Rainbow Electronics MAX5945 - Manual (Page 16)

MAX5945
Overcurrent Protection
A sense resistor (R S ), connected between SENSE_ and
V EE , monitors the load current. Under all circum-
stances, the voltage across R
S
never exceeds the
threshold V SU_LIM . If SENSE_ 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 the SENSE_ voltage exceeds
V
SU_LIM , 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 normal powered state, the
MAX5945 checks for overcurrent conditions as deter-
mined by V
FLT_LIM = ~88% of V SU_LIM . The t FAULT
counter sets the maximum allowed continuous
overcurrent period. The t
FAULT counter increases when
V SENSE exceeds V FLT_LIM and decreases at a slower
pace when V SENSE drops below V FLT_LIM . A slower
decrement for the t FAULT counter allows for detecting
repeated short-duration overcurrents. When the counter
reaches the t
FAULT
limit, the MAX5945 powers off the
port and asserts the IMAX_FLT_ bit. For a continuous
overstress, a fault latches exactly after a period of
t
FAULT . V SU_LIM , is programmable using R27h[4-7].
t FAULT is programmable using R16h[2-3] and R28[4-7].
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 MAX5945 allows the port to be powered on only
when the t
FAULT counter is at zero. This feature sets an
automatic duty-cycle protection to the external MOSFET
to avoid overheating. The duty cycle is programmable
using R16h[6-7].
The MAX5945 continuously flags when the current
exceeds the maximum current allowed for the class as
indicated in the CLASS status register. When class
overcurrent occurs, the MAX5945 sets the IVC bit in
register R09h.
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 ) > 30V. 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 EE ) > 50V (see Figure 4).
Quad Network Power Controller
for Power-Over-LAN
16 ______________________________________________________________________________________
Figure 2. Detection, Classification, and Power-Up Port
Sequence
OUT_
-4V
-9V
-18V
-48V
t
t DETI t DETII t CLASS
150ms 150ms 21.3ms
0V
0V
80ms
Figure 3. PGOOD Timing
PGOOD
POK
t PGOOD