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TPS2379
SLVSB98A – MARCH 2012 – REVISED JULY 2015
TPS2379 IEEE 802.3at PoE High-Power PD Interface With External Gate Driver
1 Features
3 Description
•
The TPS2379 device is an 8-pin integrated circuit that
contains all of the features needed to implement an
IEEE802.3at type-2 powered device (PD) such as
Detection,
Classification,
Type
2
Hardware
Classification, and 140-mA inrush current limit during
startup. The low 0.5-Ω internal switch resistance,
combined with the enhanced thermal dissipation of
the PowerPAD package, enables this controller to
continuously handle up to 0.85 A. The TPS2379
integrates a low 0.5-Ω internal switch to allow for up
to 0.85 A of continuous current through the PD during
normal operation. The TPS2379 device supports
higher-power applications through an external pass
transistor. The TPS2379 contains several protection
features such as thermal shutdown, current limit
foldback, and a robust 100-V internal switch.
1
•
•
•
•
•
•
•
IEEE 802.3at Type-2 Hardware Classification With
Status Flag
Auxiliary Gate Driver for High-Power Expansion
Robust 100-V, 0.5-Ω Hotswap MOSFET
1A (Typical) Operating Current Limit
140 mA (Typical) Inrush Current Limit
DC-DC Converter Enable
15 kV/8 kV System-level ESD Capability
PowerPAD™ HSOP Package
2 Applications
•
•
•
•
•
•
IEEE 802.3at-compliant Devices
Universal Power Over Ethernet (UPOE) Compliant
Devices
Video and VoIP Telephones
Multiband Access Points
Security Cameras
Pico-Base Stations
Device Information(1)
PART NUMBER
TPS2379
PACKAGE
HSOP (8)
BODY SIZE (NOM)
4.89 mm × 3.90 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
D1
RDEN
TPS2379
RT2P
VDD
C1
DEN
CLS
T2P
CDB
VSS
GATE
RTN
RCLS
RBLST
VC
SS
CBULK
DC/DC Converter
From Spare
Pairs or
Transformers
From Ethernet
Transformers
Typical Application Circuit
Q1
1
An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
TPS2379
SLVSB98A – MARCH 2012 – REVISED JULY 2015
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Table of Contents
1
2
3
4
5
6
7
8
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
2
3
4
6.1
6.2
6.3
6.4
6.5
6.6
4
4
4
5
5
7
Absolute Maximum Ratings ......................................
ESD Ratings..............................................................
Recommended Operating Conditions.......................
Thermal Information ..................................................
Electrical Characteristics...........................................
Typical Characteristics ..............................................
Parameter Measurement Information .................. 9
Detailed Description ............................................ 10
8.1 Overview ................................................................. 10
8.2 Functional Block Diagram ....................................... 10
8.3 Feature Description................................................. 10
8.4 Device Functional Modes........................................ 14
9
Application and Implementation ........................ 22
9.1 Application Information............................................ 22
9.2 Typical Application .................................................. 22
10 Power Supply Recommendations ..................... 25
11 Layout................................................................... 25
11.1
11.2
11.3
11.4
Layout Guidelines .................................................
Layout Example ....................................................
Thermal Considerations and OTSD......................
ESD.......................................................................
25
26
28
28
12 Device and Documentation Support ................. 29
12.1
12.2
12.3
12.4
12.5
Documentation Support ........................................
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
29
29
29
29
29
13 Mechanical, Packaging, and Orderable
Information ........................................................... 29
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Original (March 2012) to Revision A
Page
•
Added ESD Ratings table, Feature Description section, Device Functional Modes, Application and Implementation
section, Power Supply Recommendations section, Layout section, Device and Documentation Support section, and
Mechanical, Packaging, and Orderable Information section ................................................................................................. 1
•
Deleted Classification Resistor, RCLS section..................................................................................................................... 13
•
Deleted CDB Pin Interface section ....................................................................................................................................... 13
•
Deleted GATE Pin Interface section .................................................................................................................................... 13
•
Deleted External Boost Circuit (Q1, Q2, and RBLST) Considerations section ....................................................................... 13
•
Deleted T2P Pin Interface ................................................................................................................................................... 13
•
Deleted Detailed Pin Descriptions section .......................................................................................................................... 21
2
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5 Pin Configuration and Functions
DDA Package
8-Pin HSOP
Top View
VDD
1
8
GATE
DEN
2
7
T2P
CLS
3
6
CDB
VSS
4
5
RTN
Pin Functions
PIN
NAME
NO.
I/O
DESCRIPTION
VDD
1
I
DEN
2
I/O
Connect to positive PoE input power rail. Bypass with 0.1 µF to VSS.
Connect 24.9 kΩ to VDD for detection. Pull to VSS disable pass MOSFET.
CLS
3
O
Connect resistor from CLS to VSS to program classification current.
VSS
4
—
Connect to negative power rail derived from PoE source.
RTN
5
O
Drain of PoE pass MOSFET.
CDB
6
O
Opendrain converter disable output, active low, referenced to RTN.
T2P
7
O
Active low indicates type 2 PSE connected.
GATE
8
O
Auxiliary gate driver output.
PowerPAD
—
—
The PowerPAD must be connected to VSS. A large fill area is required to assist in heat dissipation.
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6 Specifications
6.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted)
Input voltage
(1)
MIN
MAX
VDD, DEN
–0.3
100
RTN (2)
–0.6
100
CLS (3)
–0.3
6.5
GATE (3)
–0.3
18
[CDB, T2P] to RTN
–0.3
100
RTN (4)
Sinking current
CDB, T2P
5
DEN
1
65
TJMAX
mA
mA
Internally limited
Storage temperature, Tstg
(2)
(3)
(4)
V
Internally limited
Sourcing current CLS
(1)
UNIT
–65
°C
150
°C
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
With I(RTN) = 0
Do not apply voltages to these pins
SOA limited to RTN = 80 V at 1.2 A.
6.2 ESD Ratings
VALUE
V(ESD)
Electrostatic discharge
Human body model (HBM), per ANSI/ESDA/JEDEC JS-001, all pins (1)
2000
Charged device model (CDM), per JEDEC specification JESD22-C101, all
pins (2)
500
IEC 61000-4-2 contact discharge
(3)
V
8000
IEC 61000-4-2 air-gap discharge (3)
(1)
(2)
(3)
UNIT
15000
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
JEDEC document JEP157 states that 250-V CDM allows safe manufacturing with a standard ESD control process.
Discharges applied to circuit of Figure 26 between RJ-45, adapter, and output voltage rails.
6.3 Recommended Operating Conditions
Voltages with respect to VSS (unless otherwise noted)
MIN
Input voltage range
Sinking current
Resistance
4
MAX
0
57
T2P or CDB to RTN
0
57
RTN
0.85
CDB, T2P
CLS (1)
–40
UNIT
V
A
2
mA
125
°C
Ω
60
Junction temperature
(1)
NOM
RTN, VDD
Voltage should not be externally applied to this pin.
