LTC4267 LINER [Linear Technology], LTC4267 Datasheet - Page 10

• Manufacturer Part Number: LTC4267
• Description: Power over Ethernet IEEE 802.3af PD Interface with Integrated Switching Regulator
• Manufacturer: LINER [Linear Technology]

LTC4267
APPLICATIO S I FOR ATIO
specifi ed to avoid corrupting the 25kΩ signature resistor
while also saving board space and cost. In addition, the in-
rush current limit requirement of the IEEE 802.3af standard
can cause large transient power dissipation in the PD. The
LTC4267 is designed to allow multiple turn-on sequences
without overheating the miniature 16-lead package. In the
event of excessive power cycling, the LTC4267 provides
thermal overload protection to keep the onboard power
MOSFET within its safe operating area.
OPERATION
The LTC4267 PD interface has several modes of opera-
tion depending on the applied input voltage as shown in
Figure 1 and summarized in Table 1. These modes satisfy
the requirements defi ned in the IEEE 802.3af specifi cation.
The input voltage is applied to the V
be negative relative to the V
sheet for the PD interface portion of the LTC4267 are with
respect to V
regulator are referenced to PGND. It is assumed that PGND
is tied to P
throughout the data sheet.
Table 1. LTC4267 Operational Mode
as a Function of Input Voltage
INPUT VOLTAGE
(V
0V to – 1.4V
–1.5V to –10V
–11V to –12.4V
–12.5V to UVLO*
UVLO* to –57V
10
* V
PORTN
Rising input threshold ≅ – 36.0V
Falling input threshold ≅ –30.5V
PORTN
with RESPECT to V
UVLO includes hysteresis.
OUT
PORTP
. Note the use of different ground symbols
U
while the voltages for the switching
PORTP
) LTC4267 MODE OF OPERATION
U
PORTP
Inactive
25kΩ Signature Resistor Detection
Classifi cation Load Current Ramps up
from 0% to 100%
Classifi cation Load Current Active
Power Applied to Switching Regulator
pin. Voltages in the data
W
PORTN
pin and must
U
PSE
I
LIM_LO
I
CLASS
–20
–30
–40
–50
–10
–20
–30
–40
–50
–10
–20
–30
–40
–50
–10
Figure 1. Output Voltage, ⎯ P ⎯ W ⎯ R ⎯ G ⎯ D and PD
Current as a Function of Input Voltage
V
I
VOLTAGES WITH RESPECT TO V
I
I
I
I
IN
LOAD
1
2
CLASS
LIM_LO
=
=
R
CLASS
I
V1 – 2 DIODE DROPS
V2 – 2 DIODE DROPS
IN
UVLO
=
OFF
DEPENDENT ON R
dV
dt
= 140mA (NOMINAL), I
R
POWER
DETECTION V1
V
LOAD
=
BAD
IN
DETECTION V2
DETECTION I
DETECTION I
I
LIMIT
CLASSIFICATION
I
25kΩ
25kΩ
C1
CLASS
CLASSIFICATION
(UP TO I
R
V
PORTN
CLASS
UVLO
TURN-ON
LTC4267
UVLO
POWER
ON
GOOD
LIM_HI
PWRGD
V
2
1
CLASS
PORTP
PGND
P
OUT
)
LIM_HI
SELECTION
PORTP
PWRGD TRACKS
V
UVLO
TURN-OFF
CURRENT
LIMIT, I
LOAD, I
τ = R
PORTN
POWER
= 375mA (NOMINAL)
UVLO
OFF
BAD
LOAD
TIME
TIME
TIME
TIME
LIM_LO
LOAD
R9
C1
(UP TO I
C1
4267 F01
V
OUT
LIM_HI
)
4267f