once the output capacitor is discharged by the downstream circuits. When
V(VDD) rises above the UVLO turnon threshold, the TPS2375 enables the internal power MOSFET with the
current limit set to the programmed inrush value. The load capacitor charges and the RTN pin voltage falls from
V(VDD) to nearly V(VSS). Once the inrush current falls about 10% below the programmed limit, the current limit
switches to the internal 450-mA operational level after a 150-µs delay. This switchover can be seen in the
operation of PG, which goes active (open drain) after inrush terminates as seen in Figure 1. The internal power
MOSFET is disabled if the input voltage drops below the lower UVLO threshold.
When in the operating current-limit state, a fault on the output or a large input transient can cause the internal
MOSFET to limit current. The RTN voltage rises above its normal operating level of less than 0.5 V while in
current-limit state. If V(RTN) rises above 10 V for more than 150 µs, the MOSFET is latched off. The PD input
voltage must drop below the lower UVLO threshold to clear this latch. If the RTN voltage does not exceed 10 V
while in current-limit state, but the condition persists long enough to overheat the TPS2375, the thermal limit
circuit activates, as described in the thermal protection section.
Practical values of R(ILIM) lie between 62.5 kΩ and 500 kΩ. The pin must not be left open. An inrush level of
140 mA, set by an R(ILIM) of 178 kΩ, should be used with TPS2377 applications for compatibility with legacy
systems. This same inrush current level suffices for many TPS2375 applications.
13
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
APPLICATION INFORMATION (continued)
The inrush limit, the bulk capacitor size, and the downstream dc/dc converter startup method must be chosen so
that the converter input current does not exceed the inrush current limit while it is active. This can be achieved by
using the PG output to enable the downstream converter after inrush finishes, by delaying the converter startup
until inrush finishes, or by increasing the value of the inrush current limit.
MAINTAIN POWER SIGNATURE
Once a valid PD has been detected and powered, the PSE uses the maintain power signature (MPS) to
determine when to remove power from the PI. The PSE removes power from that output port if it detects loss of
MPS for 300 ms or more. A valid MPS requires the PD to draw at least 10 mA and also have an ac impedance
less than 26.25 kΩ in parallel with 0.05 µF. TI's reference designs meet the requirements necessary to maintain
power.
POWER GOOD
The TPS2375 includes a power-good circuit that can be used to signal the PD circuitry that the load capacitor is
fully charged. This pin is intended for use as an enable signal for downstream circuitry. If the converter tries to
start up while inrush is active, and draws a current equal to the inrush limit, a latchup condition occurs in which
the PD never successfully starts. Using the PG pin is the safest way to assure that there are no undesired
interactions between the inrush limit, the converter startup characteristic, and the size of the bulk capacitor.
The PG pin goes to an open-drain state approximately 150 µs after the inrush current falls 10% below the
regulated value. PG pulldown current is only assured when the voltage difference between VDD and RTN
exceeds 4 V. This is not a limiting factor because the dc/dc converter should not be able to run from 4 V. The PG
output is pulled to RTN whenever the MOSFET is disabled or is in inrush current limiting.
Referencing PG to RTN simplifies the interface to the downstream dc/dc converter or other circuit because it is
referenced to RTN, not VSS. Care must be used in interfacing the PG pin to the downstream circuits. The pullup
to VDD shown in Figure 1 may not be appropriate for a particular dc/dc converter interface. The PG pin connects
to an internal open-drain, 100-V transistor capable of sinking 2 mA to a voltage below 0.4 V. The PG pin can be
left open if it is not used.
THERMAL PROTECTION
The controller may overheat after operation in current-limit state or classification for an extended period of time,
or if the ambient temperature becomes excessive. The TPS2375 protects itself by disabling the RTN and CLASS
pins when the internal die temperature reaches about 140°C. It automatically restarts when the die temperature
has fallen approximately 20°C. If this cycle occurs eight times, then the device latches off until the supply voltage
drops below the lower classification threshold. This feature prevents the part from operating indefinitely in fault,
and ensures that the PSE recognizes the fault condition when using dc MPS. Thermal protection is active
whenever the TPS2375 is not in detection.
