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PDF LTC4267-1 Data sheet ( Hoja de datos )
| Número de pieza | LTC4267-1 | |
| Descripción | Power over Ethernet IEEE 802.3af PD Interface | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC4267-1
Power over Ethernet
IEEE 802.3af PD Interface with
Integrated Switching Regulator
FEATURES
■ Complete Power Interface Port for IEEE 802®.3af
Powered Device (PD)
■ Onboard 100V, UVLO Switch
■ Precision Dual Level Inrush Current Limit
■ Integrated Current Mode Switching Regulator
■ Onboard 25k Signature Resistor with Disable
■ Programmable Classification Current (Class 0-4)
■ Thermal Overload Protection
■ Power Good Signal
■ Integrated Error Amplifier and Voltage Reference
■ Low Profile 16-Pin SSOP Package
U
APPLICATIO S
■ IP Phone Power Management
■ Wireless Access Points
■ Security Cameras
■ 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®4267-1 combines an IEEE 802.3af compliant
Powered Device (PD) interface with a current mode switch-
ing regulator, providing a complete power solution for PD
applications. The LTC4267-1 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 LTC4267-1 provides an
increased operational current limit, maximizing power
available for class 3 applications.
The current mode switching regulator is designed for
driving a 6V rated N-channel MOSFET and features pro-
grammable slope compensation, soft-start, and constant
frequency operation, minimizing noise even with light
loads. The LTC4267-1 includes an onboard error amplifier
and voltage reference allowing use in both isolated and
nonisolated configurations.
The LTC4267-1 is available in a space saving, low profile
16-pin SSOP package.
TYPICAL APPLICATIO
Class 2 PD with 3.3V Isolated Power Supply
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TYPICAL PERFOR A CE CHARACTERISTICS
LTC4267-1
Input Current vs Input Voltage
25k Detection Range
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LTC4267-1
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 LTC4267-1 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 LTC4267-1 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 LTC4267-1 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 ΔV/ΔI 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 LTC4267-1 compensates for
the two series diodes in the signature path by offsetting
the resistance so that a PD built using the LTC4267-1 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 LTC4267-1. 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.
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Figure 2. LTC4267-1 PD Front End Using
Diode Bridges on Main and Spare Inputs
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| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet LTC4267-1.PDF ] | |
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