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參數(shù)資料
| 型號： | LTC3766MPUFD#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 316 kHz SWITCHING FREQ-MAX, PQCC28 |
| 封裝： | 4 X 5 MM, LEAD FREE, PLASTIC, MO-220, QFN-28 |
| 文件頁數(shù)： | 6/60頁 |
| 文件大?。?/td> | 607K |
| 代理商： | LTC3766MPUFD#TRPBF |
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LTC3766
14
3766f
OPERATION
this scheme is constantly applying a differential voltage
across the pulse transformer. Therefore, the LTC3765 can
almost instantly detect a loss of signal and shut off the
power MOSFETs.
Forward Converter and Main Loop Operation
Once communication lock has been established between
the LTC3766 and the LTC3765, the LTC3766 will have
control over the switching of the primary-side MOSFETs.
During normal operation, the main primary-side MOSFET
(connected to PG on the LTC3765) is turned on somewhat
after the forward MOSFET on the secondary side. This
applies the input voltage across the transformer, causing
the SW node on the secondary side to rise. Since the SW
nodevoltageisgreaterthantheoutputvoltage,theinductor
current ramps upward. When the current in the inductor
has ramped up to the peak value as commanded by the
voltage on the ITH pin, the current sense comparator trips,
turning off the primary-side MOSFET. After a short delay,
the forward MOSFET is turned off and the synchronous
MOSFET is turned back on, causing the inductor current
to ramp back downwards. At the next rising edge of the
LTC3766 internal clock, the cycle repeats as the synchro-
nous MOSFET is turned off and the forward and main
primary-side MOSFETs are again turned on. The LTC3766
erroramplifiersensesthemainoutputvoltage,andadjusts
the ITH voltage to obtain the peak inductor current needed
to keep the output voltage at the desired regulation level.
In some applications, there can be considerable resistive
voltage drops between the main output voltage and the
load. To address this, the LTC3766 contains a precision
differential amplifier, which can be used to remotely sense
a load voltage as high as 15V.
Current Sensing, Slope Compensation and Blanking
TheLTC3766supportscurrentsensingeitherwithacurrent
senseresistororwithanisolatedcurrenttransformer.When
using a current sense resistor, the IS+ and IS– pins operate
differentially, and the maximum peak current threshold is
approximately 75mV. Normally, the current sense resistor
is placed in the source of the forward MOSFET to minimize
power loss. If a current transformer is used to sense the
primary-side switch current, then the IS– input should be
tied to VCC and the IS+ pin to the output of the current
transformer. This causes the gain of the internal current
sense amplifier to be reduced, so that the maximum peak
current threshold is increased to approximately 1V.
As with any PWM controller that uses constant-frequency
peak current control, slope compensation is needed to
provide current-loop stability and improve noise margin.
The LTC3766 has fixed internal slope compensation. The
amount of slope has been chosen to be adequate for a
widerangeofapplications.Normally,theuseofslopecom-
pensation would have a negative impact on the accuracy
of the current limit, but the LTC3766 uses a proprietary
circuit to nullify the effect of slope compensation on the
current limit performance.
Since the LTC3766 current loop is sensing switch cur-
rent, leading edge blanking is needed to avoid a current
comparator false trip due to the MOSFET turn-on current
spike. The LTC3766 uses the voltage on the SW pin (tied
to the drain of the synchronous MOSFET) to implement an
adaptive leading-edge blanking of approximately 180ns.
The blanking of the current comparator begins only after
the voltage on SW has risen above 1.4V. This adaptive
blanking is essential because of the potentially long delay
from the time that PT+ rises to the time that the SW node
rises,andcurrentbeginsrampingupintheoutputinductor.
Thisblankingalsominimizestheneedforexternalfiltering.
Gate Driver Delay Adjustment
As in all forward converters, the main transformer core
must be properly reset so as to maintain a balanced volt-
secondproductandpreventsaturation.Thisjobishandled
on the primary side by the LTC3765, which features an
active clamp gate driver. The active clamp MOSFET works
together with a capacitor to generate an optimal reset volt-
age for the main transformer. This optimal reset voltage
minimizesvoltagestressonthemainprimary-sideMOSFET
and maximizes the utilization of the power transformer
core by reducing the magnetic flux density excursion.
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