Datasheet

LTC4267
10
4267fc
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
PORTN
pin and must
be negative relative to the V
PORTP
pin. Voltages in the data
sheet for the PD interface portion of the LTC4267 are with
respect to V
PORTP
while the voltages for the switching
regulator are referenced to PGND. It is assumed that PGND
is tied to P
OUT
. Note the use of different ground symbols
throughout the data sheet.
Table 1. LTC4267 Operational Mode
as a Function of Input Voltage
INPUT VOLTAGE
(V
PORTN
with RESPECT to V
PORTP
) LTC4267 MODE OF OPERATION
0V to –1.4V Inactive
–1.5V to –9.5V** 25kΩ Signature Resistor Detection
–9.8V to –12.4V Classifi cation Load Current Ramps up
from 0% to 100%
–12.5V to UVLO* Classifi cation Load Current Active
UVLO* to –57V Power Applied to Switching Regulator
*V
PORTN
UVLO includes hysteresis.
Rising input threshold ≅ –36.0V
Falling input threshold ≅ –30.5V
**Measured at LTC4267 pin. The LTC4267 meets the IEEE 802.3af 10V
minimum when operating with the required diode bridges.
APPLICATIO S I FOR ATIO
WUUU
Figure 1. Output Voltage,
⎯
P
⎯
W
⎯
R
⎯
G
⎯
D and PD
Current as a Function of Input Voltage
DETECTION V1
CLASSIFICATION
UVLO
TURN-ON
UVLO
OFF
POWER
BAD
UVLO
OFF
UVLO
ON
UVLO
TURN-OFF
τ = R
LOAD
C1
PWRGD TRACKS
V
PORTN
DETECTION V2
–10
TIME
–20
–30
V
PORTN
(V)
–40
–50
–10
TIME
–20
–30
P
OUT
(V)
–40
–50
–10
TIME
–20
–30
PWRGD (V)
–40
–50
I
CLASS
PD CURRENT
I
LIM_LO
dV
dt
I
LIM_LO
C1
=
POWER
BAD
POWER
GOOD
DETECTION I
1
CLASSIFICATION
I
CLASS
DETECTION I
2
LOAD, I
LOAD
(UP TO I
LIM_HI
)
CURRENT
LIMIT, I
LIM_LO
4267 F01
I
CLASS
DEPENDENT ON R
CLASS
SELECTION
I
LIM_LO
= 140mA (NOMINAL), I
LIM_HI
= 375mA (NOMINAL)
I
1
=
V1 – 2 DIODE DROPS
25kΩ
I
LOAD
= (UP TO I
LIM_HI
)
V
OUT
R
LOAD
I
2
=
V2 – 2 DIODE DROPS
25kΩ
V
PORTP
PSE
I
IN
LTC4267
R9
R
CLASS
R
LOAD
V
OUT
C1
R
CLASS
PWRGD
P
OUT
PGND
V
PORTN
V
IN
TIME
VOLTAGES WITH RESPECT TO V
PORTP