參數(shù)資料
型號(hào)： LTC4257CS8-1
廠商： LINEAR TECHNOLOGY CORP
元件分類： 電源管理
英文描述： Power over Ethernet Interface Controller with Dual Current Limit
中文描述： 1-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO8
封裝： 0.150 INCH, PLASTIC, SO-8
文件頁(yè)數(shù)： 22/32頁(yè)
文件大?。?/td> 1975K
代理商： LTC4257CS8-1
LTC4267
22
4267f
APPLICATIU
Programmable Slope Compensation
The LTC4267 switching regulator injects a ramping current
through its SENSE pin into an external slope compensation
resistor (R
SL
in Figure 11). This current ramp starts at
zero after the NGATE pin has been high for the LTC4267’s
minimum duty cycle of 6%. The current rises linearly to-
wards a peak of 5μA at the maximum duty cycle of 80%,
shutting off once the NGATE pin goes low. A series resis-
tor (R
SL
) connecting the SENSE pin to the current sense
resistor (R
SENSE
) develops a ramping voltage drop. From
the perspective of the LTC4267 SENSE pin, this ramping
voltage adds to the voltage across the sense resistor,
effectively reducing the current comparator threshold in
proportion to duty cycle. This stabilizes the control loop
against subharmonic oscillation. The amount of reduction
in the current comparator threshold (V
SENSE
) can be
calculated using the following equation:
V
SENSE
= 5μA R
SL
[(Duty Cycle – 6%)/74%]
Note: The LTC4267 enforces 6% < Duty Cycle < 80%.
Designs not needing slope compensation may replace R
SL
with a short-circuit.
W
U
U
Applications Employing a Third Transformer Winding
A standard operating topology may employ a third
winding on the transformer’s primary side that provides
power to the LTC4267 switching regulator via its P
VCC
pin
(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
START
) in conjunc-
tion with the internal wide hysteresis undervoltage lockout
circuit that monitors the P
VCC
pin voltage.
R
START
is connected to V
PORTP
and supplies a current,
typically 100μA, to charge C
PVCC
. After some time, the
voltage on C
PVCC
reaches the P
VCC
turn-on threshold. The
LTC4267 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
PVCC
begins to decline as the
switching regulator draws its normal supply current, which
exceeds the delivery from R
START
. After some time, typically
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 switching regulator.
One potential design pitfall is under-sizing the value of
capacitor C
PVCC
. In this case, the normal supply current
drawn through P
VCC
will discharge C
PVCC
rapidly before the
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
VCC
node.
Resistor R
START
should be selected to yield a worst-case
minimum charging current greater that the maximum rated
LTC4267 start-up current to ensure there is enough current
to charge C
PVCC
to the P
VCC
turn-on threshold. R
START
should also be selected large enough to yield a worst-case
maximum charging current less than the minimum-rated
P
VCC
supply current, so that in operation, most of the
P
VCC
current is delivered through the third winding. This
results in the highest possible efficiency.
Capacitor C
PVCC
should then be made large enough to avoid
the relaxation oscillation behavior described previously.
This is difficult 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
VCC
turn-off
threshold. Also, the third winding’s nominal output voltage
should be at least 0.5V below the minimum rated P
VCC
clamp voltage to avoid running up against the LTC4267
shunt regulator, needlessly wasting power.
P
VCC
Shunt Regulator
In applications including a third transformer winding,
the internal P
VCC
shunt regulator serves to protect the
LTC4267 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 switching
regulator to be powered through a single dropping resistor
from V
PORTP
as shown in Figure 12. This simplicity comes
at the expense of reduced efficiency due to static power
dissipation in the R
START
dropping resistor.
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