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6.4 Thermal Information
TPS2379
THERMAL METRIC (1)
DDA (HSOP)
UNIT
8 PINS
RθJA
Junction-to-ambient thermal resistance
45.9
°C/W
RθJC(top)
Junction-to-case (top) thermal resistance
51.9
°C/W
RθJB
Junction-to-board thermal resistance
28.8
°C/W
ψJT
Junction-to-top characterization parameter
8.9
°C/W
ψJB
Junction-to-board characterization parameter
28.7
°C/W
RθJC(bot)
Junction-to-case (bottom) thermal resistance
6.7
°C/W
(1)
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report, SPRA953.
6.5 Electrical Characteristics
40 V ≤ VVDD ≤ 57 V, RDEN = 24.9 kΩ, CDB, CLS, GATE, T2P open; –40°C ≤ TJ ≤ 125°C. Positive currents are into pins.
Typical values are at 25°C. All voltages are with respect to VVSS (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
VDD = 1.4 V
53.8
56.5
58.3
µA
VDD = 10.1 V, Not in mark
395
410
417
3
4.8
12
DETECTION (DEN)
Measure ISUPPLY(VDD, RTN, DEN)
Detection current
VPD_DIS
Bias current
DEN open, VVDD = 10.1 V, Measure ISUPPLY, Not in mark
Disable threshold
DEN falling
3
3.7
5
50
113
200
RCLS = 1270 Ω
1.8
2.17
2.6
RCLS = 243 Ω
9.9
10.6
11.2
RCLS = 137 Ω
17.6
18.6
19.4
RCLS = 90.9 Ω
26.5
27.9
29.3
RCLS = 63.4 Ω
38
39.9
42
11.9
12.5
13
1.4
1.6
1.7
Hysteresis
µA
V
mV
CLASSIFICATION (CLS)
13 V ≤ VDD ≤ 21 V, Measure IVDD + IDEN + IRTN
ICLS
VCL_ON
Classification current
VDD rising, VCLS ↑
Class lower threshold
VCL_H
VCU_ON
VCU_H
VMSR
Class upper threshold
Hysteresis
VDD rising, VCLS↓
21
22
23
Hysteresis
0.5
0.78
0.9
mA
V
V
Mark reset threshold
VVDD falling
3
3.9
5
Mark state resistance
2-point measurement at 5 V and 10.1 V
6
10
12
kΩ
Leakage current
VDD = 57 V, VCLS = 0 V, measure ICLS
1
µA
GATE (AUXILIARY GATE OUTPUT)
Output high voltage
Sourcing current
Sinking current
8
10
12
V
VGATE = 0 V
25
38
60
µA
VGATE = 4 V, VDD = 48→ 25 V
0.6
1.25
1.75
5
23.2
30
150
365
600
0.2
0.42
0.75
Ω
30
µA
VDD = 25 V, VGATE = 0→ 4 V
Current limit delay
mA
µs
PASS DEVICE (RTN)
rDS(ON)
On resistance
Input bias current
VDD = VRTN = 30 V, measure IRTN
Current limit
VRTN =1.5 V
0.85
1
1.2
A
Inrush current limit
VRTN = 2 V, VDD: 20 V → 48 V
100
140
180
mA
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Electrical Characteristics (continued)
40 V ≤ VVDD ≤ 57 V, RDEN = 24.9 kΩ, CDB, CLS, GATE, T2P open; –40°C ≤ TJ ≤ 125°C. Positive currents are into pins.
Typical values are at 25°C. All voltages are with respect to VVSS (unless otherwise noted)
PARAMETER
TEST CONDITIONS
Inrush termination
Percentage of inrush current
Foldback threshold
VRTN rising
Foldback deglitch time
VRTN rising to when current limit changes to inrush current
limit
MIN
TYP
MAX
80%
90%
99%
UNIT
11
12.3
13.6
V
500
800
1500
µs
0.27
0.50
V
CONVERTER DISABLE (CDB)
Output low voltage
ICDB = 2 mA, VRTN = 2 V, VDD: 20 V → 48 V
Minimum voltage, V(VDD
–RTN), for CDB to be valid
VCDB = VDD, ICDB = 1 mA, in inrush
Leakage current
VCDB = 57 V, VRTN = 0 V
3
V
10
µA
0.60
V
10
µA
TYPE 2 PSE INDICATION (T2P)
VT2P
Output low voltage
IT2P = 2 mA, after 2-event classification and inrush is
complete, VRTN = 0 V
Leakage current
VT2P = 57 V, VRTN = 0 V
UVLO rising threshold
VVDD rising
36.3
38.1
40
UVLO falling threshold
VVDD falling
30.5
32
33.6
0.26
UVLO
VUVLO_R
VUVLO_H
UVLO hysteresis
6.1
V
V
THERMAL SHUTDOWN
Shutdown
TJ↑
135
Hysteresis
145
°C
20
VDD BIAS CURRENT
Operating current
6
40 V ≤ VVDD ≤ 57 V
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285
500
µA
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6.6 Typical Characteristics
7
50
6
40
TA = 25°C
Resistance (kΩ)
IVDD (µA)
5
TA = 125°C
4
3
2
Detection Resistance
30
20
10
1
TA = −40°C
0
0
1
2
3
4
5
6
V(VDD−VSS) (V)
7
8
9
0
10
1
2
3
4
5
6
7
8
9
10
V(VDD−VSS) (V)
Figure 1. Detection Bias Current vs PoE Voltage
G002
Figure 2. Detection Resistance vs PoE Voltage
22.5
13
Class Upper Threshold, On
12.5
22
V(VDD−VSS) (V)
V(VDD−VSS) (V)
0
G001
21.5
Class Lower Threshold, On
12
11.5
Class Lower Threshold, Off
Class Upper Threshold, Off
11
21
−50
−25
0
25
50
75
Junction Temperature (°C)
100
10.5
−50
125
−25
G003
Figure 3. Classification Upper Threshold vs Temperature
0
25
50
75
Junction Temperature (°C)
100
125
G004
Figure 4. Classification Lower Threshold vs Temperature
400
4.5
360
Mark Reset Threshold
TA = 25°C
V(VDD−VSS) (V)
IVDD (µA)
320
TA = 125°C
280
240
200
4
3.5
TA = −40°C
160
120
20
25
30
35
40
45
50
55
V(VDD−VSS) (V)
60
3
−50
G009
Figure 5. IVDD Bias Current vs Voltage
−25
0
25
50
75
Junction Temperature (°C)
100
125
G006
Figure 6. Mark Reset Threshold vs Temperature
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Typical Characteristics (continued)
0.7
10
Pass FET Resistance (Ω)
Mark Resistance (kΩ)
9.8
9.5
9.2
9
8.8
8.5
−50
−25
0
25
50
75
Junction Temperature (°C)
100
0.6
0.5
0.4
0.3
0.2
−50
125
0
25
50
75
Junction Temperature (°C)
100
125
G008
Figure 8. Pass FET Resistance vs Temperature
Figure 7. Mark Resistance vs Temperature
160
89
Inrush Current Termination (%)
Current Inrush Limit (mA)
−25
G007
150
140
130
120
−50
−25
0
25
50
75
Junction Temperature (°C)
100
88
87
86
85
−50
125
−25
G009
Figure 9. PoE Inrush Current Limit vs Temperature
0
25
50
75
Junction Temperature (°C)
100
125
G010
Figure 10. Inrush Termination Threshold vs Temperature
38.3
1.02
1.01
V(VDD−VSS) (V)
Current Limit (A)
38.28
1
38.26
UVLO Rising Threshold
38.24
38.22
0.99
−50
−25
0
25
50
75
Junction Temperature (°C)
100
38.2
−50
G011
Figure 11. PoE Current Limit vs Temperature
8
125
−25
0
25
50
75
Junction Temperature (°C)
100
125
G012
Figure 12. UVLO Rising Threshold vs Temperature
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Typical Characteristics (continued)
32.16
10
32.14
V(GATE−VSS) (V)
