LT8607 datasheet(15/20 Pages) AD

15 / 20 page

LTC9111

Rev. 0

For more information www.analog.com

The low-side polarity correcting MOSFETs should be sized

appropriately for the current level of the application. The

drive voltage at the LGATE1 and LGATE2 pins is compat-

ible with modern standard-level power MOSFETs. These

MOSFETs do not experience high voltage and high current

simultaneously, so SOA performance is not critical.

REMOTE SENSE PINS

The SNS1 and SNS2 pins allow the LTC9111 to sense

the port voltage outside the power rectifier for SCCP and

accurate state machine voltage thresholds.

For most applications, the pins can be connected on the

application side of the power coupling network. This pre-

vents parasitic capacitance from corrupting the data sig-

nal and filters out excessive ringing that can be observed

directly at the connector during classification. A small

snubber between the SNS pins (C3/R2, Figure 9) pre-

vents high-frequency ringing when the rectifying diodes

are reverse biased during SCCP logic transitions.

SCCP PULL-DOWN MOSFET

The LTC9111 uses external MOSFETs (M2/M3, Figure 9)

to pull-down the port voltage during classification in

order to transmit a logic low. The use of two external

MOSFETs allows successful classification regardless of

input connector polarity configuration. The drive voltage

at the SCCP pin is appropriate for low cost, logic-level

discrete MOSFETs.

While inexpensive, commodity external MOSFETs such as

the BSS123 will generally suffice for this purpose, some

care may be needed to ensure that the IEEE 802.3cg stan-

dard requirements are met while protecting the compo-

nents from overstress.

IEEE 802.3cg requires that a PD sink 30mA with a connec-

tor voltage of 0.8V while signaling a logic low. The SCCP

pull-down must be connected in series with the power

coupling network magnetics to prevent corruption of the

data link by parasitic capacitances. As such, the voltage

developed over the parasitic resistance of the coupling

network must be included when calculating the effective

pull-down strength.

Depending on the coupling network resistance and induc-

tance, the peak transient current through the pull-down

MOSFET may exceed the absolute maximum rating of

some low-cost external MOSFETs. A resistor in series with

the source of the SCCP pull-down can be used to reduce

the peak current, provided it does not compromise the

effective DC pull-down strength.

To meet the 0.8V/30mA requirement, the total SCCP

pull-down path resistance needs to be less than 26.6Ω.

This resistance includes the coupling network parasitic

resistance, MOSFET RDS(ON), and any limiting resistors.

Variations in the external MOSFET parameters should

be accounted for when selecting an appropriate discrete

MOSFET.

PD LOAD DYNAMICS

In order to protect PHY data integrity and avoid EMI

issues, IEEE 802.3cg establishes specific limitations for

input current dI/dt, input voltage dV/dt, and ripple voltage

amplitude measured at the PI. The coupling network and

PD capacitance form a filter between the application and

the connector, so that AC dynamics in the application are

attenuated at the connector. Additional filtering elements

may be required in applications with large dynamic loads.

Filtering can be achieved with bulk capacitance at the

hot swap output, a higher-order series filter network, or

capacitance at the output of a step-down converter ser-

vicing the application load, as shown in Figure 10.

In addition to filtering high frequency noise generated by,

for example, switching in the application circuitry, these

elements play an important role in case of large transient

load steps. Because of the large inductors in the power

coupling network, the current provided by the PSE can-

not change instantaneously. The application must have

enough bulk capacitance to prevent the line voltage from

browning out during a load step.

The optimal configuration of these filtering elements

depends on the specific application—how much high-fre-

quency noise it generates, and the magnitude and speed

of the worst-case change in load during operation.

APPLICATIONS INFORMATION

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