Linear Technology LTC4267 Manuale utente

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 859

MULTIPLE OUTPUT PD

LTC4267

DESCRIPTION

Demonstration circuit 859 is a Multiple Output PD fea-

turing the LTC4267. The board provides a complete

IEEE 802.3af power device (PD) interface and isolated

power supply solution for use in Power over Ethernet

(PoE) applications. It generates 1.8V @ 2.5A, 2.5V @

1.5A and 3.3V @ 0.5A.

The LTC4267 integrates the 25k

Ω

signature resistor,

classification current source, thermal overload protec-

tion, signature disable and power good signal along with

an undervoltage lockout optimized for use with the IEEE

required diode bridge. The precision dual level input

current limit allows the LTC4267 to charge load capaci-

tors and interface with legacy PoE systems.

The LTC4267 combines the above features with a cur-

rent mode switching controller designed for driving a 6V

rated N-channel MOSFET. It features programmable

slope compensation, soft-start, and constant frequency

operation, minimizing electrical noise even with light

loads.

Design files for this circuit board are available. Call

the LTC factory.

, LTC and LT are registered trademarks of Linear Technology Corporation.

Table 1.

Performance Summary (T

= 25°C)

PARAMETER CONDITION VALUE

Turn-on Voltage Input from PSE -36V maximum

Maximum Turn-off Voltage Input from PSE, PD switch turns off -32V

Power Converter Input operating range V

OUT

=3.3V, I

OUT

= 0 to 2.6A -37V to -57V

Maximum Input Current Input from PSE, PD high level current limit 375 mA, typical

Maximum Output Current

OUT1

=1.8V

OUT2

=2.5V

OUT3

=3.3V

2.5A

1.5A

0.5A

Output Voltage V

=48VDC from PSE

1.79V, typical

2.5V, typical

3.3V, typical

Output Regulation

OUT1

= 1.8V, V

= -37 to -57VDC, zero to full load

OUT2

= 2.5V, V

= -37 to -57VDC, zero to full load

OUT3

= 3.3V, V

= -37 to -57VDC, zero to full load

+ 5%

+ 2%

+ 5%

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 859

MULTIPLE OUTPUT PD

QUICK START PROCEDURE

Demonstration circuit 859 is easy to set up to evaluate

the performance of the LTC4267. For proper equipment

setup, refer to figure 1 and follow the procedure below:

With the power source to the PSE turned off, con-

nect the input power supply to the board through the

J1 filtered Ethernet connector.

In addition to a PSE, the DC859 board can be pow-

ered by an alternate input power supply through the

V+ (TP5) and VPORTN (TP14) terminals. Do not

connect more than one power source.

Set the SIGNATURE jumper (JP1) to the ENABLE

position.

Turn on the PSE or alternate input power supply and

increase the voltage until the power converter turns

on. Be careful not to exceed 57VDC. NOTE: Make

sure that the input voltage does not exceed 57VDC.

If a higher voltage is required, power components

with higher voltage ratings should be used.

Verify proper classification and signature detection.

Check the output voltages. They should be as listed

in table 1. If there is no output, temporarily discon-

nect the load to make sure that the load is not set

too high.

Once the proper output voltage is established, adjust

the load current within the appropriate range and

observe the output regulation, ripple voltage, effi-

ciency and other parameters.

OPERATION

Demonstration circuit 859 interfaces with a customer’s

Power-Over-Ethernet test setup per Figure 1. The front

end of the demo circuit implements the required

Ethernet input interface transformer coupling and

common-mode termination through the integrated

connector J1. The demonstration circuit is set up to

allow data to pass in and back out of the demo circuit

while the DC859 performs IEEE 802.3af interface func-

tions. The Power Sourcing Equipment (PSE) is con-

nected to J1 and the PHY is optionally connected to

J2.

The PD is required to have 0.1uF of capacitance during

detection; this is provided by C2. It is also required to

have at least 5uF of capacitance after the in-rush cir-

cuit, provided by capacitors C1 and C9.

This demo circuit allows detection and power classifi-

cation of the PD per the IEEE 802.3af specification.

During the detection process of a PD, the LTC4267

displays the proper 25k

Ω

signature resistor. Signa-

ture detection may be disabled, if so desired, by set-

ting the SIGNATURE jumper (JP1) to the DISABLE po-

sition. If signature classification is disabled, all inter-

face functions of the LTC4267 are disabled. Signature

detection, classification and the internal power

MOSFET switch are all disabled.

Classification is programmed by the selection of a sin-

gle external resistor, RClass, connected to the RCLASS

pin on the LTC4267.

After detection and classification, the PD is powered

up when the input voltage exceeds the LTC4267 turn-

on under-voltage lock out (UVLO) through a dual-level

current-limited power switch. While the voltage be-

tween POUT and VPORTN is above the Power Good

trip point, the amperage through the power switch is

held below the low-level current limit. When the volt-

age between POUT and VPORTN falls below the Power

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 859

MULTIPLE OUTPUT PD

Good trip point, the Power Good signal goes active

low and the amperage through the power switch is

held below the high-level current limit.

For the PD to remain powered on, it must present to

the PSE both AC and DC components of the Maintain

Power Signature (MPS). The PD must hold the DC

MPS by drawing at least 10mA or the PSE may dis-

connect power. The DC859 demo board does not im-

plement a discrete minimum load. The synchronous

switching design satisfies the requirement.

The synchronous Flyback converter operates at a typi-

cal switching frequency of 200kHz, controlled by the

current mode controller portion of the LT4267. Gal-

vanic isolation is achieved through transformer T1 and

opto-isolator ISO2.

The primary side power path is comprised of C1, C9,

½ of T1, Q2, and R11. These components should be

as close to each other as possible when laying out the

printed circuit board. The secondary side power path

for the 1.8V output is made up of part of T1, Q5, and

C4. The 2.5V power path is made up of part of T1, Q4,

C6 and C7. The 3.3V power path is made up of part of

T1, Q6, C11 ad C12. These parts, as grouped in the

above description, should also be laid out as close to

each other as possible, without overlapping any of the

circuitry or traces of the primary side.

IN ORDER TO ENSURE PROPER OPERATION, THE

DESIGNER MUST ENSURE THAT THE PD INPUT

CURRENT REQUIREMENT DOES NOT EXCEED THE

LTC4267 CURRENT LIMIT OVER THE UNIT’S

OPERATING VOLTAGE RANGE.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 859

MULTIPLE OUTPUT PD

Figure 1.

Proper Measurement Equipment Setup

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 859

MULTIPLE OUTPUT PD

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 859

MULTIPLE OUTPUT PD

Mouser Electronics

Authorized Distributor

Click to View Pricing, Inventory, Delivery & Lifecycle Information:

Analog Devices Inc.:

DC859A

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