V(VDD−VSS) (V)
9.9
32.12
UVLO Falling Threshold
32.1
32.08
Auxilary Gate Voltage
9.8
9.7
32.06
32.04
−50
−25
0
25
50
75
Junction Temperature (°C)
100
125
9.6
−50
−25
G013
Figure 13. UVLO Falling Threshold vs Temperature
0
25
50
75
Junction Temperature (°C)
100
125
G015
Figure 14. Auxiliary Gate Voltage vs Temperature
7 Parameter Measurement Information
Mark
Reset
Idle
Detect
Detect
Mark
Class
Between
Ranges
Class
Between
Ranges
Class
Between
Ranges
UVLO
Falling
UVLO
Rising
Operating
T2P
open-drain
TYPE 1 PSE
Hardware Class
PoE Startup Sequence
Mark
Class
Between
Ranges
UVLO
Rising
Operating
T2P low
TYPE 2 PSE
Hardware Class
UVLO
Falling
Figure 15. PD Class State Diagram
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8 Detailed Description
8.1 Overview
The TPS2379 device is an 8-pin integrated circuit that contains all of the features needed to implement an
IEEE802.3at type-2 powered device (PD) such as Detection, Classification, Type 2 Hardware Classification, and
140mA inrush current limit during start-up.
The TPS2379 integrates a low 0.5-Ω internal switch to allow for up to 0.85 A of continuous current through the
PD during normal operation.
The TPS2379 supports higher power PoE applications through the use of an external pass transistor.
The TPS2379 contains several protection features such as thermal shutdown, current limit foldback, and a robust
100V internal switch.
8.2 Functional Block Diagram
12V &
10V
VDD
1
Detection
Comp.
Class
Comp.
4V
Class
Comp.
VSS
22V &
21.25V
5V & 4V
Mark
Comp.
2.5V
REG.
800ms
800ms
12V
UVLO Comp Output
R
OTSD
VSS
T2P
RTN
6
CDB
R
6V
S
UVLO
Comp.
CLS
Type 2
State Eng.
1 = inrush
0 = current limit
Inrush latch
38.1V &
32V
3
7
Mark Comp Output
Q
DEN
VSS
RTN
S
2
Inrush limit
threshold 1
Q
RTN
1
Current limit
0
threshold
8
GATE
5
RTN
GATE DRIVER
0
High if over
temperauture
I RTN sense
4
Hotswap
MOSFET
Signals referenced to VSS unless otherwise noted
365ms
IRTN sense,1 if < 90% of inrush current limit
8.3 Feature Description
8.3.1 CDB Converter Disable Bar Pin Interface
CDB is an active low output that is pulled to RTN when the device is in inrush current limiting, going open when
the inrush period has completed once the GATE output has become higher than 6 V. This ensures the external
pass transistor is enhanced before the load is enabled. It remains in a high impedance state at all other times.
This pin is an opendrain output and may require a pullup resistor or other interface to the downstream load. CDB
may be left open if unused.
10
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Feature Description (continued)
The CDB pin can inhibit downstream converter start up by keeping the soft start pin low. Figure 16 shows where
the CDB connects to the SS pin of a UCC2897A DC-DC controller. Because CDB is an open drain output, it will
not affect the soft start capacitor charge time when it deasserts. The CDB pin can also enable a converter with
an active-high enable input. In this case, CDB may require a pullup resistor to either VDD, or to a bias supply,
depending on the requirements of the controller enable pin.
TPS2379
UCC2897A
SS
CDB
CSS
RTN
GND
Figure 16. CDB Interface
8.3.2 CLS Classification
An external resistor (RCLS) connected between the CLS pin and VSS provides a classification signature to the
PSE. The controller places a voltage of approximately 2.5 V across the external resistor whenever the voltage
differential between VDD and VSS is between about 10.9 V and 22 V. The current drawn by this resistor,
combined with the internal current drain of the controller and any leakage through the internal pass MOSFET,
creates the classification current. Table 1 lists the external resistor values required for each of the PD power
ranges defined by IEEE802.3at. The maximum average power drawn by the PD combined with the power
supplied to the downstream load should not exceed the maximum power indicated in Table 1.
Table 1. Class Resistor Selection
CLASS
MINIMUM POWER AT PD (W)
MAXIMUM POWER
AT PD
RESISTOR RCLS (Ω)
0
0.44
12.95
1270
1
0.44
3.84
243
2
3.84
6.49
137
3
6.49
12.95
90.9
4
12.95
25.5
63.4
8.3.3 DEN Detection and Enable
DEN pin implements two separate functions. A resistor (RDEN) connected between VDD and DEN generates a
detection signature whenever the voltage differential between VDD and VSS is from 1.4 V and 10.9 V. Beyond
this range, the controller disconnects this resistor to save power. The IEEE 802.3at standard specifies a
detection signature resistance, RDEN from 23.75 kΩ to 26.25 kΩ, or 25 kΩ ± 5%. A resistor of 24.9 kΩ ± 1% is
recommended for RDEN.
If the resistance connected between VDD and DEN is divided into two roughly equal portions, then the application
circuit can disable the PD by grounding the tap point between the two resistances. This action simultaneously
spoils the detection signature and thereby signals the PSE that the PD no longer requires power.
8.3.4 GATE Auxiliary Gate Driver
GATE pin allows the connection of an external pass MOSFET in parallel with the internal pass transistor. The
GATE pin enables the external transistor after inrush has completed. The current is divided between the external
MOSFET and the internal transistor as a function of their respective resistances. The addition of a balancing
resistor (RBLST) in series with RTN and the external MOSFET can ensure the desired distribution of the two
currents. When the RTN current exceeds the current limit threshold, the GATE pin will pull low after a 365 μs
delay. The GATE pin is pulled low in thermal shutdown. After the controller cools down and the inrush cycle is
complete, the GATE pin rises again.
