Datasheet

LTC4274A/LTC4274C
17
4274acfc
OPERATION
When a PD is detected, the PSE optionally looks for a
classification signature that tells the PSE the maximum
power the PD will draw. The PSE can use this information
to allocate power among several ports, police the current
consumption of the PD, or to reject a PD that will draw
more power that the PSE has available. For a 802.3af PSE,
the classification step is optional; if a PSE chooses not to
classify a PD, it must assume that the PD is a 13W (full
802.3af power) device.
New in 802.3at
The newer 802.3at standard supersedes 802.3af and brings
several new features:
• A PD may draw as much as 25.5W. Such PDs (and the
PSEs that support them) are known as Type 2. Older
13W 802.3af equipment is classified as Type 1. Type 1
PDs will work with all PSEs; Type 2 PDs may require
Type 2 PSEs to work properly. The LTC4274A/LTC4274C
is designed to work in both Type 1 and Type 2 PSE de-
signs, and also supports non-standard configurations
at higher power levels.
• The Classification protocol is expanded to allow Type 2
PSEs to detect Type 2 PDs, and to allow Type 2 PDs to
determine if they are connected to a Type 2 PSE. Two
versions of the new Classification protocol are avail-
able: an expanded version of the 802.3af Class Pulse
protocol, and an alternate method integrated with the
existing LLDP protocol (using the Ethernet data path).
The LTC4274A/LTC4274C fully supports the new Class
Pulse protocol and is also compatible with the LLDP
protocol (which is implemented in the data communica-
tions layer, not in the PoE circuitry).
• Fault protection current levels and timing are adjusted
to reduce peak power in the MOSFET during a fault;
this allows the new 25.5W power levels to be reached
using the same MOSFETs as older 13W designs.
Extended Power LTPoE
++
The LTC4274A adds the capability to autonomously deliver
up to 90W of power to the PD. LTPoE
++
PDs may forego
802.3 LLDP support and rely solely on the LTPoE
++
Physi-
cal Classification to negotiate power with LTPoE
++
PSEs;
this greatly simplifies high-power PD implementations.
LTPoE
++
classification may be optionally enabled for the
LTC4274A by setting both the High Power Enable and
LTPoE
++
Enable bits.
The higher levels of LTPoE
++
delivery impose additional
layout and component selection constraints. The LTC4274A
is offered in four power levels (-1, -2, -3, and -4) which
allows the AUTO pin mode LTC4274A to autonomously
power up to supported power levels. If the AUTO pin is
high, internal circuitry determines the maximum deliver-
able power. PDs requesting more than the available power
limits are not powered.
Table 1. LTPoE
++
Auto Pin Mode Maximum Delivered
Power Capabilities
PART PAIRS PD POWER
LTC4274A-1 4 38.7W
LTC4274A-2 4 52.7W
LTC4274A-3 4 70W
LTC4274A-4 4 90W