Datasheet

LTC4267-1
13
42671fa
For more information www.linear.com/4267-1
applicaTions inForMaTion
the classification current is reenabled. C1 will discharge
through the PD circuitry and the P
OUT
pin will go to a high
impedance state.
limit because the load capacitor is charged with a current
below the IEEE inrush current limit specification.
As the LTC4267-1 switches from the low to high level
current limit, the current will increase momentarily. This
current spike is a result of the LTC4267-1 charging the
last 1.5V at the high level current limit. When charging a
10µF capacitor, the current spike is typically 100µs wide
and 125% of the nominal low level current limit.
The LTC4267-1 stays in the high level current limit mode
until the input voltage drops below the UVLO turn-off
threshold. This dual level current limit provides the sys-
tem designer with the flexibility to design PDs which are
compatible with legacy PSEs while also being able to take
advantage of the higher power allocation available in an
IEEE 802.3af system.
During the current limited turn on, a large amount of power
is dissipated in the power MOSFET. The LTC4267-1 PD
interface is designed to accept this thermal load and is
thermally protected to avoid damage to the
onboard power
MOSFET.
Note that in order to adhere to the IEEE 802.3af
standard, it is necessary for the PD designer to ensure the
PD steady state power consumption falls within the limits
shown in Table 2. In addition, the steady state current
must be less than I
LIM_HI
.
Power Good
The LTC4267-1 PD Interface includes a power good circuit
(Figure 6) that is used to indicate that load capacitor C1
is fully charged and that the switching regulator can start
operation. The power good circuit monitors the voltage
across the internal UVLO power MOSFET and PWRGD is
asserted when the voltage falls below 1.5V. The power
good circuit includes hysteresis to allow the LTC4267-1 to
operate near the current limit point without inadvertently
disabling PWRGD. The MOSFET voltage must increase to
3V before PWRGD is disabled.
If a sudden increase in voltage appears on the input line,
this voltage step will be transferred through capacitor C1
and appear across the power MOSFET. The response of
the LTC4267-1 will depend on the magnitude of the volt-
age step, the rise time of the step, the value of capacitor
C1 and the switching regulator load. For fast rising inputs,
Input Current
Limit
IEEE 802.3
af specifies a maximum inrush current and
also specifies a minimum load capacitor between the
V
PORTP
and P
OUT
pins. To control turn-on surge current
in the system, the LTC4267-1 integrates a dual level cur-
rent limit circuit with an onboard power MOSFET and
sense resistor to provide a complete inrush control circuit
without additional external components. At turn-on, the
LTC4267-1 will limit the input current to the low level,
allowing the load capacitor to ramp up to the line voltage
in a controlled manner.
The LTC4267-1 has been specifically designed to interface
with legacy PSEs which do not meet the inrush current
requirement of the IEEE 802.3af specification. At turn-on
the LTC4267-1 current limit is set to the lower level. After
C1 is charged up and the P
OUT
– V
PORTN
voltage difference
is below the power good threshold, the LTC4267-1 switches
to the high level current limit. The dual level current limit
allows legacy PSEs with limited current sourcing capability
to power up the PD while also allowing the PD to draw full
power from an IEEE 802.3af PSE. The dual level current
limit also allows use of arbitrarily large load capacitors.
The IEEE 802.3af specification mandates that at turn-on
the PD not exceed the inrush current limit for more than
50ms. The LTC4267-1 is not restricted to the 50ms time
Figure 5. LTC4267-1 V
PORTN
Undervoltage Lockout
C1
5µF
MIN
V
PORTN
V
PORTP
P
OUT
PGND
LTC4267-1
42671 F05
TO
PSE
UNDERVOLTAGE
LOCKOUT
CIRCUIT
CURRENT-LIMITED
TURN ON
+
INPUT LTC4267-1
VOLTAGE POWER MOSFET
0V TO UVLO* OFF
>UVLO* ON
*UVLO INCLUDES HYSTERESIS
RISING INPUT THRESHOLD ≅ –36V
FALLING INPUT THRESHOLD ≅ –30.5V