LTC4292 datasheet(26/40 Pages) AD

26 / 40 page

LTC4291-1/LTC4292

Rev 0

For more information www.analog.com

APPLICATIONS INFORMATION

Auto Mode Maximum PSE Power

In auto mode the LTC4291-1/LTC4292 automatically

detects, classifies and powers all connected valid PDs.

In order to do this, the PSE must be configured for its

maximum power allocation. The maximum power alloca-

tion is a reflection of the power supply and power path

capability. The PWRMD pins must be set appropriately

to reflect the PSE system’s power delivery capabilities.

These pins are sampled at reset.

Table 10. Auto Mode Maximum Delivered Power Capabilities

PWRMD1 PWRMD0

MAX PORT POWER

(SINGLE-SIGNATURE)

MAX PAIRSET POWER

(DUAL-SIGNATURE)

40W

13W

51W

25.5W

62W

25.5W

71.3W

35.6W

POWER CONTROL

The primary function of the LTC4291-1/LTC4292 is to con-

trol power delivery to the PSE port. With the LTC4291-1/

LTC4292, a PSE port is composed of two power chan-

nels; each power channel controls power delivery over a

pairset. Within this section, power delivery and control

are defined per-channel.

The LTC4291-1/LTC4292 delivers power by controlling

the gate drive voltage of an external power MOSFET while

monitoring the current (through an external sense resis-

tor) and the output voltage (across the OUT pin).

The LTC4291-1/LTC4292 connects the VEE power sup-

ply to the PSE port in a controlled manner, meeting the

power demands of the PD while minimizing power dis-

sipation in the external MOSFET and disturbances to the

VEE backplane.

Inrush Control

When commanded to apply power to a port, the

LTC4291-1/LTC4292 ramps up the GATE pin of one or

both channels (as commanded), raising the external

MOSFET gate voltage in a controlled manner.

During a typical inrush, the MOSFET gate voltage will rise

until the external MOSFET is fully enhanced or the channel

reaches the inrush current limit (IINRUSH-2P). IINRUSH-2P is

set automatically by the PSE. When the PSE is applying

4-pair power to a single-signature PD assigned Class 0

to Class 4, IINRUSH-2P is 212.5mA (typical) per channel

(LIMn = 08h). Otherwise, IINRUSH-2P is 425mA (typical)

per channel (LIMn = 80h).

The GATE pin will be servoed if channel current exceeds

IINRUSH-2P, actively limiting current to IINRUSH-2P. When

the GATE pin is not being servoed, the final VGS is 12V

(typical).

During inrush, each powered channel runs a timer (tSTART).

Each powered channel stays in inrush until tSTART expires.

When tSTART expires, the PSE inspects channel voltage

and current. When the PSE is applying power to a PD,

inrush is successful if the channel(s) are drawing current

below IINRUSH-2P, as appropriate per the PD configuration

and Class.

If inrush is not successful, power is removed and the

corresponding tSTART faults are set. Otherwise, the port

or channel, as appropriate, advances to power on and

the programmed current limiting thresholds are used as

described in the Current Limit section.

Port Power Policing

The power policing threshold (PCUT-4P) is monitored on a

per-port basis, up to 128W in 0.5W increments (typical).

When the total output power over a one second moving

average exceeds the specified threshold, power will be

removed from the port and the corresponding tCUT faults

are set.

In particular, the port policing feature may be used to

ensure delivery of PD Class power while staying below

100W Limited Power Source (LPS) requirements.

Current Cutoff and Limit

Each LTC4291-1/LTC4292 port includes two current lim-

iting thresholds (ICUT-2P and ILIM-2P), each with a cor-

responding timer (tCUT and tLIM). Setting the ICUT-2P and

ILIM-2P thresholds depends on several factors: the PD

LTC4292 Datasheet(PDF) 26 Page - Analog Devices

LTC4292 Datasheet(HTML) 26 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Datasheet Download

Link URL