MAX5965A_11 Datasheet, PDF (21/54 Pages) Maxim Integrated Products – High-Power, Quad, Monolithic, PSE Controllers for Power over Ethernet

High-Power, Quad, Monolithic, PSE Controllers
for Power over Ethernet
POK_
t_PGOOD
PGOOD_
Figure 2. PGOOD_ Timing
Overcurrent Protection
A sense resistor R_S connected between SENSE_ and
V_EE monitors the load current. Under normal operating
conditions, the voltage across R_S (V_RS) never exceeds
the threshold V_{SU_LIM}. If V_RS exceeds V_{SU_LIM}, an
internal current-limiting circuit regulates the GATE_ volt-
age, limiting the current to I_LIM = V_{SU_LIM}/R_S. During
transient conditions, if V_RS exceeds V_{SU_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, t_START, 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 V_{FLT_LIM} = ~88% of V_{SU_LIM}.
The t_FAULT counter sets the maximum allowed continu-
ous overcurrent period. The t_FAULT counter increases
when V_RS exceeds V_{FLT_LIM} and decreases at a slower
pace when V_RS drops below V_{FLT_LIM}. A slower decre-
ment for the t_FAULT counter allows for detecting repeat-
ed short-duration overcurrents. When the counter
reaches the t_FAULT 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 t_FAULT. V_{SU_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 t_FAULT 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 t_FAULT 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 Register Map and Description section.
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_EE) > 28V. The foldback
function helps to reduce the power dissipation on the
FET. The current limit eventually reduces down to 1/3 of
I_LIM when (V_OUT_ - V_EE ) > 48V (see Figure 3a). For
high-power mode, the foldback starts when (V_OUT_ -
V_EE ) > 10V (see Figure 3b). In high-power mode, the
current limit (I_LIM) is reduced down to minimum fold-
back current (V_{TH_FB}/R_S) when (V_OUT_ - V_EE ) > 48V.
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