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参数资料
| 型号： | LTC3803MPS6#TRPBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 1 A SWITCHING CONTROLLER, 240 kHz SWITCHING FREQ-MAX, PDSO6 |
| 封装： | LEAD FREE, PLASTIC, MO-193, TSOT-23, 6 PIN |
| 文件页数： | 14/16页 |
| 文件大小： | 164K |
| 代理商： | LTC3803MPS6#TRPBF |
LTC3803
7
3803fc
OPERATION
The LTC3803 is a constant frequency current mode control-
ler for yback and DC/DC boost converter applications in
a tiny ThinSOT package. The LTC3803 is designed so that
none of its pins need to come in contact with the input or
output voltages of the power supply circuit of which it is
a part, allowing the conversion of voltages well beyond
the LTC3803’s absolute maximum ratings.
Main Control Loop
Due to space limitations, the basics of current mode
DC/DC conversion will not be discussed here; instead, the
reader is referred to the detailed treatment in Application
Note 19, or in texts such as Abraham Pressman’s
Switch-
ing Power Supply Design.
Please refer to the Block Diagram and the Typical Ap-
plication on the front page of this data sheet. An external
resistive voltage divider presents a fraction of the output
voltage to the VFB pin. The divider must be designed so
that when the output is at the desired voltage, the VFB pin
voltage will equal the 800mV from the internal reference.
If the load current increases, the output voltage will de-
crease slightly, causing the VFB pin voltage to fall below
800mV. The error amplier responds by feeding current
into the ITH/RUN pin. If the load current decreases, the
VFB voltage will rise above 800mV and the error amplier
will sink current away from the ITH/RUN pin.
The voltage at the ITH/RUN pin commands the pulse-width
modulator formed by the oscillator, current comparator
and RS latch. Specically, the voltage at the ITH/RUN pin
sets the current comparator’s trip threshold. The current
comparator monitors the voltage across a current sense
resistor in series with the source terminal of the external
MOSFET. The LTC3803 turns on the external power MOSFET
when the internal free-running 200kHz oscillator sets
the RS latch. It turns off the MOSFET when the current
comparator resets the latch or when 80% duty cycle is
reached, whichever happens rst. In this way, the peak
current levels through the yback transformer’s primary
and secondary are controlled by the ITH/RUN voltage.
Since the ITH/RUN voltage is increased by the error ampli-
er whenever the output voltage is below nominal, and
decreased whenever output voltage exceeds nominal, the
voltage regulation loop is closed. For example, whenever
the load current increases, output voltage will decrease
slightly, and sensing this, the error amplier raises the
ITH/RUN voltage by sourcing current into the ITH/RUN pin,
raising the current comparator threshold, thus increasing
the peak currents through the transformer primary and
secondary. This delivers more current to the load, bringing
the output voltage back up.
The ITH/RUN pin serves as the compensation point for
the control loop. Typically, an external series RC network
is connected from ITH/RUN to ground and is chosen for
optimal response to load and line transients. The impedance
of this RC network converts the output
current of the error
amplier to the ITH/RUN voltage which sets the current
comparator threshold and commands considerable inu-
ence over the dynamics of the voltage regulation loop.
Start-Up/Shutdown
The LTC3803 has two shutdown mechanisms to disable
and enable operation: an undervoltage lockout on the
VCC supply pin voltage, and a forced shutdown whenever
external circuitry drives the ITH/RUN pin low. The LTC3803
transitions into and out of shutdown according to the state
diagram (Figure 1).
Figure 1. Start-Up/Shutdown State Diagram
LTC3803
SHUT DOWN
VITH/RUN < VITHSHDN
(NOMINALLY 0.28V)
3803 F01
VCC < VTURNOFF
(NOMINALLY 5.7V)
VITH/RUN > VITHSHDN
AND VCC > VTURNON
(NOMINALLY 8.7V)
LTC3803
ENABLED
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