Figure 20 shows how the TPS2375 responds when it is enabled into a short. The TPS2375 starts in the inrush
current-limit state when the input voltage exceeds the upper UVLO limit. A power dissipation of over 5 W heats
the die from 25°C to 140°C in approximately 400 ms. The TPS2375 then shuts down until the die temperature
drops to about 120°C, which occurs in about 20 ms. This process repeats eight times before the TPS2375
latches off. The PG pin is high because RTN is tied to VDD.
14
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
APPLICATION INFORMATION (continued)
V
(PI)
= 44 V, R
= 178 kW
(ILIM)
Ch1: VDD @ 50 V/div
CH2: RTN @ 50 V/div
CH3: V
@ 50 V/div
(PG)
Iin @ 100 mA/div
Figure 20. TPS2375 Started Into Short
POWER SYSTEM DESIGN
The PSE is a power and current limited source, which imposes certain constraints on the PD power supply
design. DC/DC converters have both a constant input power characteristic that causes them to draw high
currents at low voltage, and they tend to go to a full input power mode during start-up that is often 25% or more
above their rated power. Improper design of the power system can cause the PD to not start up with all
combinations of Ethernet lines and PSE sources.
The following guidelines should be used:
1. Set the TPS2375 inrush to a moderate value as previously discussed.
2. Hold the dc/dc converter off during inrush as previously discussed.
3. The converter should have a softstart that keeps the peak input start-up current below 400 mA, and
preferably only a modest amount over the operating current, with a 44-V PSE source and a 20-Ω loop.
4. If step 3 cannot be met, the bulk input capacitor should not discharge more than 8 V during converter start
up from a 400-mA limited, 44-V source with a 20-Ω line. Start-up must be completed in less than 50 mS
Step 4 requires a balance between the converter output capacitance, load, and input bulk capacitance. While
there are some cases which may not require all these measures, such as a 1-W PD with minimal converter
output capacitance, it is always a good practice to follow them.
AUXILIARY POWER SOURCE ORING
Many PoE capable devices are designed to operate from either a wall adapter or PoE power. A local power
solution adds cost and complexity, but allows a product to be used regardless of PoE availability. Attempting to
create solutions where the two power sources coexist in a specific controlled manner results in additional
complexity, and is not generally recommended. Figure 21 demonstrates three methods of diode ORing external
power into a PD. Option 1 inserts power on the output side of the PoE power conversion. Option 2 inserts power
on the TPS2375 output. Option 3 applies power to the TPS2375 input. Each of these options has advantages
and disadvantages. The wall adapter must meet a minimum 1500-Vac dielectric withstand test voltage to the ac
input power and to ground for options 2 and 3.
15
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
APPLICATION INFORMATION (continued)
Inserting a Diode in This Location
With Option 2, Allows PoE To Start
With Aux Power Present.
~
~
+
−
Main
DC/DC
Converter
Output
R
(DET)
DC/DC
Converter
22 µF
DET
ILIM
UCC3809
or
UCC3813
R
(ILIM)
~
~
+
−
CLASS
RTN
Option 3
R
(CLASS)
For Option 2,
The Capacitor Must Be
Right At The Output
To Control The
Auxiliary
Power
Input
Option 2
Option 1
Transients.
Use only
one option
Optional
Regulator
A Full Wave Bridge
Gives Flexibility To
Use Supply With Either
Polarity
See TI Document SLVR030 For ATypical
Application Circuit.
Figure 21. Auxiliary Power ORing
Option 1 consists of ORing power to the output of the PoE dc/dc converter. This option is preferred in cases
where PoE is added to an existing design that uses a low-voltage wall adapter. The relatively large PD
capacitance reduces the potential for harmful transients when the adapter is plugged in. The wall adapter output
may be grounded if the PD incorporates an isolated converter. This solution requires two separate regulators, but
low-voltage adapters are readily available. The PoE power can be given priority by setting its output voltage
above that from the auxiliary source.
