Datasheet

LTC4265
10
4265fa
APPLICATIONS INFORMATION
PD STABILITY DURING CLASSIFICATION
Classifi cation presents a challenging stability problem due
to the wide range of possible classifi cation load current.
The onset of the classifi cation load current introduces a
voltage drop across the cable and increases the forward
voltage of the input diode bridge. This may cause the PD
to oscillate between detection and classifi cation with the
onset and removal of the classifi cation load current.
The LTC4265 prevents this oscillation by introducing a
voltage hysteresis window between the detection and clas-
sifi cation ranges. The hysteresis window accommodates
the voltage changes a PD encounters at the onset of the
classifi cation load current, thus providing a trouble-free
transition between detection and classifi cation modes.
The LTC4265 also maintains a positive I-V slope throughout
the classifi cation ranges up to the ON voltage. In the event
a PSE overshoots beyond the classifi cation voltage range,
the available load current aids in returning the PD back
into the classifi cation voltage range. (The PD input may
otherwise be “trapped” by a reverse-biased diode bridge
and the voltage held by the 0.1F capacitor.)
INRUSH CURRENT
Once the PSE detects and optionally classifi es the PD, the
PSE then applies powers on the PD. When the LTC4265
input voltage rises above the ON voltage threshold,
LTC4265 connects V
OUT
to V
IN
through the internal power
MOSFET.
To control the power-on surge currents in the system, the
LTC4265 provides a fi xed inrush current, allowing C1 to
ramp up to the line voltage in a controlled manner.
The LTC4265 keeps the PD inrush current below the
PSE current limit to provide a well-controlled power-up
Figure 5. LTC4265 Undervoltage and Overvoltage Lockout
GND
C1
5µF
MIN
V
IN
V
OUT
LTC4265
4265 F05
TO
PSE
UNDERVOLTAGE
OVERVOLTAGE
LOCKOUT
CIRCUIT
PD
LOAD
CURRENT-LIMITED
TURN ON
+
INPUT LTC4265
VOLTAGE POWER MOSFET
0V TO ON* OFF
>ON* ON
<UVLO* OFF
>OVLO OFF
*INCLUDES ON-UVLO HYSTERESIS
ON THRESHOLD ≅ 36.1V
UVLO THRESHOLD ≅ 30.7V
OVLO THRESHOLD ≅ 71.0V
characteristic that is independent of the PSE behavior.
This ensures a PD using the LTC4265 interoperability
with any PSE.
UNDERVOLTAGE LOCKOUT
The IEEE 802.3af/at specifi cation for the PD dictates a
maximum turn-on voltage of 42V and a minimum turn-off
voltage of 30V. This specifi cation provides an adequate
voltage to begin PD operation, and to discontinue PD op-
eration when the input voltage is too low. In addition, this
specifi cation allows PD designs to incorporate an on-off
hysteresis window to prevent start-up oscillations.
The LTC4265 features an ON-undervoltage lockout (UVLO)
hysteresis window (See Figure 5) that conforms with the
IEEE 802.3af/at specifi cations and accommodates the
voltage drop in the cable and input diode bridge at the
onset of the inrush current.