MAX5952CUAX+【PDF 20/50 页】IC PSE CNTRLR FOR POE 36-SSOP

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Sheet目录43

16页 17页 18页 19页 [20页]21页 22页 23页 24页

High-Power, Quad, PSE Controller

for Power-Over-Ethernet

20 ______________________________________________________________________________________

Figure 3a. Foldback Current Characteristics

48V

28V

SU_LIM

/ 3

SENSE_

- V

)

OUT_

- V

)

Figure 3b. Foldback Current Characteristics for High-Power

Mode

48V

10V

SU_LIM

/ 3

- V

)

OUT_

- V

)

High-Power Mode

When CL_DISC (R17h[2]) is set to 0, high-power mode

is configured by setting the ICUT bits to any combina-

tion other than 000, 110, or 111 (note that 000 is the

default value for the IEEE standard limit). See Table 3

and the Register Map and Description section.

Foldback Current

During startup and normal operation, an internal circuit

senses the voltage at OUT_ and reduces the current-

limit value when (V

OUT

_ - V

) > 28V. The foldback

function helps to reduce the power dissipation on the

FET. The current limit eventually reduces to 1/3 of I

LIM

when (V

OUT

_ - V

) > 48V (see Figure 3a). For high-

power mode, the foldback starts when (V

OUT

_ - V

) >

10V (see Figure 3b). In high-power mode, the current

limit (I

LIM

) is reduced up to 1/8 of its programmed value

when (V

OUT

_ - V

) > 48V.

MOSFET Gate Driver

Connect the gate of the external n-channel MOSFET to

GATE_. An internal 50礎 current source pulls GATE_ to

+ 10V) to turn on the MOSFET. An internal 40礎

current source pulls down GATE_ to V

to turn off the

MOSFET.

The pullup and pulldown current controls the maximum

slew rate at the output during turn-on or turn-off. Use

the following equation to set the maximum slew rate:

where C

is the total capacitance between GATE and

DRAIN of the external MOSFET. Current limit and the

capacitive load at the drain control the slew rate during

startup. During current-limit regulation, the MAX5952

manipulates the GATE_ voltage to control the voltage at

SENSE_ (V

). A fast pulldown activates if V

over-

shoots the limit threshold (V

SU_LIM

). The fast pulldown

current increases with the amount of overshoot. The

maximum fast pulldown current is 100mA.

During turn-off, when the GATE voltage reaches a value

lower than 1.2V, a strong pulldown switch is activated

to keep the MOSFET securely off.

OUT

GATE

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