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A nonstandard PoE system can be designed to meet extended power requirements and retain the PoE benefits,
such as protection of non-PoE devices and fault tolerance. This type of solution will not comply with IEEE802.3at
and should be designed and operated as stand-alone system. The TPS2379 GATE pin controls an external pass
MOSFET as shown in Figure 17. When the inrush is complete, GATE sources 38 μA to enable Q1, the external
pass MOSFET. When Q1 is fully enhanced, CDB deasserts and enables the load. Delaying the deassertion of
CDB until Q1 becomes fully enhanced prevents nuisance overcurrent faults that could occur with heavy start-up
loads. A resistor from GATE to VSS is not required to ensure that Q1 turns off. If a resistor from GATE to VSS is
used, choose a value large enough so that the GATE sourcing current can fully enhance Q1.
VDD
T2P
CDB
I2379
GATE
VSS
CBULK
IL
RTN
DC/DC Converter
VC
TPS2379
Q1
Q2
RBLST
Figure 17. GATE Interface
8.3.4.1 External Boost Circuit (Q1, Q2, and RBLST) Considerations
The IEEE802.3at template bounds the peak PSE output current from 50 A for 10 μs and 1.75 A for 75 ms for a
two-pair system. In a nonstandard, four-pair system, these current levels may double. During an overload event,
the TPS2379 device will limit current to approximately 1 A and the rest of the current will flow through Q1 and
RBLST. Ignoring the ballast resistor and parasitic impedances, the current through Q1 could be as high as 99 A.
Actual system level behavior will be influenced by the circuit parasitic impedances, diode bridge impedance,
contact resistances, external MOSFET resistance, and input voltage droop during the overload event. The
impedances act to reduce the peak current as well as drop the voltage across Q1 during the overload event.
Evaluate the overload performance of your system and ensure that the selected external MOSFET safe
operating area (SOA) is not violated during the output overload. The duration of the overload can be terminated if
the input voltage droop to the TPS2379 goes below the UVLO falling threshold (typically 32 V). When UVLO
occurs, the internal MOSFET is disabled, GATE goes low and the external MOSFET is disabled. This shortened
overload duration is beneficial when evaluating the external MOSFET SOA performance.
Additional limiting and control of the external output overload current can be achieved by using the ballast
resistor, RBLST. RBLST helps balance the internal and external MOSFET load currents and implements external
current limiting through Q2. The load current, IL, divides between the external Q1 and the internal pass MOSFET
of the TPS2379 as shown Equation 1.
RBLST + RQ1
I2379 = IL ´
RBLST + RQ1 + R2379
(1)
RQ1 is the ON resistance of Q1 and R2379 is the ON resistance of the TPS2379. Q2 can be used to force Q1 to
limit its current when the voltage across RBLST exceeds VBEON of Q2. For further discussion of these details and
additional considerations involving PD classification, see the application report titled Implementing a 60-W Endto-End PoE System (SLVA498).
8.3.5 Internal Pass MOSFET
RTN pin provides the negative power return path for the load. When VDD exceeds the UVLO threshold, the
internal pass MOSFET pulls RTN to VSS. Inrush limiting prevents the RTN current from exceeding 140 mA until
the bulk capacitance (CBULK in Figure 26) is fully charged. Inrush ends when the RTN current drops below about
125 mA. The RTN current is subsequently limited to about 1 A. CDB pulls low to signal the downstream load that
the bulk capacitance is fully charged. If RTN ever exceeds about 12 V for longer than 800 μs, the TPS2379
returns to inrush limiting.
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8.3.6 T2P Type-2 PSE Indicator
The TPS2379 pulls T2P to RTN when type-2 hardware classification has been observed. The T2P output will
return to a high-impedance state if the part enters thermal shutdown, the pass MOSFET enters inrush limiting, or
if a type-2 PSE was not detected. The circuitry that watches for type-2 hardware classification latches its result
when the VDD-to-VSS voltage differential rises above the upper classification threshold. This circuit resets when
the VDD-to-VSS voltage differential drops below the mark threshold. The T2P pin can be left unconnected if
unused.
The T2P pin is an active-low, opendrain output which indicates that a high power source is available. An
optocoupler can interface the T2P pin to circuitry on the secondary side of the converter. A high-gain optocoupler
and a high-impedance (that is, CMOS) receiver are recommended. Figure 18 presents the design of the T2P
optocoupler interface.
VOUT
RT2P
IT2P-OUT
IT2P
VC
RT2P-OUT
VT2P-OUT
VT2P Low
Indicates
Type 2
T2P From
TPS2379
Figure 18. T2P Interface
To design the T2P octocoupler interface, do the following:
1. Let VC = 12 V, VOUT = 5 V, RT2P-OUT = 10 kΩ, VT2P = 260 mV, VT2P-OUT = 400 mV.
V
- VT2P -OUT 5 - 0.4
IT2P -OUT = OUT
=
= 0.46mA
RT2P -OUT
10000
(2)
2. The optocoupler current transfer ratio, CTR, will be needed to determine RT2P. A device with a minimum CTR
of 100% at 1 mA LED bias current, IT2P, is selected. CTR will vary with temperature, LED bias current, and
aging. These variations may require some iteration using the CTR-versus-IDIODE curve on the optocoupler
data sheet.
(a) The approximate forward voltage of the optocoupler diode, VFWLED, is 1.1 V from the data sheet.
(b) Select a 10.7-kΩ resistor.
I
0.46mA
IT2P-MIN = T2P-OUT =
= 0.46mA, Select IT2P = 1mA
CTR
1.00
V - VT2P - VFWLED 12 V - 0.26 V - 1.1 V
RT2P = C
=
= 10.6kΩ
IT2P
1mA
(3)
(c) Select a 10.7-kΩ resistor.
8.3.7 VDD Supply Voltage
VDD pin connects to the positive side of the input supply. It provides operating power to the PD controller and
allows monitoring of the input line voltage.
8.3.8 VSS
VSS pin is the input supply negative rail that serves as a local ground. The PowerPAD must be connected to this
pin to ensure proper operation.
8.3.9 PowerPAD
The PowerPAD is internally connected to VSS.The PowerPAD should be tied to a large VSS copper area on the
PCB to provide a low resistance thermal path to the circuit board. TI recommends that a clearance of 0.025” be
maintained between VSS and high-voltage signals such as VDD.
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8.4 Device Functional Modes
8.4.1 PoE Overview
The following text is intended as an aid in understanding the operation of the TPS2379 but not as a substitute for
the IEEE 802.3at standard. The IEEE 802.3at standard is an update to IEEE 802.3-2008 clause 33 (PoE),
adding high-power options and enhanced classification. Generally speaking, a device compliant to IEEE 802.32008 is referred to as a type 1 device, and devices with high power and enhanced classification will be referred
to as type 2 devices. Standards change and should always be referenced when making design decisions.
