MAX5971AETI+【PDF 17/22 页】IC CTRLR PD IEEE 802.3AF 28WQFN

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______________________________________________________________________________________ 17

Single-Port, 40W, IEEE 802.3af/802.3at PSE

Controller with Integrated MOSFET

Overcurrent Protection

The MAX5971A has an internal sense resistor, R

SENSE

(see the Simplified Diagram), connected between the

source of the internal MOSFET and V

to monitor the

load current. Under normal operating conditions, the

current through R

SENSE

RSENSE

) never exceeds the

threshold I

LIM

. If I

RSENSE

exceeds I

LIM

, an internal

current-limiting circuit regulates the gate voltage of the

internal MOSFET, limiting the current. During transient

conditions, if I

RSENSE

exceeds I

LIM

by more than 2A, a

fast pulldown circuit activates to quickly recover from the

current overshoot.

In the normal powered state, the MAX5971A checks for

overcurrent conditions, as determined by I

CUT

= ~88%

of I

LIM

. The t

FAULT

counter sets the maximum allowed

continuous overcurrent period. This timer is incremented

both in startup and in normal powered state, but under

different conditions. During startup, the counter increas-

es when I

RSENSE

exceeds I

LIM

, while in the normal pow-

ered state the counter increases when I

RSENSE

exceeds

CUT

. It decreases at a slower pace when I

RSENSE

drops below I

LIM

or I

CUT

. A slower decrement for the

FAULT

counter allows for detection of repeated short-

duration overcurrent events. When the counter reaches

the t

FAULT

limit, the MAX5971A powers down the port.

For a continuous overstress, a fault occurs exactly after

a period of t

FAULT

After a power-off due to an overcurrent fault, the t

FAULT

timer is not immediately reset but starts decrementing.

The MAX5971A allows the port to be powered on only

when the t

FAULT

counter reaches zero. This feature sets

an automatic port power duty-cycle protection to the

internal MOSFET to avoid overheating.

In the normal powered state, the I

LIM

and I

CUT

thresh-

olds are set automatically according to the classification

result (see Table 2 for classification results based on

detection current, and the Electrical Characteristics table

for the corresponding thresholds). During startup, I

LIM

always set to 420mA regardless of the detected class.

Foldback Current

During startup and normal operation, an internal circuit

senses the port voltage and reduces the current-limit

value and the overcurrent threshold when (V

AGND

OUT

) < 27V. The foldback function helps to reduce the

power dissipation on the internal MOSFET. The current

limit eventually reduces down to I

TH_FB

when (V

AGND

OUT

) < 10V (see Figure 3).

Figure 3. Foldback Current Characteristics

AGND

- V

OUT

LIM

TH_FB

10V

27V

RSENSE

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