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參數(shù)資料
| 型號(hào)： | LTC3766MPUFD#PBF |
| 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)： | 4/60頁 |
| 文件大?。?/td> | 607K |
| 代理商： | LTC3766MPUFD#PBF |
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LTC3766
12
3766f
OPERATION
TheLTC3766isasecondary-sidePWMcontrollerdesigned
for use in a forward converter with active clamp reset and
synchronous rectification. When used in conjunction with
the LTC3765 active-clamp forward controller and gate
driver, it forms a highly efficient and robust isolated power
supply with a minimum number of external components.
Bymakinguseofasecondary-sidecontrolarchitecture,the
LTC3766 is able to provide exceptional transient response
whiledirectlymonitoringtheloadtoensurethatbothoutput
voltage and output current are precisely controlled. This
architecture provides superior performance and greater
simplicity, and is particularly well suited to high power
battery charger applications.
Self-Starting Start-Up
In most applications, the LTC3766 will be used with the
LTC3765 to create a self-starting forward converter with
secondary-side control. Since there is initially no bias
voltage available on the secondary side, the LTC3765
must manage the start-up in an open-loop fashion on the
primary side. When power is first applied on the primary
side, the LTC3765 begins an open-loop soft-start using its
own internal oscillator. Power is supplied to the secondary
by switching the main primary-side MOSFET with a gradu-
ally increasing duty cycle from 0% to 70%, as controlled
by the rate of rise of the voltage on the SSFLT pin. On the
secondaryside,biasvoltagecanbegenerateddirectlyfrom
the main transformer using a peak charge circuit, or other
techniqueasappropriate.WhentheLTC3766hasadequate
voltage to satisfy its start-up requirements, it provides
duty cycle information through the pulse transformer as
shown in Figure 2. The LTC3765 detects this signal and
transfers control of the gate drivers to the LTC3766, which
continues the soft-start of the output voltage. Typically,
this hand-off from primary to secondary occurs when the
output voltage is less than one half of its final level. The
LTC3765 then turns off the linear regulator and, through
an on-chip rectifier, extracts bias power for the primary-
side MOSFETs from this signal.
Linear Regulators
In general, the bias voltage generated on the secondary
side is higher than the level desired for operation of the
forward and synchronous MOSFETs. Consequently, the
LTC3766containsahighvoltagelinearregulatorcontroller
as well as a 15V VAUX bypass regulator with an internal
PMOS, either of which can be used to regulate the volt-
age on the VCC pin. The linear regulator controller is used
by tying the NDRV pin to the base or gate of an external
N-type pass device. The LTC3766 VIN pin provides bias to
the linear regulator controller as well as to internal standby
and monitoring circuitry. If adequate voltage is detected
on the VAUX pin, then the VAUX bypass regulator will be
activated and the high voltage linear regulator controller
will be shut down to reduce power loss. Alternatively, if
only the VAUX regulator is needed, then the NDRV pin can
be tied off to VCC, while VIN is tied to VAUX. This flexible
arrangement of two linear regulators allows for the con-
venient and efficient generation of VCC bias voltage for a
wide array of applications.
Using the MODE pin, the output voltage of both linear
regulators can be set to either 7V or 8.5V, depending on
the level needed to drive the gates of the forward and
synchronous MOSFETs. Note that the undervoltage lock-
out (UVLO) set points as well as VAUX switchover levels
are adjusted along with the VCC regulation levels. This
ensures that the MOSFETs are only switched when there
is adequate gate drive voltage.
Run Control and Soft-Start
The main on/off control for the LTC3766 is the RUN pin.
This pin features precision thresholds with both internal
and externally adjustable hysteresis. This pin can be used
to monitor the secondary-side bias voltage or main output
voltage, thereby controlling the point at which hand-off
from primary to secondary side occurs. Alternatively, it
can be driven directly with a control signal. In nonisolated
applicationswhentheLTC3766isusedstandalone,thispin
can be used as an undervoltage lockout by monitoring the
mainpowersupplyinputvoltage.SeeNonisolatedApplica-
tions in the Applications Information section for details.
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