ltc4269cdkd-2 Linear Technology Corporation, ltc4269cdkd-2 Datasheet - Page 20

• Manufacturer Part Number: ltc4269cdkd-2
• Description: Ieee 802.3at High Power Pd And Synchronous Forward Controller With Aux Support
• Manufacturer: Linear Technology Corporation

LTC4269-2
APPLICATIONS INFORMATION
Shutdown Interface
To corrupt the signature resistance, the SHDN pin can be
driven high with respect to V
SHDN directly to V
Exposed Pad
The LTC4269-2 uses a thermally enhanced DFN12 package
that includes an Exposed Pad. The Exposed Pad should
be electrically connected to the GND pin’s PCB copper
plane. This plane should be large enough to serve as the
heat sink for the LTC4269-2.
Auxiliary Power Source
In some applications, it is desirable to power the PD from
an auxiliary power source such as a wall adapter. Auxiliary
power can be injected into the PD at several locations with
priority chosen between PoE or auxiliary power sources.
These options come with various trade-offs and design
considerations. Contact Linear Technology applications
support for detailed information on implementing custom
auxiliary power sources.
IEEE 802.3AT SYSTEM POWER-UP REQUIREMENT
Under the IEEE 802.3at standard, a PD must operate
under 12.95W in accordance with IEEE 802.3at standard
20
OPTION 2: SHUNT CONFIGURATION FOR ACTIVE HIGH/OPEN COLLECTOR OUTPUT
OPTION 1: SERIES CONFIGURATION FOR ACTIVE LOW/LOW IMPEDANCE OUTPUT
PSE
PSE
TO
TO
–54V
–54V
Figure 8. T2P Interface Examples
V
V
PORTN
PORTN
LTC4269-2
LTC4269-2
PORTN
V
V
PORTP
PORTP
V
T2P
T2P
NEG
.
PORTN
V
V
+
+
R
R
P
P
. If unused, connect
42692 F08
TO PD’s
MICROPROCESSOR
TO PD’s
MICROPROCESSOR
until it recognizes a Type 2 PSE. Initializing PD operation
in 12.95W mode eliminates interoperability issue in case
a Type 2 PD connects to a Type 1 PSE. Once the PD rec-
ognizes a Type 2 PSE, the IEEE 802.3at standard requires
the PD to wait 80ms in 12.95W operation before 25.5W
operation can commence.
MAINTAIN POWER SIGNATURE
In an IEEE 802.3af/at system, the PSE uses the maintain
power signature (MPS) to determine if a PD continues to
require power. The MPS requires the PD to periodically draw
at least 10mA and also have an AC impedance less than
26.25k in parallel with 0.05μF . If one of these conditions
is not met, the PSE may disconnect power to the PD.
Isolation
The 802.3 standard requires Ethernet ports to be electrically
isolated from all other conductors that are user accessible.
This includes the metal chassis, other connectors, and any
auxiliary power connection. For PDs, there are two com-
mon methods to meet the isolation requirement. If there
are any user-accessible connections to the PD, then an
isolated DC/DC converter is necessary to meet the isolation
requirements. If user connections can be avoided, then it
is possible to meet the safety requirement by completely
enclosing the PD in an insulated housing.
Switcher Controller Operation
The LTC4269-2 has a current mode synchronous PWM
controller optimized for control of a forward converter
topology. The LTC4269-2 is ideal for power systems
where very high effi ciency and reliability, low complexity
and cost are required in a small space. Key features of the
LTC4269-2 include an adaptive maximum duty cycle clamp.
An additional output signal is included for synchronous
rectifi er control or active clamp control. A precision 107mV
threshold senses overcurrent conditions and triggers soft-
start for low stress short-circuit protection and control.
The key functions of the LTC4269-2 PWM controller are
shown in the Block Diagrams.
42692f