The IEEE 802.3at standard defines a method of safely powering a PD (powered device) over a cable by power
sourcing equipment (PSE), and then removing power if a PD is disconnected. The process proceeds through an
idle state and three operational states of detection, classification, and operation. The PSE leaves the cable
unpowered (idle state) while it periodically looks to see if something has been plugged in; this is referred to as
detection. The low power levels used during detection are unlikely to damage devices not designed for PoE. If a
valid PD signature is present, the PSE may inquire how much power the PD requires; this is referred to as
classification. The PSE may then power the PD if it has adequate capacity.
Type 2 PSEs are required to do type 1 hardware classification plus a (new) data-layer classification, or an
enhanced type 2 hardware classification. Type 1 PSEs are not required to do hardware or data link layer (DLL)
classification. A type 2 PD must do type 2 hardware classification as well as DLL classification. The PD may
return the default, 13W current-encoded class, or one of four other choices. DLL classification occurs after
power-on and the Ethernet data link has been established.
Shutdown
Classify
Detect
6.9
Maximum Input
Voltage
Must Turn On byVoltage Rising
Lower Limit Operating Range
Must Turn Off by Voltage Falling
Classification
Upper Limit
Classification
Lower Limit
Detection
Upper Limit
Detection
Lower Limit
IEEE 802-2008
When started, the PD must present a maintain power signature (MPS) to assure the PSE that it is still present.
The PSE monitors its output for a valid MPS, and turns the port off if it loses the MPS. Loss of the MPS returns
the PSE to the idle state. Figure 19 shows the operational states as a function of PD input voltage. The upper
half is for IEEE 802.3-2008, and the lower half shows specific differences for IEEE 802.3at. The dashed lines in
the lower half indicate these are the same (that is, Detect and Class) for both.
Normal Operation
42.5
0
30
37
57 PI Voltage (V)
42
Normal Operation
250ms
Transient
Class-Mark
Transition
Mark
20.5
Lower Limit 13W Op.
10.1 14.5
T2 Reset
Range
IEEE 802.3at
2.7
Figure 19. Threshold Voltages
The PD input, typically an RJ-45 eight-lead connector, is referred to as the power interface (PI). PD input
requirements differ from PSE output requirements to account for voltage drops and operating margin. The
standard allots the maximum loss to the cable regardless of the actual installation to simplify implementation.
IEEE 802.3-2008 was designed to run over infrastructure including ISO/IEC 11801 class C (CAT3 per TIA/EIA568) that may have had AWG 26 conductors. IEEE 802.3at type 2 cabling power loss allotments and voltage
drops have been adjusted for 12.5 Ω power loops per ISO/IEC11801 class D (CAT5 or higher per TIA/EIA-568,
typically AWG #24 conductors). Table 2 shows key operational limits broken out for the two revisions of the
standard.
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Device Functional Modes (continued)
Table 2. Comparison of Operational Limits
STANDARD
POWER LOOP
RESISTANCE
(max)
PSE OUTPUT
POWER
(min)
PSE STATIC
OUTPUT VOLTAGE
(min)
PD INPUT
POWER
(max)
20Ω
15.4W
44V
12.95W
37V – 57V
N/A
12.5Ω
30W
50V
25.5W
37V – 57V
42.5V – 57V
IEEE802.3at-2008
802.3at (Type 1)
802.3at (Type 2)
STATIC PD INPUT VOLTAGE
Power ≤12.95W Power >12.95W
The PSE can apply voltage either between the RX and TX pairs (pins 1 - 2 and 3 - 6 for 10baseT or 100baseT),
or between the two spare pairs (4 - 5 and 7 - 8). Power application to the same pin combinations in 1000baseT
systems is recognized in IEEE 802.3at. 1000baseT systems can handle data on all pairs, eliminating the spare
pair terminology. The PSE may only apply voltage to one set of pairs at a 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 create a difference between the standard limits at the PI and the TPS2379 specifications.
A compliant type 2 PD has power management requirements not present with a type 1 PD. These requirements
include the following:
1. Must interpret type 2 hardware classification,
2. Must present hardware class 4,
3. Must implement DLL negotiation,
4. Must behave like a type 1 PD during inrush and start-up,
5. Must not draw more than 13W for 80ms after the PSE applies operating voltage (power up),
6. Must not draw more than 13W if it has not received a type 2 hardware classification or received permission
through DLL,
7. Must meet various operating and transient templates, and
8. Optionally monitor for the presence or absence of an adapter (assume high power).
As a result of these requirements, the PD must be able to dynamically control its loading, and monitor T2P for
changes. In cases where the design needs to know specifically if an adapter is plugged in and operational, the
adapter should be individually monitored, typically with an optocoupler.
8.4.1.1 Threshold Voltages
The TPS2379 has a number of internal comparators with hysteresis for stable switching between the various
states. Figure 20 relates the parameters in the Electrical Characteristics section to the PoE states. The mode
labeled Idle between Classification and Operation implies that the DEN, CLS, and RTN pins are all high
impedance. The state labeled Mark, which is drawn in dashed lines, is part of the new type 2 hardware class
state machine.
Type 1
Mark
Classification
Type 2
Functional
State
PD Powered
Idle
VDD-VSS
Detection
VCL_H
VMSR
VCL_ON
VCU_H
VUVLO_H
VCU_OFF
VUVLO_R
Note: Variable names refer to Electrical Characteristic Table parameters
Figure 20. Threshold Voltages
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8.4.1.2 PoE Start-Up Sequence
Current: 100 mA/div
The waveforms of Figure 21 demonstrate detection, classification, and start-up from a PSE with type 2 hardware
classification. The key waveforms shown are V(VDD-VSS), V(RTN-VSS), and IPI. IEEE 802.3at requires a
minimum of two detection levels, two class and mark cycles, and start-up from the second mark event. VRTN to
VSS falls as the TPS2379 charges CBULK following application of full voltage. In Figure 21, deassertion of the CDB
signal is delayed and used to enable load current as seen in the IPI waveform.
Load enabled using
CDB plus delay
Inrush
IPI
Voltage: 10 V/div
VVDD-VSS
Class
Mark
Detect
VRTN-VSS
Time : 50 ms/div
Figure 21. Start-up
8.4.1.3 Detection
The TPS2379 pulls DEN to VSS whenever V(VDD-VSS) is below the lower classification threshold. When the input
voltage rises above VCL-ON, the DEN pin goes to an opendrain condition to conserve power. While in detection,
RTN is high impedance, and almost all the internal circuits are disabled. An RDEN of 24.9 kΩ (±1%), presents the
correct signature. It may be a small, low-power resistor because it only sees a stress of about 5 mW. A valid PD
detection signature is an incremental resistance ( ΔV/ΔI ) between 23.75 kΩ and 26.25 kΩ at the PI.
