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PDF LTC4267-3 Data sheet ( Hoja de datos )
| Número de pieza | LTC4267-3 | |
| Descripción | Power over Ethernet IEEE 802.3af PD Interface | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC4267-3
Power over Ethernet
IEEE 802.3af PD Interface with
Integrated Switching Regulator
FEATURES
n Complete Power Interface Port for IEEE 802®.3af
Powered Device (PD)
n Onboard 100V, UVLO Switch
n Constant-Frequency 300kHz Operation
n Precision Dual Level Inrush Current Limit
n Integrated Current Mode Switching Regulator
n Onboard 25k Signature Resistor with Disable
n Programmable Classification Current (Class 0-4)
n Thermal Overload Protection
n Power Good Signal
n Integrated Error Amplifier and Voltage Reference
n Low Profile 16-Pin SSOP or DFN Packages
U
APPLICATIO S
n IP Phone Power Management
n Wireless Access Points
n Security Cameras
n Power over Ethernet
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
DESCRIPTIO
The LTC®4267-3 combines an IEEE 802.3af compliant
Powered Device (PD) interface with a 300kHz current mode
switching regulator, providing a complete power solution
for PD applications. The LTC4267-3 integrates the 25k
signature resistor, classification current source, thermal
overload protection, signature disable and power good
signal along with an undervoltage lockout optimized for
use with the IEEE-required diode bridge. The LTC4267-3
provides an increased operational current limit, maximizing
power available for class 3 applications.
The 300kHz current mode switching regulator provides
higher output power or smaller external size compared
to its lower frequency counterparts. The LTC4267-3 is
designed for driving a 6V rated N-channel MOSFET and
features programmable slope compensation, soft-start,
and constant-frequency operation, minimizing noise even
with light loads. The LTC4267-3 includes an onboard er-
ror amplifier and voltage reference allowing use in both
isolated and nonisolated configurations.
The LTC4267-3 is available in a space saving, low profile
16-pin SSOP or DFN packages.
TYPICAL APPLICATIO
Class 2 PD with 3.3V Isolated Power Supply
PA1133 SBM1040
–48V
FROM
DATA PAIR
+
HD01
–
–48V
FROM
SPARE PAIR
+
HD01
–
VPORTP
PVCC
SMAJ58A
PWRGD
LTC4267-3
NGATE
0.1µF
SENSE
RCLASS
68.1Ω
1%
RCLASS ITH/RUN
VFB
SIGDISA PGND
VPORTN
POUT
10k
+ PVCC +
4.7µF
•
5µF
MIN
320µF
• MIN
10k Si3440
PVCC
0.1Ω
6.8k
BAS516
PS2911
470Ω
22nF
TLV431
3.3V
1.5A
CHASSIS
100k
60.4k
42671 TA01
42673f
1
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TYPICAL PERFOR A CE CHARACTERISTICS
LTC4267-3
Input Current vs Input Voltage
25k Detection Range
0.5
TA = 25°C
0.4
0.3
0.2
0.1
0
0
–2 –4 –6 –8
VPORTN VOLTAGE (V)
–10
42673 G01
Input Current vs Input Voltage
3
EXCLUDES ANY LOAD CURRENT
TA = 25°C
2
1
0
–40
–45 –50 –55
VPORTN VOLTAGE (V)
–60
42673 G04
Power Good Output Low Voltage
vs Current
4
TA = 25°C
3
2
1
0
0 2 4 6 8 10
CURRENT (mA)
42673 G07
Input Current vs Input Voltage
Classification Range
50
TA = 25°C
CLASS 4
40
30 CLASS 3
20
10
0
0
CLASS 2
CLASS 1
CLASS 0
–10 –20 –30 –40
VPORTN VOLTAGE (V)
–50 –60
42673 G02
Signature Resistance
vs Input Voltage
28
27
RESISTANCE
=
∆V
∆I
=
V2
I2
–
–
V1
I1
DIODES: S1B
TA = 25°C
IEEE UPPER LIMIT
26
25 LTC4267-3 + 2 DIODES
24
LTC4267-3 ONLY
23
IEEE LOWER LIMIT
22
V1: –1
V2: –2
–3 –5 –7
–4 –6 –8
VPORTN VOLTAGE (V)
–9
–10
42673 G05
POUT Leakage Current
120
VIN = 0V
TA = 25°C
90
60
30
0
0 20 40 60
