LT8607 datasheet(16/20 Pages) AD

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LTC9111

Rev. 0

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ILOAD

LTC9111

GATE

GND

VREG BULK CAP

BUCK

CONVERTER

2ND-ORDER FILTER

VPDBYPASS

9111 F10

Figure 10. Filtering and Bypass for Dynamic Loads

AUXILIARY POWER

An auxiliary DC supply can be connected at the LTC9111

OUT pin or down-stream DC/DC converter output with a

diode, bypassing the LTC9111 features. In this case, the

auxiliary supply is responsible for inrush.

FAULT PROTECTION

PD Overload Condition

In the event of a PD current overload condition, it is antic-

ipated that the PSE will limit the port current as mandated

by IEEE 802.3cg, causing the voltage at the PD input to

collapse. When the PD input voltage drops below the VOFF

threshold, the LTC9111 will disable the external MOSFET

by pulling the GATE pin to the OUT pin and thus isolate

the overload from the cable.

PD Input Short

If the inputs of the PD are shorted together after appli-

cation of power, the high-side polarity correction diodes

prevent the PD bulk capacitor from backfeeding through

the coupling magnetics, thus protecting the LTC9111. A

voltage clamp such as a TVS diode may be necessary to

protect the PHY coupling capacitors and magnetics.

PD Load Dump

If the application produces a large load dump (e.g., the PD

current is quickly reduced to zero from a large value), the

input voltage to the LTC9111 may overshoot as the power

coupling network inductors discharge. The LTC9111 pro-

vides a large transient voltage capability of 100V in order

to accommodate such transients.

If other elements in the system do not have margin to

cover such transients, additional application bulk capac-

itance can absorb the energy from the charged induc-

tance, limiting the voltage stress experienced by the

application circuitry.

PD Disconnect

If a compliant PD is disconnected from a PSE, the 802.3cg

standard requires specific limits on the energy that can be

back-fed at the connector. In the recommended applica-

tion schematic, the high-side rectifier diodes isolate the

PD bulk capacitance from the connector, guaranteeing

compliance with this specification.

PD Connected to Voltage Source

The LTC9111 is designed primarily for connection to an

802.3cg complaint, classifying PSE. When a PD is pow-

ered directly by an auxiliary power supply, the state will

advance to the MDI_POWER2 state and enable the appli-

cation, provided the supply voltage exceeds VON for the

configured class. Note that the LTC9111 does not imple-

ment current limit or circuit breaker functionality.

OV Protection for 24V Classes

When the LTC9111 is configured for Class 10 through 12

(i.e., CLASSV pin is tied low), an overvoltage protection

feature is enabled to protect downstream elements in the

application from exposure to excessive voltage. When

the input voltage exceeds VOV (40V max), the gate of the

external hot swap MOSFET will be turned off, regardless

of the port state.

APPLICATIONS INFORMATION

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