ltc4267-3 Linear Technology Corporation, ltc4267-3 Datasheet - Page 14

• Manufacturer Part Number: ltc4267-3
• Description: Power Over Ethernet Ieee 802.3af Pd Interface With Integrated 300khz Switching Regulator
• Manufacturer: Linear Technology Corporation

LTC4267-3
APPLICATIO S I FOR ATIO
14
sequence, the PD circuitry will rob current intended for
charging up the load capacitor and create a slow rising
input, possibly causing the LTC4267-3 to go into thermal
shutdown.
The PWRGD pin connects to an internal open drain, 100V
transistor capable of sinking 1mA. Low impedance to
V
ance during signature and classifi cation probing and in
the event of a thermal overload. During turn-off, PWRGD
is deactivated when the input voltage drops below 30V.
In addition, PWRGD may go active briefl y at turn-on for
fast rising input waveforms. PWRGD is referenced to the
V
tential. Connect the PWRGD pin to the switching regulator
circuitry as shown in Figure 7.
the LTC4267-3 will attempt to quickly charge capacitor C1
using an internal secondary current limit circuit. In this
scenario, the PSE current limit should provide the overall
limit for the circuit. For slower rising inputs, the 375mA
current limit in the LTC4267-3 will set the charge rate of
the capacitor C1. In either case, the PWRGD signal may
go inactive briefl y while the capacitor is charged up to the
new line voltage. In the design of a PD, it is necessary
to determine if a step in the input voltage will cause the
PWRGD signal to go inactive and how to respond to this
event. In some designs, it may be desirable to fi lter the
PWRGD signal so that intermittent power bad conditions
are ignored. Figure 7 demonstrates a method to insert a
lowpass fi lter on the power good interface.
For PD designs that use a large load capacitor and also
consume a lot of power, it is important to delay activation
of the switching regulator with the PWRGD signal. If the
regulator is not disabled during the current-limited turn-on
PSE
PORTN
PORTN
TO
V
LTC4267-3
PORTN
indicates power is good. PWRGD is high imped-
pin and when active, will be near the V
Figure 6. LTC4267-3 Power Good
U
THERMAL SHUTDOWN
UVLO
U
–
+
+
–
1.125V
300k
W
PWRGD
I
TH
PGND
/RUN
300k
P
42673 F06
OUT
R9
100k
U
PORTN
MIN
5µF
C1
+
PGND
po-
PD Interface Thermal Protection
The LTC4267-3 PD Interface includes thermal overload
protection in order to provide full device functionality
in a miniature package while maintaining safe operating
temperatures. Several factors create the possibility of
signifi cant power dissipation within the LTC4267-3. At
turn-on, before the load capacitor has charged up, the
instantaneous power dissipated by the LTC4267-3 can be
as much as 10W. As the load capacitor charges up, the
power dissipation in the LTC4267-3 will decrease until it
reaches a steady-state value dependent on the DC load
current. The size of the load capacitor determines how fast
the power dissipation in the LTC4267-3 will subside. At
room temperature, the LTC4267-3 can typically handle load
capacitors as large as 800µF without going into thermal
shutdown. With large load capacitors, the LTC4267-3 die
temperature will increase by as much as 50°C during a
single turn-on sequence. If for some reason power were
removed from the part and then quickly reapplied so that
the LTC4267-3 had to charge up the load capacitor again,
the temperature rise would be excessive if safety precau-
tions were not implemented.
The LTC4267-3 PD interface protects itself from thermal
damage by monitoring the die temperature. If the die
PSE
TO
PSE
TO
–48V
–48V
ALTERNATE ACTIVE-HIGH ENABLE FOR P
C15 AND C17 OPTIONAL
SEE APPLICATIONS INFORMATION SECTION
I
V
TH
PORTN
Figure 7. Power Good Interface Examples
/RUN
P
LTC4267-3
V
VCC
PORTN
LTC4267-3
PWRGD
ACTIVE-HIGH ENABLE FOR RUN PIN WITH INTERNAL PULL-UP
V
PORTP
PGND
P
PWRGD
OUT
V
PGND
PORTP
P
OUT
+
+
C1
5µF
100V
R9
100k
C1
5µF
100V
PGND
R18
10k
R9
100k
PGND
VCC
R18
10k
PIN
D6
MMBD4148
0.047µF
D6
MMBD4148
0.047µF
C15
C15
FMMT2222
FMMT2222
Q1
Q1
C17
R
START
C
PVCC
REGULATOR
I
GND
SWITCHING
TH
AUXILIARY
LTC3803
OPTIONAL
/RUN
42673f
42673 F07