The detection resistance seen by the PSE at the PI is the result of the input bridge resistance in series with the
parallel combination of RDEN and internal VDD loading. The input diode bridge’s incremental resistance may be
hundreds of ohms at the very low currents drawn when 2.7 V is applied to the PI. The input bridge resistance is
partially compensated by the TPS2379's effective resistance during detection.
The type 2 hardware classification protocol of IEEE 802.3at specifies that a type 2 PSE drops its output voltage
into the detection range during the classification sequence. The PD is required to have an incorrect detection
signature in this condition, which is referred to as a mark event (see Figure 21). After the first mark event, the
TPS2379 will present a signature less than 12 kΩ until it has experienced a V(VDD-VSS) voltage below the mark
reset threshold (VMSR). This is explained more fully under Hardware Classification.
8.4.1.4 Hardware Classification
Hardware classification allows a PSE to determine a PD’s power requirements before powering, and helps with
power management once power is applied. Type 2 hardware classification permits high power PSEs and PDs to
determine whether the connected device can support high-power operation. A type 2 PD presents class 4 in
hardware to indicate that it is a high-power device. A type 1 PSE will treat a class 4 device like a class 0 device,
allotting 13 W if it chooses to power the PD. A PD that receives a 2-event class understands that it is powered
from a high-power PSE and it may draw up to 25.5 W immediately after the 80 ms start-up period completes. A
type 2 PD that does not receive a 2-event hardware classification may choose to not start, or must start in a 13
W condition and request more power through the DLL after start-up. The standard requires a type 2 PD to
indicate that it is underpowered if this occurs. Start-up of a high-power PD under 13 W implicitly requires some
form of powering down sections of the application circuits.
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The maximum power entries in Table 1 determine the class the PD must advertise. The PSE may disconnect a
PD if it draws more than its stated Class power, which may be the hardware class or a lower DLL-derived power
level. The standard permits the PD to draw limited current peaks that increase the instantaneous power above
the Table 1 limit, however the average power requirement always applies.
The TPS2379 implements two-event classification. Selecting an RCLS of 63.4 Ω provides a valid type 2
signature. TPS2379 may be used as a compatible type 1 device simply by programming class 0–3 per Table 1.
DLL communication is implemented by the Ethernet communication system in the PD and is not implemented by
the TPS2379.
The TPS2379 disables classification above VCU_OFF to avoid excessive power dissipation. CLS voltage is turned
off during PD thermal limiting or when DEN is active. The CLS output is inherently current-limited, but should not
be shorted to VSS for long periods of time.
Figure 22 shows how classification works for the TPS2379. Transition from state-to-state occurs when
comparator thresholds are crossed (see Figure 19 and Figure 20). These comparators have hysteresis, which
adds inherent memory to the machine. Operation begins at idle (unpowered by PSE) and proceeds with
increasing voltage from left to right. A 2-event classification follows the (heavy lined) path towards the bottom,
ending up with a latched type 2 decode along the lower branch that is highlighted. This state results in a low T2P
during normal operation. Once the valid path to type 2 PSE detection is broken, the input voltage must transition
below the mark reset threshold to start anew.
Idle
Detect
Mark
Reset
Mark
Class
Between
Ranges
Class
Between
Ranges
Class
Between
Ranges
UVLO
Falling
UVLO
Rising
Operating
T2P
open-drain
TYPE 1 PSE
Hardware Class
PoE Startup Sequence
Mark
Class
Between
Ranges
UVLO
Rising
Operating
T2P low
TYPE 2 PSE
Hardware Class
UVLO
Falling
Figure 22. Two-Event Class Internal States
8.4.1.5 Inrush and Start-up
IEEE 802.3at has a start-up current and time limitation, providing type 2 PSE compatibility for type 1 PDs. A type
2 PSE limits output current to between 400 mA and 450 mA for up to 75 ms after power up (applying “48 V” to
the PI) to mirror type 1 PSE functionality. The type 2 PSE will support higher output current after 75 ms. The
TPS2379 implements a 140 mA inrush current, which is compatible with all PSE types. A high-power PD must
limit its converter start-up peak current. The operational current cannot exceed 400 mA for a period of 80 ms or
longer. This requirement implicitly requires some form of powering down sections of the application circuits.
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8.4.1.6 Maintain Power Signature
The MPS is an electrical signature presented by the PD to assure the PSE that it is still present after operating
voltage is applied. A valid MPS consists of a minimum DC current of 10 mA (or a 10 mA pulsed current for at
least 75 ms every 325 ms) and an AC impedance lower than 26.3 kΩ in parallel with 0.05 μF. The AC
impedance is usually accomplished by the minimum operating CBULK requirement of 5 μF. When DEN is used to
force the hotswap switch off, the DC MPS will not be met. A PSE that monitors the DC MPS will remove power
from the PD when this occurs. A PSE that monitors only the ac MPS may remove power from the PD.
8.4.1.7 Start-up and Operation
The internal PoE UVLO (Under Voltage Lock Out) circuit holds the hotswap switch off before the PSE provides
full voltage to the PD. This prevents the downstream converter circuits from loading the PoE input during
detection and classification. The converter circuits will discharge CBULK while the PD is unpowered. Thus V(VDDRTN) will be a small voltage just after full voltage is applied to the PD, as seen in Figure 21. The PSE drives the PI
voltage to the operating range once it has decided to power up the PD. When VVDD rises above the UVLO turnon
threshold (VUVLO-R, approximately 38 V) with RTN high, the TPS2379 enables the hotswap MOSFET with an
approximate 140 mA (inrush) current limit as seen in Figure 23. The CDB pin is active while CBULK charges and
VRTN falls from VVDD to nearly VVSS. Once the inrush current falls about 10% below the inrush current limit, the PD
current limit switches to the operational level (approximately 1000 mA) and CDB is deasserted to allow
downstream converter circuitry to start. The TPS2379 asserts GATE after inrush is complete to enable an
external pass MOSFET if used. In Figure 23, T2P is active because a type 2 PSE is plugged in.
50V/div
VVDD-RTN
10V/div
V(GATE -VSS)
Type 1 PSE
50V/div
VT2P-RTN
10V/div
VCDB-RTN
PI powered
Type 2 PSE
Load enabled using
CDB plus delay
Inrush
100mA/div
IPI
Time: 5ms/div
Figure 23. Power Up and Start
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8.4.1.8 PD Hotswap Operation
IEEE 802.3at has taken a new approach to PSE output limiting. A type 2 PSE must meet an output current
versus time template with specified minimum and maximum sourcing boundaries. The peak output current may
be as high as 50 A for 10 μs or 1.75 A for 75 ms. This makes robust protection of the PD device even more
important than it was in IEEE 802.3-2008.
