參數(shù)資料
型號(hào)： DC804A-B
廠商： Linear Technology
文件頁(yè)數(shù)： 15/32頁(yè)
文件大?。?/td> 0K
描述： EVAL BOARD FOR LTC4267
設(shè)計(jì)資源： DC804A Design File
標(biāo)準(zhǔn)包裝： 1
主要目的： 電源管理，以太網(wǎng)供電（POE）
嵌入式： 否
已用 IC / 零件： LTC4267
已供物品： 板
LTC4267
22
4267fc
APPLICATIO S I FOR ATIO
WU
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Programmable Slope Compensation
The LTC4267 switching regulator injects a ramping current
through its SENSE pin into an external slope compensation
resistor (RSL 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 5A at the maximum duty cycle of 80%,
shutting off once the NGATE pin goes low. A series resis-
tor (RSL) connecting the SENSE pin to the current sense
resistor (RSENSE) 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 (VSENSE) can be
calculated using the following equation:
VSENSE = 5A RSL [(Duty Cycle – 6%)/74%]
Note: The LTC4267 enforces 6% < Duty Cycle < 80%.
Designs not needing slope compensation may replace RSL
with a short-circuit.
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 PVCC 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 (RSTART) in conjunc-
tion with the internal wide hysteresis undervoltage lockout
circuit that monitors the PVCC pin voltage.
RSTART is connected to VPORTP and supplies a current,
typically 100A, to charge CPVCC. After some time, the
voltage on CPVCC reaches the PVCC 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 CPVCC begins to decline as the
switching regulator draws its normal supply current, which
exceeds the delivery from RSTART.Aftersometime,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 CPVCC. In this case, the normal supply current
drawn through PVCC will discharge CPVCC 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 PVCC node.
Resistor RSTART 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 CPVCC to the PVCC turn-on threshold. RSTART
should also be selected large enough to yield a worst-case
maximum charging current less than the minimum-rated
PVCC supply current, so that in operation, most of the
PVCC current is delivered through the third winding. This
results in the highest possible efciency.
Capacitor CPVCCshouldthenbemadelargeenoughtoavoid
the relaxation oscillation behavior described previously.
This is difcult 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 PVCC turn-off
threshold. Also, the third winding’s nominal output voltage
should be at least 0.5V below the minimum rated PVCC
clamp voltage to avoid running up against the LTC4267
shunt regulator, needlessly wasting power.
PVCC Shunt Regulator
In applications including a third transformer winding,
the internal PVCC 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 VPORTP as shown in Figure 12. This simplicity comes
at the expense of reduced efciency due to static power
dissipation in the RSTART dropping resistor.
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