MAX5986AETE+【PDF 14/23 页】IC POE SWITCH PSE 16TQFN

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MAX5986AMAX5986C/MAX5987A

IEEE 802.3af-Compliant, High-Efficiency, Class 1/

Class 2, PDs with Integrated DC-DC Converter

Maxim Integrated

Sleep and Ultra-Low Power Modes

(MAX5986AMAX5986C)

The MAX5986AMAX5986C feature a sleep mode and

an ultra-low power mode in which the internal p-channel

isolation MOSFET is kept on and the buck regulator is off.

In sleep mode, the LED driver output (LED) pulse width

modulates the LED current with a 25% duty cycle. The

peak LED current (I

LED

) is set by an external resistor R

. To enable sleep mode, apply a falling edge to SL with

ULP disconnected or high impedance. Sleep mode can

only be entered from wake mode.

Ultra-low power mode allows the devices to reduce

power consumption lower than sleep mode, while main-

taining the power signature of the IEEE standard. The

ultra-low power-mode enable input ULP is internally held

high with a 50k?pullup resistor to the internal 5V bias of

the device. To enable ultra-low power mode, apply a fall-

ing edge to SL with ULP = LOW. Ultra-low power mode

can only be entered from wake mode.

To exit from sleep mode or ultra-low power mode and

resume normal operation, apply a falling edge on the

wake-mode enable input (WK).

Thermal-Shutdown Protection

If the devices die temperature reaches 143癈, an over-

temperature fault is generated and the device shuts

down. The die temperature must cool down below

+127癈 to remove the overtemperature fault condition.

After a thermal shutdown condition clears, the device is

reset.

WAD Description

For applications where an auxiliary power source such

as a wall power adapter is used to power the PD, the

devices feature wall power adapter detection.

The wall power adapter is connected from WAD to PGND.

The devices detect the wall power adapter when the volt-

age from WAD to PGND is greater than 8.8V. When a wall

power adapter is detected, the internal isolation MOSFET

is turned off, classification current is disabled.

Connect the auxiliar power source to WAD, connect a

diode from WAD to V

, and connect a diode from WAD

to V

. See the typical application circuit in Figure 2.

The application circuit must ensure that the auxiliary

power source can provide power to V

and V

means of external diodes. The voltage on V

must be

within the V

voltage range to allow the DC-DC to oper-

ate. To allow operation of the DC-DC converter, the V

and V

voltage must be greater than 8.7V, on the rising

edge, while on the falling edge the V

and V

may fall

down to 7.3V keeping the DC-DC converter on.

Note: When operating solely with a wall power adapter,

the WAD voltage must be able to meet the condition V

> 8.7V, that likely results in WAD > 9.4V.

Internal Linear Regulator and Back Bias

An internal voltage regulator provides VDRV to internal

circuitry. The VDRV output is filtered by a 1礔 capaci-

tor connected from VDRV to GND. The regulator is for

internal use only and cannot be used to provide power to

external circuits. VDRV can be powered by either V

AUX

, depending on V

AUX

. The internal regulator is used

for both PD and buck converter operations.

OUT

can be used to back bias the VDRV voltage regu-

lator if V

OUT

is greater than 4.75V. Back biasing VDRV

increases device efficiency by drawing current from

OUT

instead of V

. If V

OUT

is used as back bias,

connect AUX directly to V

OUT

. In this configuration, the

DRV

source switches from V

to V

AUX

after the buck

converters output has reached its regulation voltage.

Cable Discharge Event Protection (CDE)

A 70V voltage clamp is integrated to protect the internal

circuits from a cable discharge event.

DC-DC Buck Converter

The DC-DC buck converter uses a PWM, peak current-

mode, fixed-frequency control scheme providing an

easy-to-implement architecture without sacrificing a fast

transient response. The buck converter operates in a

wide input voltage range from 8.7V to 60V and sup-

ports up to 3.84W (MAX5986A) or 6.49W (MAX5986B/

MAX5986C/MAX5987A) of input power. The devices pro-

vide a wide array of protection features including UVLO,

overtemperature shutdown, short-circuit protection with

hiccup runaway current limit, cycle-by-cycle peak cur-

rent protection, and cycle-by-cycle output overvoltage

protection, for enhanced performance and reliability. A

frequency foldback scheme is implemented to reduce

the switching frequency to half at light loads to increase

the efficiency.

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