The internal hotswap MOSFET is protected against output faults and input voltage steps with a current limit and
deglitched (time-delay filtered) foldback. An overload on the pass MOSFET engages the current limit, with VRTNVVSS rising as a result. GATE is pulled down about 300 μs after RTN current reaches the current limit level. If
VRTN rises above approximately 12 V for longer than about 800 μs, the current limit reverts to the inrush value.
The 800 μs deglitch feature prevents momentary transients from causing a PD reset, provided that recovery lies
within the bounds of the hotswap and PSE protection. Figure 24 shows an example of the RTN current profile
during VDD to RTN short circuit when only the internal hotswap MOSFET is used. The hotswap MOSFET goes
into current limit, causing the RTN voltage to increase. Once VRTN exceeds 12V, IRTN which was clamped to the
current limit drops to the level of inrush current limit after 800µs.
VRTN-VSS > 12V
VRTN-VSS
20V/div
VCDB-VSS
20V/div
VGATE -VSS
5V/div
Inrush
500mA/div
I PI
Time: 200us/div
Figure 24. Response to PD Output Short Circuit Without AUX MOSFET
Figure 25 shows an example of the RTN current profile during VDD to RTN short circuit when the external
MOSFET is used. The circuit is depicted in Figure 26. The current will divide between the internal and external
MOSFETs. During the short circuit, the hotswap MOSFET goes into current limit, causing the RTN voltage to
increase. When the internal MOSFET exceeds current limit for about 300 µs, GATE will deassert and shut off the
auxiliary MOSFET. VRTN will rise quickly and the internal MOSFET will go into current limit for approximately 800
µs (after VRTN > about 12 V) and then IRTN which was clamped to the current limit drops into the inrush current
limit.
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V RTN-VSS
20V/div
V RTN-VSS > 12V
V GATE-VSS
5V/div
Inrush
500mA/div
5A/div
IINT
IEXT
Current limit
5A/div
I PI
Time: 200us/div
Figure 25. Response to PD Output Short Circuit With AUX MOSFET
The PD control has a thermal sensor that protects the internal hotswap MOSFET. Conditions like start-up or
operation into a VDD -to-RTN short cause high power dissipation in the MOSFET. An over-temperature shutdown
(OTSD) turns off the hotswap MOSFET and class regulator, which are restarted after the device cools. The
hotswap MOSFET will be re-enabled with the inrush current limit when exiting from an over-temperature event.
Pulling DEN to VSS during powered operation causes the internal hotswap MOSFET to turn off.
The hotswap switch will be forced off under the following conditions:
1. V(DEN –VSS) < VPD_DIS when VVDD – VVSS is in the operational range,
2. PD is over-temperature, or
3. V(DEN – VSS) PoE UVLO falling threshold (about 32 V).
8.4.1.9 CDB and T2P
CDB (converter disable) is an active-low pin that indicates when the internal hotswap MOSFET is inrush limiting.
CDB deasserts when inrush is over and can be used to enable a downstream converter to start up. Common
interfaces to the converter controller include the soft start or enable pins.
T2P (type 2 PSE) is an active-low multifunction pin that indicates if (PSE = Type_2) and (PD current limit ≠
Inrush).
The usage of T2P is demonstrated in Figure 18. When PSE applies and PD observes a type 2 hardware
classification, T2P pin is pulled to RTN as a indication of the type of PSE.
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8.4.1.10 Auxiliary Pass MOSFET Control
The TPS2379 can be used in non-standard applications requiring power significantly above the IEEE802.3at,
type 2 levels. This implementation can be achieved by utilizing all four Ethernet wire pairs and boosting the
TPS2379 hotswap MOSFET operating current limit. Boosting the TPS2379 operating current limit is achieved by
adding an external pass MOSFET to share the total load current with the internal hotswap MOSFET. The
external pass MOSFET is enabled by the GATE pin after the internal hotswap MOSFET inrush is complete. The
GATE pin will deassert if the TPS2379 internal current limit is exceeded in excess of 300 µs.
A comprehensive high power POE design example is discussed in application report Implementing a 60-W, Endto-End PoE System (SLVA498).
8.4.1.11 Using DEN to Disable PoE
The DEN pin may be used to turn the PoE hotswap switch off by pulling it to VSS while in the operational state,
or to prevent detection when in the idle state. A low voltage on DEN forces the hotswap MOSFET off during
normal operation.
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9 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
The TPS2379 has the flexibility to be implemented in IEEE802.3at PDs, Universal Power Over Ethernet (UPOE)
PDs, or high power non-standard PDs. Therefore, it can be used in a wide range applications such as video and
VoIP telephones, multiband access points, security cameras, and pico-base stations.
D1
RDEN
C1
From Spare
Pairs or
Transformers
TPS2379
RT2P
VDD
DEN
CLS
T2P
CDB
VSS
GATE
RTN
RCLS
RBLST
VC
SS
CBULK
DC/DC Converter
From Ethernet
Transformers
9.2 Typical Application
Q1
Figure 26. Typical Application Circuit
9.2.1 Design Requirements
Table 3. TPS2379EVM Electrical and Performance Specifications
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
POWER INTERFACE
Input voltage
Applied to the power pins of connectors J1 or J3
Operating voltage
After start-up
Input UVLO
0
57
V
30
57
V
Rising input voltage at device terminals
40
Falling input voltage
30.5
Detection voltage
At device terminals
1.4
Classification voltage
At device terminals
11.9
Detection signature
Classification current
23
24.9
Class 4
Inrush current limit
Operating current limit
10.1
Internal plus external
V
V
V
kΩ
38
42
mA
100
180
mA
2260
mA
9.2.2 Detailed Design Procedures
Given in Equation 5, RBLST can be calulated using Equation 4
I 2379
1A
R BLST =
R2379 - RQ1 =
0.42W - 0.064W = 0.27W
I L - I 2379
2.26A - 1A
(4)
FMMT493TC can be used for Q2.
22
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9.2.2.1 Input Bridges and Schottky Diodes
Using Schottky diodes instead of PN junction diodes for the PoE input bridges will reduce the power dissipation
in these devices by about 30%. There are, however, some things to consider when using them.
The IEEE standard specifies a maximum backfeed voltage of 2.8 V. A 100 kΩ resistor is placed between the
unpowered pairs and the voltage is measured across the resistor. Schottky diodes often have a higher reverse
leakage current than PN diodes, making this a harder requirement to meet. To compensate, use conservative
design for diode operating temperature, select lower-leakage devices where possible, and match leakage and
temperatures by using packaged bridges.
Schottky diode leakage currents and lower dynamic resistances can impact the detection signature. Setting
reasonable expectations for the temperature range over which the detection signature is accurate is the simplest
solution. Increasing RDEN slightly may also help meet the requirement.
