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LTC4269CDKD-1#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P36P37-P39P40-P42P43-P44
LTC4269-1
16
42691fc
APPLICATIONS INFORMATION
To control the power-on surge currents in the system, the
LTC4269-1 provides a fi xed inrush current, allowing C1 to
ramp up to the line voltage in a controlled manner.
The LTC4269-1 keeps the PD inrush current below the
PSE current limit to provide a well controlled power-up
characteristic that is independent of the PSE behavior.
This ensures a PD using the LTC4269-1 interoperability
with any PSE.
TURN-ON/ TURN-OFF THRESHOLD
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 LTC4269-1 features an ON/OFF hysteresis window (see
Figure 5) that conforms with the IEEE 802.3af/at specifi ca-
tion and accommodates the voltage drop in the cable and
input diode bridge at the onset of the inrush current.
Once C1 is fully charged, the LTC4269-1 turns on is internal
MOSFET and passes power to the PD load. The LTC4269-1
continues to power the PD load as long as the input voltage
Figure 5. LTC4269-1 ON/OFF and Overvoltage Lockout
V
PORTP
C1
5µF
MIN
V
PORTN
V
NEG
LTC4269-1
42691 F05
TO
PSE
ON/OFF AND
OVERVOLTAGE
LOCKOUT
CIRCUIT
PD
LOAD
CURRENT-LIMITED
TURN ON
+
V
PORTP
–
V
PORTN
LTC4269-1
VOLTAGE POWER MOSFET
0V TO ON* OFF
>ON* ON
<OFF* OFF
>OVLO OFF
*INCLUDES ON/OFF HYSTERESIS
ON THRESHOLD 36.1V
OFF THRESHOLD 30.7V
OVLO THRESHOLD 71.0V
Figure 6. LTC4269-1 Power Good Functional and State Diagram
42691 F06
BOLD LINE INDICATES HIGH CURRENT PATH
PWRGD
POWER
NOT
GOOD
INRUSH COMPLETE
ON < V
PORTP
< OVLO
AND NOT IN THERMAL SHUTDOWN
V
PORTP
< OFF
V
PORTP
> OVLO
OR THERMAL SHUTDOWN
POWER
GOOD
29
PWRGD
LTC4269-1
30
V
NEG
27
V
NEG
26
V
PORTN
6
V
PORTN
OVLO
ON/OFF
TSD
5
CONTROL
CIRCUIT
does not fall below the OFF threshold. When the LTC4269-1
input voltage falls below the OFF threshold, the PD load
is disconnected, and classifi cation mode resumes. C1
discharges through the LTC4269-1 circuitry.
COMPLEMENTARY POWER GOOD
When LTC4269-1 fully charges the load capacitor (C1),
power good is declared and the LTC4269-1 load can safely
begin operation. The LTC4269-1 provides complementary
power good signals that remain active during normal op-
eration and are de-asserted when the input voltage falls
below the OFF threshold, when the input voltage exceeds
the overvoltage lockout (OVLO) threshold, or in the event
of a thermal shutdown (see Figure 6).
The PWRGD pin features an open collector output refer-
enced to V
NEG
which can interface directly with the UVLO
pin. When power good is declared and active, the PWRGD
pin is high impedance with respect to V
NEG
. An internal
14V clamp limits the PWRGD pin voltage. Connecting the
PWRGD pin to the UVLO prevents the DC/DC converter
LTC4269-1
17
42691fc
from commencing operation before the PD interface
completely charges the reservoir capacitor, C1.
The active low PWRGD pin connects to an internal, open-
drain MOSFET referenced to V
PORTN
and may be used as an
indicator bit when power good is declared and active. The
PWRGD pin is low impedance with respect to V
PORTN
.
PWRGD PIN WHEN SHDN IS INVOKED
In PD applications where an auxiliary power supply invokes
the SHDN feature, the PWRGD pin becomes high imped-
ance. This prevents the PWRGD pin that is connected to
the UVLO pin from interfering with the DC/DC converter
operations when powered by an auxiliary power supply.
OVERVOLTAGE LOCKOUT
The LTC4269-1 includes an overvoltage lockout (OVLO)
feature (Figure 6) which protects the LTC4269-1 and its
load from an overvoltage event. If the input voltage ex-
ceeds the OVLO threshold, the LTC4269-1 discontinues
PD operation. Normal operations resume when the input
voltage falls below the OVLO threshold and when C1 is
charged up.
THERMAL PROTECTION
The IEEE 802.3af/at specifi cation requires a PD to withstand
any applied voltage from 0V to 57V indefi nitely. However,
there are several possible scenarios where a PD may
encounter excessive heating.
During classifi cation, excessive heating may occur if the
PSE exceeds the 75ms probing time limit. At turn-on, when
the load capacitor begins to charge, the instantaneous
power dissipated by the PD interface can be large before
it reaches the line voltage. And if the PD experiences a
fast input positive voltage step in its operational mode
(for example, from 37V to 57V), the instantaneous power
dissipated by the PD Interface can be large.
