MAX5965AEAX datasheet(21/54 Pages) MAXIM

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High-Power, Quad, Monolithic, PSE Controllers

for Power over Ethernet

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Overcurrent Protection

A sense resistor RS connected between SENSE_ and

VEE monitors the load current. Under normal operating

conditions, the voltage across RS (VRS) never exceeds

the threshold VSU_LIM. If VRS exceeds VSU_LIM, an

internal current-limiting circuit regulates the GATE_ volt-

age, limiting the current to ILIM = VSU_LIM/RS. During

transient conditions, if VRS exceeds VSU_LIM by more

than 1V, a fast pulldown circuit activates to quickly

recover from the current overshoot. During startup, if

the current-limit condition persists, when the startup

timer, tSTART, times out, the port shuts off, and the

STRT_FLT_ bit is set. In the normal powered state, the

MAX5965A/MAX5965B check for overcurrent condi-

tions as determined by VFLT_LIM = ~88% of VSU_LIM.

The tFAULT counter sets the maximum allowed continu-

ous overcurrent period. The tFAULT counter increases

when VRS exceeds VFLT_LIM and decreases at a slower

pace when VRS drops below VFLT_LIM. A slower decre-

ment for the tFAULT counter allows for detecting repeat-

ed short-duration overcurrents. When the counter

reaches the tFAULT limit, the MAX5965A/MAX5965B

power off the port and assert the IMAX_FLT_ bit. For a

continuous overstress, a fault latches exactly after a

period of tFAULT. VSU_LIM is programmable through the

ICUT registers R2Ah[6:4], R2Ah[2:0], R2Bh[6:4],

R2Bh[2:0], and the IVEE bits in register R29h[1:0]. See

the

High-Power Mode section for more information on

the ICUT register.

After power-off due to an overcurrent fault, and if the

RSTR_EN bit is set, the tFAULT timer is not immediately

reset but starts decrementing at the same slower pace.

The MAX5965A/MAX5965B allow the port to be pow-

ered on only when the tFAULT counter is at zero. This

feature sets an automatic duty-cycle protection to the

external MOSFET avoiding overheating.

The MAX5965A/MAX5965B continuously flag when the

current exceeds the maximum current allowed for the

class as indicated in the CLASS status register. When

class overcurrent occurs, the MAX5965A/MAX5965B

set the IVC_ bit in register R09h.

ICUT Register and High-Power Mode

ICUT Register

The ICUT register determines the maximum current lim-

its allowed for each port of the MAX5965A/MAX5965B.

The 3 ICUT bits (R2Ah[6:4], R2Ah[2:0], R2Bh[6:4], and

R2Bh[2:0]) allow programming of the current-limit and

overcurrent thresholds in excess of the IEEE standard

limit (see Tables 34a, 34b, and 34c). The ICUT regis-

ters can be written to directly through the I2C interface

when CL_DISC (R17h[2]) is set to 0 (see Table 3). In

this case, the current limit of the port is configured

regardless of the status of the classification.

By setting the CL_DISC bit to 1, the MAX5965A/

MAX5965B automatically set the ICUT register based

upon the classification result of the port. See Table 3

and the

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

Foldback Current

During startup and normal operation, an internal circuit

senses the voltage at OUT_ and reduces the current-

limit value when (VOUT_ - VEE) > 28V. The foldback

function helps to reduce the power dissipation on the

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

ILIM when (VOUT_ - VEE ) > 48V (see Figure 3a). For

high-power mode, the foldback starts when (VOUT_ -

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

current limit (ILIM) is reduced down to minimum fold-

back current (VTH_FB/RS) when (VOUT_ - VEE ) > 48V.

Figure 2. PGOOD_ Timing

PGOOD_

POK_

tPGOOD

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