LTC4269-1 Linear Technology Corporation, LTC4269-1 Datasheet - Page 18

• Manufacturer Part Number: LTC4269-1
• Description: IEEE 802.3 At PD And Synchronous No-Opto Flyback Controller
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
LTC4269-1
The PWRGD pin features an open collector output refer-
enced to V
pin. When power good is declared and active, the PWRGD
pin is high impedance with respect to V
clamp protects the UVLO pin from an excessive voltage.
The active low PWRGD pin connects to an internal, open-
drain MOSFET referenced to V
indicator bit when power good is declared and active. The
PWRGD pin is low impedance with respect to V
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.
18
NEG
which can interface directly with the UVLO
PORTN
and may be used as an
NEG
. An internal 14V
PORTN
.
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
Bel Fuse Inc.
Coilcraft Inc.
Halo Electronics
Pulse Engineering
Tyco Electronics
CONTACT INFORMATION
206 Van Vorst Street
Jersey City, NJ 07302
Tel: [phone]
www.belfuse.com
1102 Silver Lake Road
Gary, IL 60013
Tel: [phone]
www.coilcraft.com
1861 Landings Drive
Mountain View, CA 94043
Tel: [phone]
www.haloelectronics.com
12220 World Trade Drive
San Diego, CA 92128
Tel: [phone]
www.pulseeng.com
308 Constitution Drive
Menlo Park, CA 94025-1164
Tel: [phone]
www.circuitprotection.com
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