max5952auaxt Maxim Integrated Products, Inc., max5952auaxt Datasheet - Page 20

• Manufacturer Part Number: max5952auaxt
• Description: Max5952 High-power, Quad, Pse Controller For Power-over-ethernet
• Manufacturer: Maxim Integrated Products, Inc.

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.
During startup and normal operation, an internal circuit
senses the voltage at OUT_ and reduces the current-
limit value when (V
function helps to reduce the power dissipation on the
FET. The current limit eventually reduces to 1/3 of I
when (V
power mode, the foldback starts when (V
10V (see Figure 3b). In high-power mode, the current
limit (I
when (V
Connect the gate of the external n-channel MOSFET to
GATE_. An internal 50µA current source pulls GATE_ to
(V
High-Power, Quad, PSE Controller
for Power-Over-Ethernet
Figure 3a. Foldback Current Characteristics
20
(V
EE
SENSE_
V
SU_LIM
______________________________________________________________________________________
+ 10V) to turn on the MOSFET. An internal 40µA
V
LIM
SU_LIM
- V
OUT
OUT
EE
/ 3
) is reduced up to 1/8 of its programmed value
)
_ - V
_ - V
EE
EE
) > 48V.
OUT
) > 48V (see Figure 3a). For high-
28V
_ - V
MOSFET Gate Driver
EE
) > 28V. The foldback
Foldback Current
High-Power Mode
48V
OUT
_ - V
(V
OUT_
EE
- V
LIM
EE
) >
)
current source pulls down GATE_ to V
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
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
shoots the limit threshold (V
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.
Figure 3b. Foldback Current Characteristics for High-Power
Mode
V
(V
SU_LIM
RS
V
SU_LIM
- V
EE
GD
/ 3
)
RS
is the total capacitance between GATE and
). A fast pulldown activates if V
Δ
10V
V
Δ
OUT
t
=
SU_LIM
I
GATE
C
GD
). The fast pulldown
48V
EE
to turn off the
(V
OUT_
RS
- V
over-
EE
)