Schottky diodes have proven less robust to the stresses of ESD transients than PN junction diodes. After
exposure to ESD, Schottky diodes may become shorted or leak. Take care to provide adequate protection in line
with the exposure levels. This protection may be as simple as ferrite beads and capacitors.
As a general recommendation, use 1 A or 2 A, 100 V rated discrete or bridge diodes for the input rectifiers.
9.2.2.2 Protection, D1
A TVS, D1, across the rectified PoE voltage per Figure 26 must be used. A SMAJ58A, or equivalent, is
recommended for general indoor applications. Adequate capacitive filtering or a TVS must limit input transient
voltage to within the absolute maximum ratings. Outdoor transient levels or special applications require additional
protection.
9.2.2.3 Capacitor, C1
The IEEE 802.3at standard specifies an input bypass capacitor (from VDD to VSS) of 0.05 μF to 0.12 μF.
Typically a 0.1 μF, 100 V, 10% ceramic capacitor is used.
9.2.2.4 Detection Resistor, RDEN
The IEEE 802.3at standard specifies a detection signature resistance, RDEN between 23.75 kΩ and 26.25 kΩ, or
25 kΩ ± 5%. A resistor of 24.9 kΩ ± 1% is recommended for RDEN.
9.2.2.5 Classification Resistor, RCLS
Select RCLS according to Table 1. For Class 4, choose RCLS = 63.4 Ω.
9.2.2.6 External Boost Circuit
For a PD application requiring current limit of 2.26 A at the PD input (approximately 96 W) and using Q1 FET
BUK7275-100A
RBLST + RQ1
I2379 = IL ´
RBLST + RQ1 + R2379
(5)
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50V/div
Current: 100 mA/div
9.2.3 Application Curves
VVDD-RTN
10V/div
V(GATE -VSS)
Load enabled using
CDB plus delay
Inrush
IPI
Type 1 PSE
VT2P-RTN
10V/div
VCDB-RTN
PI powered
Type 2 PSE
Load enabled using
CDB plus delay
Inrush
100mA/div
VVDD-VSS
Voltage: 10 V/div
50V/div
Class
Mark
Detect
VRTN-VSS
IPI
Time : 50 ms/div
Figure 28. Start-up
Time: 5ms/div
Figure 27. Power Up and Start
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10 Power Supply Recommendations
The TPS2379 will typically be followed by a power supply such as an isolated flyback or active clamp forward
converter or a non-isolated buck converter. The input voltage of the converter should be capable of operating
within the IEEE802.3at recommended input voltage as shown in Table 2.
11 Layout
11.1 Layout Guidelines
The layout of the PoE front end should follow power and EMI/ESD best practice guidelines. A basic set of
recommendations include:
• Parts placement must be driven by power flow in a point-to-point manner; RJ-45, Ethernet transformer, diode
bridges, TVS and 0.1-μF capacitor, and TPS2379.
• All leads should be as short as possible with wide power traces and paired signal and return.
• There should not be any crossovers of signals from one part of the flow to another.
• 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.
• The TPS2379 should be located over split, local ground planes referenced to VSS for the PoE input and to
RTN for the switched output.
• Large copper fills and traces should be used on SMT power-dissipating devices, and wide traces or overlay
copper fills should be used in the power path.
11.1.1 EMI Containment
•
•
•
•
•
•
•
•
•
•
•
•
Use compact loops for dv/dt and di/dt circuit paths (power loops and gate drives)
Use minimal, yet thermally adequate, copper areas for heat sinking of components tied to switching nodes
(minimize exposed radiating surface).
Use copper ground planes (possible stitching) and top layer copper floods (surround circuitry with ground
floods)
Use 4 layer PCB if economically feasible (for better grounding)
Minimize the amount of copper area associated with input traces (to minimize radiated pickup)
Use Bob Smith terminations, Bob Smith EFT capacitor, and Bob Smith plane
Use Bob Smith plane as ground shield on input side of PCB (creating a phantom or literal earth ground)
Use of ferrite beads on input (allow for possible use of beads or 0 ohm resistors)
Maintain physical separation between input-related circuitry and power circuitry (use ferrite beads as
boundary line)
Possible use of common-mode inductors
Possible use of integrated RJ-45 jacks (shielded with internal transformer and Bob Smith terminations)
End-product enclosure considerations (shielding)
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11.2 Layout Example
Figure 29 and Figure 30 show the top and bottom layer and assemblies of the TPS2378EVM-106 as a reference
for optimum parts placement. A detailed PCB layout can be found in the user’s guide of the TPS2378EVM-106.
Figure 29. Recommended Layout Top View
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Layout Example (continued)
Figure 30. Recommended Layout Bottom View
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11.3 Thermal Considerations and OTSD
Sources of nearby local PCB heating should be considered during the thermal design. Typical calculations
assume that the TPS2379 device is the only heat source contributing to the PCB temperature rise. It is possible
for a normally operating TPS2379 device to experience an OTSD event if it is excessively heated by a nearby
device.
11.4 ESD
ESD requirements for a unit that incorporates the TPS2379 device have a much broader scope and operational
implications than are used in TI’s testing. Unit-level requirements should not be confused with reference design
testing that only validates the ruggedness of the TPS2379 device.
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12 Device and Documentation Support
12.1 Documentation Support
12.1.1 Related Documentation
For related documentation see the following:
• Implementing a 60-W, End-to-End PoE System, SLVA498
12.2 Community Resources
The following links connect to TI community resources. Linked contents are provided "AS IS" by the respective
contributors. They do not constitute TI specifications and do not necessarily reflect TI's views; see TI's Terms of
Use.
TI E2E™ Online Community TI's Engineer-to-Engineer (E2E) Community. Created to foster collaboration
among engineers. At e2e.ti.com, you can ask questions, share knowledge, explore ideas and help
solve problems with fellow engineers.
Design Support TI's Design Support Quickly find helpful E2E forums along with design support tools and
contact information for technical support.
12.3 Trademarks
PowerPAD, E2E are trademarks of Texas Instruments.
All other trademarks are the property of their respective owners.
12.4 Electrostatic Discharge Caution
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
12.5 Glossary
SLYZ022 — TI Glossary.
This glossary lists and explains terms, acronyms, and definitions.
13 Mechanical, Packaging, and Orderable Information
The following pages include mechanical, packaging, and orderable information. This information is the most
current data available for the designated devices. This data is subject to change without notice and revision of
this document. For browser-based versions of this data sheet, refer to the left-hand navigation.
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PACKAGE OPTION ADDENDUM
www.ti.com
10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
TPS2379DDA
ACTIVE SO PowerPAD
DDA
8
75
RoHS & Green
NIPDAUAG
Level-2-260C-1 YEAR
-40 to 85
2379
TPS2379DDAR
ACTIVE SO PowerPAD
DDA
8
2500
RoHS & Green
NIPDAUAG
Level-2-260C-1 YEAR
-40 to 85
2379
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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