POUT PIN VOLTAGE (V)
42673 G08
Input Current vs Input Voltage
12.0
CLASS 1 OPERATION
11.5
11.0
85°C
10.5
–40°C
10.0
9.5
9.0
–12
–14 –16 –18 –20
VPORTN VOLTAGE (V)
–22
42673 G03
Normalized UVLO Threshold
vs Temperature
2
APPLICABLE TO TURN-ON
AND TURN-0FF THRESHOLDS
1
0
–1
–2
–40 –20
0 20 40 60
TEMPERATURE (°C)
80
42673 G06
Current Limit vs Input Voltage
400
85°C
– 40°C
HIGH CURRENT MODE
300
200
100
–40
LOW CURRENT MODE
85°C
– 40°C
–45 –50 –55
VPORTN VOLTAGE (V)
–60
42673 G09
42673f
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LTC4267-3
APPLICATIO S I FOR ATIO
Series Diodes
The IEEE 802.3af-defined operating modes for a PD refer-
ence the input voltage at the RJ45 connector on the PD.
The PD must be able to accept power of either polarity
at each of its inputs, so it is common to install diode
bridges (Figure 2). The LTC4267-3 takes this into account
by compensating for these diode drops in the threshold
points for each range of operation. A similar adjustment
is made for the UVLO voltages.
Detection
During detection, the PSE will apply a voltage in the
range of –2.8V to –10V on the cable and look for a 25k
signature resistor. This identifies the device at the end of
the cable as a PD. With the terminal voltage in this range,
the LTC4267-3 connects an internal 25k resistor between
the VPORTP and VPORTN pins. This precision, temperature
compensated resistor presents the proper signature to
alert the PSE that a PD is present and desires power to be
applied. The internal low-leakage UVLO switch prevents
the switching regulator circuitry from affecting the detec-
tion signature.
The LTC4267-3 is designed to compensate for the voltage
and resistance effects of the IEEE required diode bridge.
The signature range extends below the IEEE range to ac-
commodate the voltage drop of the two diodes. The IEEE
specification requires the PSE to use a ∆V/∆I measurement
technique to keep the DC offset of these diodes from af-
fecting the signature resistance measurement. However,
the diode resistance appears in series with the signature
resistor and must be included in the overall signature
resistance of the PD. The LTC4267-3 compensates for
the two series diodes in the signature path by offsetting
the resistance so that a PD built using the LTC4267-3 will
meet the IEEE specification.
In some applications it is necessary to control whether
or not the PD is detected. In this case, the 25k signature
resistor can be enabled and disabled with the use of the
SIGDISA pin (Figure 3). Disabling the signature via the
SIGDISA pin will change the signature resistor to 9k
(typical) which is an invalid signature per the IEEE 802.3af
specification. This invalid signature is present for PD input
voltages from –2.8V to –10V. If the input rises above –10V,
the signature resistor reverts to 25k to minimize power
dissipation in the LTC4267-3. To disable the signature,
tie SIGDISA to VPORTP. Alternately, the SIGDISA pin can
be driven high with respect to VPORTN. When SIGDISA is
high, all functions of the PD interface are disabled.
RJ45 TX+
1
TX–
2 RX+
3
POWERED DEVICE (PD)
INTERFACE
AS DEFINED
BY IEEE 802.3af
RX–
6
SPARE+
4
5
7 SPARE–
8
T1
TO PHY
BR2
BR1
VPORTP
LTC4267-3
D3
VPORTN
42673 F02
Figure 2. LTC4267-3 PD Front End Using
Diode Bridges on Main and Spare Inputs
42673f
11
11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC4267-3.PDF ] | |
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