The LTC4269-1 includes a thermal protection feature
which protects the LTC4269-1 from excessive heating.
If the LTC4269-1 junction temperature exceeds the over-
temperature threshold, the LTC4269-1 discontinues PD
operations and power good becomes inactive. Normal
operation resumes when the junction temperature falls
below the overtemperature threshold and when C1 is
charged up.
EXTERNAL INTERFACE AND COMPONENT SELECTION
Transformer
Nodes on an Ethernet network commonly interface to the
outside world via an isolation transformer. For PDs, the
isolation transformer must also include a center tap on
the RJ45 connector side (see Figure 7).
The increased current levels in a Type 2 PD over a Type 1
increase the current imbalance in the magnetics which
can interfere with data transmission. In addition, proper
termination is also required around the transformer to
provide correct impedance matching and to avoid radiated
and conducted emissions. Transformer vendors such as
Bel Fuse, Coilcraft, Halo, Pulse, and Tyco (Table 4) can
assist in selecting an appropriate isolation transformer
and proper termination methods.
Table 4. Power over Ethernet Transformer Vendors
VENDOR CONTACT INFORMATION
Bel Fuse Inc. 206 Van Vorst Street
Jersey City, NJ 07302
Tel: 201-432-0463
www.belfuse.com
Coilcraft Inc. 1102 Silver Lake Road
Gary, IL 60013
Tel: 847-639-6400
www.coilcraft.com
Halo Electronics 1861 Landings Drive
Mountain View, CA 94043
Tel: 650-903-3800
www.haloelectronics.com
PCA Electronics 16799 Schoenborn Street
North Hills, CA 91343
Tel: 818-892-0761
www.pca.com
Pulse Engineering 12220 World Trade Drive
San Diego, CA 92128
Tel: 858-674-8100
www.pulseeng.com
Tyco Electronics 308 Constitution Drive
Menlo Park, CA 94025-1164
Tel: 800-227-7040
www.circuitprotection.com
APPLICATIONS INFORMATION
LTC4269-1
18
42691fc
Input Diode Bridge
Figure 2 shows how two diode bridges are typically con-
nected in a PD application. One bridge is dedicated to the
data pair while the other bridge is dedicated to the spare
pair. The LTC4269-1 supports the use of either silicon or
Schottky input diode bridges. However, there are trade-offs
in the choice of diode bridges.
An input diode bridge must be rated above the maximum
current the PD application will encounter at the tempera-
ture the PD will operate. Diode bridge vendors typically
call out the operating current at room temperature, but
derate the maximum current with increasing temperature.
Consult the diode bridge vendors for the operating current
derating curve.
A silicon diode bridge can consume over 4% of the available
power in some PD applications. Using Schottky diodes can
help reduce the power loss with a lower forward voltage.
A Schottky bridge may not be suitable for some high
temperature PD application. The leakage current has a
voltage dependency that can reduce the perceived signature
resistance. In addition, the IEEE 802.3af/at specifi cation
mandates the leakage back-feeding through the unused
bridge cannot generate more than 2.8V across a 100k
resistor when a PD is powered with 57V.
Sharing Input Diode Bridges
At higher temperatures, a PD design may be forced to
consider larger bridges in a bigger package because the
maximum operating current for the input diode bridge is
drastically derated. The larger package may not be accept-
able in some space-limited environments.
One solution to consider is to reconnect the diode bridges
so that only one of the four diodes conducts current in
each package. This confi guration extends the maximum
operating current while maintaining a smaller package
profi le. Figure 7 shows how to reconnect the two diode
bridges. Consult the diode bridge vendors for the derating
curve when only one of four diodes is in operation.
Input Capacitor
The IEEE 802.3af/at standard includes an impedance
requirement in order to implement the AC disconnect
function. A 0.1µF capacitor (C14 in Figure 7) is used to
meet this AC impedance requirement.
Input Series Resistance
Linear Technology has seen the customer community cable
discharge requirements increase by nearly 500,000 times
the original test levels. The PD must survive and operate
APPLICATIONS INFORMATION
14
13
12
1
2
3
RX
–
6
RX
+
3
TX
–
2
TX
+
RJ45
T1
COILCRAFT
ETHI - 230LD
42691 F07
1
7
8
5
4
10
9
11
5
6
4
D3
SMAJ58A
TVS
BR1
HD01
10
BR2
HD01
TO PHY
V
PORTP
LTC4269-1
C1
V
PORTN
V
NEG
SPARE
–
SPARE
+
C14
0.1µF
100V
Figure 7. PD Front-End with Isolation Transformer, Diode Bridges,
Capacitors, and a Transient Voltage Suppressor (TVS).
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P36P37-P39P40-P42P43-P44

LTC4269CDKD-1#PBF

Mfr. #:
Buy LTC4269CDKD-1#PBF
Manufacturer:
Analog Devices Inc.
Description:
Power Switch ICs - POE / LAN IEEE 802.3at High Power PD Controller with Flyback Switcher
Lifecycle:
New from this manufacturer.
Delivery:
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