Option 2 has the benefits that the adapter voltage may be lower than the TPS2375 UVLO, and that the bulk
capacitor shown can control voltage transients caused by plugging an adapter in. The capacitor size and location
are chosen to control the amount of ringing that can occur on this node, which can be affected by additional
filtering components specific to a dc/dc converter design. The optional diode blocks the adapter voltage from
reverse biasing the input, and allows a PoE source to apply power provided that the PSE output voltage is
greater than the adapter voltage. The penalty of the diode is an additional power loss when running from PSE
power. The PSE may not be able to detect and start powering without the diode. This means that the adapter
may continue to power the PD until removed. Auxiliary voltage sources can be selected to be above or below the
PoE operational voltage range. If automatic PoE precedence is desired when using the low-voltage auxiliary
source option, make sure that the TPS2375 inrush program limit is set higher than the maximum converter input
current at its lowest operating voltage. It is difficult to use PG with the low-voltage auxiliary source because the
converter must operate during a condition when the TPS2375 would normally disable it. Circuits may be
designed to force operation from one source or the other depending on the desired operation and the auxiliary
source voltage chosen. However, they are not recommended because they increase complexity and thus cost.
Option 3 inserts the power before the TPS2375. It is necessary for the adapter to meet the TPS2375 UVLO
turnon requirement and to limit the maximum voltage to 57 V. This option provides a valid power-good signal and
simplifies power priority issues. The disadvantage of this method is that it is the most likely to cause transient
voltage problems. Plugging a powered adapter in applies a step input voltage to a node that has little
capacitance to control the dv/dt and voltage ringing. If the wall mount supply applies power to the PD before the
PSE, it prevents the PSE from detecting the PD. If the PSE is already powering the PD when the auxiliary source
is plugged in, priority is given to the higher supply voltage.
16
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
APPLICATION INFORMATION (continued)
ESD
The TPS2375 has been tested using the surge of EN61000-4-2 in an evaluation module (EVM) using the circuit
in Figure 1. The levels used were 8-kV contact discharge and 15-kV air discharge. Surges were applied between
the RJ-45 and the dc EVM outputs, and between an auxiliary power input jack and the dc outputs. No failures
were observed.
ESD requirements for a unit that incorporates the TPS2375 have much broader scope and operational
implications than those used in TI’s testing. Unit level requirements should not be confused with EVM testing that
only validated the TPS2375.
EXTERNAL COMPONENTS
Transformer
Nodes on an Ethernet network commonly interface to the outside world via an isolation transformer per IEEE
802.3 requirements, see Figure 1. For powered devices, the isolation transformer must include a center tap on
the media (cable) side. Proper termination is required around the transformer to provide correct impedance
matching and to avoid radiated and conducted emissions. Transformers must be specifically rated to work with
the Ethernet chipset, and the IEEE 802.3af standard.
Input Diodes or Diode Bridges
The IEEE 802.3af requires the PD to accept power on either set of input pairs in either polarity. This requirement
is satisfied by using two full-wave input bridge rectifiers as shown in Figure 1. Silicon p-n diodes with a 1-A or
1.5-A rating and a minimum breakdown of 100 V are recommended. Diodes exhibit large dynamic resistance
under low-current operating conditions such as in detection. The diodes should be tested for their behavior under
this condition. The diode forward drops must be less than 1.5 V at 500 µA and at the lowest operating
temperature.
Input Capacitor
The IEEE 802.3af requires a PD input capacitance between 0.05 µF and 0.12 µF during detection. This capacitor
should be located directly adjacent to the TPS2375 as shown in Figure 1. A 100-V, 10%, X7R ceramic capacitor
meets the specification over a wide temperature range.
