ltc4259a Linear Technology Corporation, ltc4259a Datasheet - Page 27

• Manufacturer Part Number: ltc4259a
• Description: Quad Ieee 802.3af Power Over Ethernet Controller With Ac Disconnect
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
System Software Strategy
Control of the LTC4259A hinges on one decision, the
LTC4259A’s operating mode. The three choices are de-
scribed under Operating Modes. In Auto mode the
LTC4259A can operate autonomously without direction
from a host controller. Because LTC4259As running in
Auto mode will power every valid PD connected to them,
the PSE must have 15.4W/port available. To reduce the
power requirements of the –48V supply, PSE systems
can track power usage, only turning on ports when
sufficient power is available. The IEEE describes this as a
power allocation algorithm and places two limitations: the
PSE shall not power a PD unless it can supply the
guaranteed power for that PD’s class (see Table 2) and
power allocation may not be based solely on a history of
each PD’s power consumption. In order for a PSE to
implement power allocation, the PSE’s processor/con-
troller must control whether ports are powered—the
LTC4259A cannot be allowed to operate in Auto mode.
Semiauto mode fits the bill as the LTC4259A automati-
cally detects and classifies PDs, then makes this informa-
tion available to the host controller, which decides to
apply power or not. Operating the LTC4259A in Manual
mode also lets the controller decide whether to power the
ports but the controller must also control detection and
classification. If the host controller operates near the limit
of its computing resources, it may not be able to guide a
Manual mode LTC4259A through detect, class and port
turn-on in less than the IEEE mandated maximum of
950ms.
In a typical PSE, the LTC4259As will operate in Semiauto
mode as this allows the controller to decide to power a
port without unduly burdening the controller. With an
interrupt mask of F4h, the LTC4259A will signal to the
host after it has successfully detected and classed a PD,
at which point the host can decide whether enough power
is available and command the LTC4259A to turn that port
on. Similarly, the LTC4259A will generate interrupts when
a port’s power is turned off. By reading the LTC4259A’s
interrupt register, the host can determine if a port was
turned off due to overcurrent (t
because the PD was removed (Disconnect event). The
host then updates the amount of available power to reflect
U
U
START
W
or t
ICUT
U
faults) or
the power no longer consumed by the disconnected PD.
Setting the MSB of the interrupt mask causes the LTC4259A
to communicate fault conditions caused by failures within
the PSE, so the host does not need to poll to check that the
LTC4259As are operating properly. This interrupt driven
system architecture provides the controller with the final
say on powering ports at the same time, minimizing the
controller’s computation requirements because inter-
rupts are only generated when a PD is detected or on a
fault condition.
The LTC4259A can also be used to power older powered
Ethernet devices that are not 802.3af compliant and may
be detected with other methods. Although the LTC4259A
does not implement these older detection methods auto-
matically, if software or external circuitry can detect the
noncompliant devices, the host controller may command
the LTC4259A to power the port, bypassing IEEE compli-
ant detection and classification and sending power to the
noncompliant device.
LOGIC LEVEL SUPPLY
In additon to the 48V used to source power to each port,
a logic level supply is required to power the digital portion
of the LTC4259A. To simplify design and meet voltage
isolation requirements, the logic level supply can be
generated from the isolated – 48V supply. Figure 21
shows an example method using an LT
a –48V to 3.3V current mode supply. This boost con-
verter topology uses the LT1619 current mode controller
and a current mirror which reflects the 3.3V output
voltage to the –48V rail, improving the regulation toler-
ance over the more traditional large resistor voltage
divider. This approach achieves high accuracy with a
transformerless design.
IEEE 802.3af COMPLIANCE AND EXTERNAL
COMPONENT SELECTION
The LTC4259A is designed to control power delivery in
IEEE 802.3af compliant Power Sourcing Equipment (PSE).
Because proper operation of the LTC4259A may depend
on external signals and power sources, like the –48V
supply (V
EE
) or the OSCIN oscillator source, external
LTC4259A
®
1619 to control
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