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Data Sheet

LTC9105

THEORY OF OPERATION

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Rev. A | 13 of 22

DUAL-INPUT PRIORITIZATION

The LTC9105 supports dual-input power path prioritized solutions.

The first input is a primary LTC9105 and the second input may be

either an auxiliary supply (see Figure 17) or a secondary LTC9105.

The LTC9105 targets seamless switchover topologies, which only

draw power from one input at a time, switching over to the secon-

dary input when power is removed from the primary input.

Details on PD/auxiliary seamless switchover are provided in this

data sheet. Contact Analog Devices for information and support

regarding dual PD seamless switchover, to enable hitless failover

between dual-port PoE supplies.

LTC9105 systems enable three power priority modes, as shown in

Table 9. An LTC9105 configured in Primary mode provides power to

the load. An LTC9105 configured in Hot Secondary mode enables

seamless switchover, providing power to the load when power is

removed from the first input. An LTC9105 may be ineligible to

provide seamless switchover due to insufficient power allocation; in

this case, an LTC9105 may be configured in Cold Secondary mode

to prevent seamless switchover when power is removed from the

first input, forcing a system reboot and power allocation refresh.

If the AUX pin is low (AUX < VAUX_TH) at LTC9105 power on,

the LTC9105 sets its priority bit and clears its stdby_dis bit,

entering Primary mode.

If the AUX pin is high (AUX > VAUX_TH) at LTC9105 power on, the

LTC9105 automatically clears its priority and stdby_dis bits,

entering Hot Secondary mode.

In Hot Secondary mode, the LTC9105 will automatically transition to

Primary mode when AUX pin goes low. This seamless switchover

ensures power is maintained at the load.

In Cold Secondary mode, the LTC9105 automatically removes MPS

when the AUX pin goes low. This behavior enables a port power

cycle through the IEEE DC MPS feature to force a system reboot

and power allocation refresh. Cold Secondary mode is used to

ensure a higher power primary input does not fail-over to a lower

power secondary input. Applications implementing Cold Secondary

mode must take care to reduce total current below the minimum

IEEE DC MPS threshold (4mA) to meet IEEE requirements for

power removal.

Current is limited by the PSE during a seamless switchover event.

The switchover threshold from primary to secondary is set using an

external AUX circuit. The switchover threshold should be set just

under the VPORT_PD-2P,MIN threshold for the application's requested

PoE Class as specified in IEEE 802.3bt standard. The IEEE stand-

ard specifies VPORT_PD-2P at the RJ-45 interface. Remember to

consider any extra voltage drops due to diode forward voltage or

resistance between the RJ-45 connector and the AUX circuit.

Figure 17. LTC9105 with Dual-Input Auxiliary Circuit

Table 9. Power Priority Modes and Behaviors

Mode

priority

stdby_dis

MOSFET State

Description

Primary

MPS current is drawn automatically if load current drops below MPS.

Hot Secondary

OFF

MPS current is drawn automatically, to keep the PoE input powered.

If the primary input drops, as sensed at AUX pin, the secondary input

automatically becomes the primary input by enabling the LTC9105’s

MOSFET and setting the priority bit.

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