Load Capacitor
The IEEE 802.3af specification requires that the PD maintain a minimum load capacitance of 5 µF. It is
permissible to have a much larger load capacitor, and the TPS2375 can charge in excess of 470 µF before
thermal issues become a problem. However, if the load capacitor is too large, the PD design may violate IEEE
802.3af requirements.
If the load capacitor is too large, there can be a problem with inadvertent power shutdown by the PSE caused by
failure to meet the MPS. This is caused by having a long input current dropout due to a drop in input voltage with
a large capacitance-to-load ratio. The standard gives Equation 2:
I
180
(PD)
C v
10 mA
(2)
where C is the bulk capacitance in µF and I(PD) is the PD load current in mA.
A particular design may have a tendency to cause ringing at the RTN pin during startup, inadvertent hot-plugs of
the PoE input, or plugging in a wall adapter. It is recommended that a minimum value of 1 µF be used at the
output of the TPS2375 if downstream filtering prevents placing the larger bulk capacitor right on the output. When
using ORing option 2, it is recommended that a large capacitor such as a 22 µF be placed across the TPS2375
output.
17
TPS2375
TPS2376
TPS2377
www.ti.com
SLVS525A–APRIL 2004–REVISED SEPTEMBER 2004
APPLICATION INFORMATION (continued)
Transient Suppressor
Voltage transients on the TPS2375 can be caused by connecting or disconnecting the PD, or by other
environmental conditions like ESD. The TPS2375 is specified to operate with absolute maximum voltages
V(VDD-VSS) and V(RTN-VSS) of 100 V. A transient voltage suppressor, such as the SMAJ58A, should be installed
after the bridge and across the TPS2375 input as shown in Figure 1. Various configurations of output filters and
the insertion of local power sources across either the TPS2375 input or output have the potential to cause
stresses outside the absolute maximum ratings of the device. Designers should be aware of this possibility and
account for it in their circuit designs. For example, use adequate capacitance across the output to limit the
magnitude of voltage ringing caused by downstream filters. Plugging an external power source across the output
may cause ESD-like events. Some form of protection should be considered based on a study of the specific
waveforms seen in an application circuit.
Layout
The layout of the PoE front end must use good practices for power and EMI/ESD. A basic set of
recommendations include:
1. The parts placement must be driven by the power flow in a point-to-point manner such as RJ-45 → Ethernet
transformer → diode bridges → TVS and 0.1-µF capacitor → TPS2375 → output capacitor.
2. There should not be any crossovers of signals from one part of the flow to another.
3. All leads should be as short as possible with wide power traces and paired signal and return.
4. Spacing consistent with safety standards like IEC60950 must be observed between the 48-V input voltage
rails and between the input and an isolated converter output.
5. The TPS2375 should be over a local ground plane or fill area referenced to VSS to aid high-speed operation.
6. Large SMT component pads should be used on power dissipating devices such as the diodes and the
TPS2375.
Use of added copper on local power and ground to help the PCB spread and dissipate the heat is recommended.
Pin 4 of the TPS2375 has the lowest thermal resistance to the die.
18
MECHANICAL DATA
MTSS001C – JANUARY 1995 – REVISED FEBRUARY 1999
PW (R-PDSO-G**)
PLASTIC SMALL-OUTLINE PACKAGE
14 PINS SHOWN
0,30
0,19
M
0,10
0,65
14
8
0,15 NOM
4,50
4,30
6,60
6,20
Gage Plane
0,25
1
7
0°–8°
A
0,75
0,50
Seating Plane
0,10
0,15
0,05
1,20 MAX
PINS **
8
14
16
20
24
28
DIM
3,10
2,90
5,10
4,90
5,10
4,90
6,60
6,40
7,90
9,80
9,60
A MAX
A MIN
7,70
4040064/F 01/97
NOTES: A. All linear dimensions are in millimeters.
B. This drawing is subject to change without notice.
C. Body dimensions do not include mold flash or protrusion not to exceed 0,15.
D. Falls within JEDEC MO-153
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
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