ltc4267-3 Linear Technology Corporation, ltc4267-3 Datasheet - Page 23

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

Applications Employing a Third Transformer Winding
A standard operating topology may employ a third wind-
ing on the transformer’s primary side that provides power
to the LTC4267-3 switching regulator via its P
(Figure 11). However, this arrangement is not inherently
self-starting. Start-up is usually implemented by the use of
an external “trickle-charge” resistor (R
tion with the internal wide hysteresis undervoltage lockout
circuit that monitors the P
R
typically 100µA, to charge C
voltage on C
LTC4267-3 switching regulator then turns on abruptly and
draws its normal supply current. The NGATE pin begins
switching and the external MOSFET (Q1) begins to deliver
power. The voltage on C
switching regulator draws its normal supply current, which
exceeds the delivery from R
tens of milliseconds, the output voltage approaches the
desired value. By this time, the third transformer winding
is providing virtually all the supply current required by the
LTC4267-3 switching regulator.
One potential design pitfall is under-sizing the value of
capacitor C
drawn through P
third winding drive becomes effective. Depending on the
particular situation, this may result in either several off-on
cycles before proper operation is reached or permanent
relaxation oscillation at the P
Resistor R
minimum charging current greater that the maximum rated
LTC4267-3 start-up current to ensure there is enough cur-
rent to charge C
should also be selected large enough to yield a worst-case
maximum charging current less than the minimum-rated
APPLICATIO S I FOR ATIO
START
is connected to V
START
PVCC
PVCC
PVCC
. In this case, the normal supply current
VCC
should be selected to yield a worst-case
reaches the P
will discharge C
U
to the P
PVCC
VCC
PORTP
U
START
VCC
VCC
PVCC
pin voltage.
VCC
begins to decline as the
turn-on threshold. R
. After some time, typically
and supplies a current,
node.
. After some time, the
turn-on threshold. The
PVCC
W
START
rapidly before the
) in conjunc-
U
VCC
START
pin
P
current is delivered through the third winding. This results
in the highest possible effi ciency.
Capacitor C
the relaxation oscillation behavior described previously.
This is diffi cult to determine theoretically as it depends on
the particulars of the secondary circuit and load behavior.
Empirical testing is recommended.
The third transformer winding should be designed so
that its output voltage, after accounting for the forward
diode voltage drop, exceeds the maximum P
threshold. Also, the third winding’s nominal output voltage
should be at least 0.5V below the minimum rated P
clamp voltage to avoid running up against the LTC4267-3
shunt regulator, needlessly wasting power.
P
In applications including a third transformer winding,
the internal P
LTC4267-3 switching regulator from overvoltage transients
as the third winding is powering up.
If a third transformer winding is undesirable or unavail-
able, the shunt regulator allows the LTC4267-3 switching
regulator to be powered through a single dropping resistor
from V
at the expense of reduced effi ciency due to static power
dissipation in the R
The shunt regulator can sink up to 5mA through the P
pin to PGND. The values of R
selected for the application to withstand the worst-case
load conditions and drop on P
turn-off threshold is not reached. C
suffi ciently to handle the switching current needed to drive
NGATE while maintaining minimum switching voltage.
VCC
VCC
supply current, so that in operation, most of the P
Shunt Regulator
PORTP
PVCC
as shown in Figure 12. This simplicity comes
VCC
should then be made large enough to avoid
shunt regulator serves to protect the
START
dropping resistor.
VCC
START
, ensuring that the P
LTC4267-3
PVCC
and C
should be sized
PVCC
VCC
must be
turn-off
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VCC
VCC
VCC
VCC