LTC4292 datasheet(14/22 Pages) AD

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Data Sheet

LTC9105

THEORY OF OPERATION

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Rev. A | 14 of 22

Table 9. Power Priority Modes and Behaviors (Continued)

Mode

priority

stdby_dis

MOSFET State

Description

Cold Secondary

OFF

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

primary input drops, as sensed at AUX pin, the MPS current is removed

and the LTC9105’s MOSFET remains disabled, enabling a port power

cycle.

AUTOMATIC MAINTAIN POWER SIGNATURE

A PSE removes power when a connected PD no longer draws

a minimum amount of current, known as the maintain power

signature (MPS) current. The LTC9105 implements an Automatic

MPS feature, which detects when the current falls below the DC

MPS threshold (20mA typ., RSNSP-SNSN = 50mΩ) and automatically

generates a current pulse to maintain PoE power. This feature is

enabled by default at power-up.

Figure 18 shows an example of Automatic MPS timing. The

LTC9105's MPS state machine works on a 100ms (typ.) time base.

An MPS pulse is issued when the average current over a 100ms

(typ.) period drops below the DC MPS threshold. The MPS pulse

current has an amplitude greater than 20mA and a duration of

100ms (typ.). No pulses are issued in the following 200ms (typ.).

The LTC9105 stops drawing MPS pulse current once the average

current is above the DC MPS threshold.

Automatic MPS is disabled in one of two conditions. First is when

the inrush timer expires, that is, if power up of the load does not

complete within 93.75ms (typ.). And second, when stdby_dis is

set to 1 and an auxiliary supply is not present (when aux_status

reports 0; when AUX < VAUX_TH).

Figure 18. MPS Timing

ANALOG-TO-DIGITAL CONVERTERS (ADC)

The LTC9105 has two first order delta sigma analog-to-digital

converters (ADCs) that simultaneously measure application current

and supply voltage. The converters operate at an over sample rate

(OSR) of 2.5MHz (typ.) and complete a full conversion every 100ms

(typ.). First order delta sigma converters provide superior perform-

ance for power monitoring applications because the measured sig-

nals are averaged with even weight across the conversion period.

Delta-sigma converters produce a mean average by integrating

the area under the signal curve across the conversion period. Fur-

thermore, simultaneous conversion of voltage and current allows

high accuracy power calculations, ensuring the voltage and current

factors are sampled during the same period.

Measuring application power at the PD controller input captures

the DC/DC conversion losses missed by traditional power monitors

after the converter. Analog Devices recommends using the LT4321

ideal diode bridge for minimal bridge voltage drop and increased

telemetry accuracy with respect to the RJ45 input (see PoE Input

Bridge in External Component Selection).

VIN Readback

When configured to report voltage telemetry by clearing the

padc_en bit, the LTC9105 continuously measures the VIN volt-

age with a dedicated A/D converter. This measurement is fully

synchronized to the current measurement and can continue to

monitor until VIN drops below VOFF. This telemetry is updated every

100ms.

When the padc_data bit reports '0', the voltage_power field

contains voltage readings.

Power Readback

When configured to report power telemetry by setting the padc_en

bit, the LTC9105 continuously measures the PD power by inter-

nally multiplying port voltage and current, and reports the power

telemetry to the voltage_power field. This measurement is fully

synchronized to the current and voltage measurements and can

continue to monitor until VIN drops below VOFF. This telemetry is

updated every 100ms.

When the padc_data bit reports '1', the voltage_power field

contains power readings.

Current Readback

The LTC9105 measures the voltage across the SNSP and SNSN

pins through a 50mΩ sense resistor. This measurement is fully

synchronized to the power/voltage measurements and can continue

to monitor until VIN drops below VOFF. Current telemetry is reported

to the current field. This telemetry is updated every 100ms.

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