LTC9111

14

Rev. 0

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APPLICATIONS INFORMATION

their nominal capacitance when biased at higher voltages.

If the effective capacitance is reduced under DC bias, the

LC tank formed by the power coupling magnetics and the

bypass capacitor will be resonant.

INPUT POLARITY CORRECTION

IEEE 802.3cg requires polarity insensitive PDs to facili-

tate fool-proof field installation with screw terminals. The

LTC9111 drives external MOSFETs (M4/M5, Figure 9) for

polarity correction on the low-side and relies on external

Schottky diodes (D1/D2) to rectify the high-side voltage.

Active low-side rectification minimizes power losses and

also helps overcome circuit start-up challenges posed by

the constraints of the standard.

When the port is first initialized, a compliant PoDL PSE

will bias the port at a voltage of VSLEEP, which may be

as low as 3.1V. The voltage available to bias the PD con-

troller is even lower, due to the drop (VD(Schottky)) across

the high-side rectifying diodes and the body diode of the

low-side MOSFET. The LTC9111 is guaranteed to enhance

the low-side rectifying MOSFET at startup, as long as the

voltage between IN and GND is greater than 1.6V. Once

the low-side rectifying MOSFET is on, the voltage at the

input to the LTC9111 increases according to Equation 2.

VIN = VSLEEP – VD(Schottky) – I(PORT) • RDS(ON)

(2)

Since port currents are in the mA range while the port

is in the WAKEUP and CLASSIFICATION states, the drop

over the external MOSFET is expected to be very small. In

order to ensure that the LTC9111 provides a valid wakeup

signature, the voltage at the IN pin should be at least 2.3V

in the WAKEUP state. The drop across a traditional silicon

diode may be too high, especially at cold temperatures.

For this reason, Schottky diodes are recommended for

high-side rectification, even in cases where power dis-

sipation is not critical. Since the full application current

flows through the high-side Schottky diodes once the port

is powered up, they must meet the current and power-dis-

sipation requirements of the application.

C1

R1

C2

LTC9111

IN

GND

SCCP

VPD1

VPD2

HIGH-SIDE POWER

RECTIFICATION

LOW-SIDE POWER

RECTIFICATION

SCCP SIGNALING

POWER

COUPLING

NETWORK

SENSE

SNUBBER

INPUT

SNUBBER

LGATE1

LGATE2

REMOTE SENSE

SNS1

SNS2

D1

D2

M2

M3

M4

M5

C3

R2

R3

R4

9111 F09

Figure 9. External Components for Power Rectification, Sensing, and SCCP