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LTC4263

4263fe

APPLICATIONS INFORMATION

Power Management

The LTC4263 includes a power management feature

allowing simple implementation of power management

across multiple ports driven by a single power supply. The

PWRMGT pins of all LTC4263 devices are tied together

along with an RC network to prevent over-allocation of

power in a multi-port system.

Immediately following classiﬁcation, the LTC4263 performs

a power management check to ensure power is available

to supply the newly classed PD. The allocated power is

represented by the voltage on the shared PWRMGT node

and the LTC4263 checks the allocated power by measur-

ing this voltage. If the PWRMGT voltage is less than 1V,

there is power available and the power needs of the new

PD are added to the already allocated power on the node.

To allocate power, a current proportional to the power

needs for the new PD is sourced out of the PWRMGT pin

(Table 2).

Table 2. LTC4263 Power Management

IEEE 802.3af

CLASS

PSE OUTPUT

POWER REQUIRED

LTC4263 PWRMGT

CURRENT

0, 3, 4

15.4W

–72.3μA

–32.8μA

–18.8μA

For multiple LTC4263s implementing power management,

the PWRMGT pins are connected together and to a RC

network connected to VSSasshowninFigure5.Thevalueof

RPM represents the full load output capability of the system

power supply (PFULL_LOAD). Select a 1% resistor to set the

full load output power using the following formula:

FULL LOAD

213 Ω •

The LTC4263 power management uses pulse width

modulation to set the power requirements of each PD.

Capacitor CPM is used as a lowpass ﬁlter to generate the

average power requirement for all PDs in the system. Set

CPM to 1μF.

If power management is not used, tie PWRMGT to VSS.

When additional current is added to the PWRMGT node,

the voltage rises toward the 1V threshold. After adding

current, the LTC4263 veriﬁes that the power supply is not

over-allocated by verifying the node voltage remains below

1V. If the voltage is below 1V, the LTC4263 proceeds to

power the port. If over 1V, the current is removed from

the node, port powering is aborted, and the LTC4263 goes

back into detection mode.

Figure 5. PWRMGT Pin Connections

PWRMGT

VSS

LTC4263

PWRMGT

VSS

LTC4263

PWRMGT

VSS

LTC4263

PWRMGT

VSS

LTC4263

4263 F05

CPM

1μF

RPM

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