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参数资料
| 型号： | LTC3803IS6-3#TRMPBF |
| 厂商： | Linear Technology |
| 文件页数： | 7/16页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR FLYBK PWM TSOT23-6 |
| 标准包装： | 500 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 330kHz |
| 占空比： | 90% |
| 电源电压： | 5.7 V ~ 8 V |
| 降压： | 无 |
| 升压： | 无 |
| 回扫： | 是 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | SOT-23-6 细型，TSOT-23-6 |
| 包装： | 带卷 (TR) |
| 相关产品： | 732-2263-6-ND - TRANS FLYBACK LTC3803 16.9UH 732-2263-1-ND - TRANS FLYBACK LTC3803 16.9UH 732-2263-2-ND - TRANS FLYBACK LTC3803 16.9UH |
LTC3803-3
OPERATION
The LTC3803-3 is a constant frequency current mode con-
troller for ?yback and DC/DC boost converter applications
in a tiny ThinSOT package. The LTC3803-3 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 LTC3803-3’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 Switching 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 V FB pin. The divider must be designed so
that when the output is at the desired voltage, the V FB pin
voltage will equal the 800mV from the internal reference.
If the load current increases, the output voltage will de-
crease slightly, causing the V FB pin voltage to fall below
800mV. The error ampli?er responds by feeding current
into the I TH /RUN pin. If the load current decreases, the
V FB voltage will rise above 800mV and the error ampli?er
will sink current away from the I TH /RUN pin.
The voltage at the I TH /RUN pin commands the pulse-width
modulator formed by the oscillator, current comparator
and RS latch. Speci?cally, the voltage at the I TH /RUN pin
sets the current comparator’s trip threshold. The current
voltage regulation loop is closed. For example, whenever
the load current increases, output voltage will decrease
slightly, and sensing this, the error ampli?er raises the
I TH /RUN voltage by sourcing current into the I TH /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 I TH /RUN pin serves as the compensation point for
the control loop. Typically, an external series RC network
is connected from I TH /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
ampli?er to the I TH /RUN voltage which sets the current
comparator threshold and commands considerable in?u-
ence over the dynamics of the voltage regulation loop.
Start-Up/Shutdown
The LTC3803-3 has two shutdown mechanisms to disable
and enable operation: an undervoltage lockout on the V CC
supply pin voltage, and a forced shutdown whenever ex-
ternal circuitry drives the I TH /RUN pin low. The LTC3803-3
transitions into and out of shutdown according to the state
diagram (Figure 1).
LTC3803-3
SHUT DOWN
comparator monitors the voltage across a current sense
resistor in series with the source terminal of the external
MOSFET. The LTC3803-3 turns on the external power
V CC < V TURNOFF
(NOMINALLY 5.7V)
V ITH/RUN < V ITHSHDN
(NOMINALLY 0.28V)
V ITH/RUN > V ITHSHDN
AND V CC > V TURNON
(NOMINALLY 8.7V)
MOSFET when the internal free-running 300kHz oscillator
sets the RS latch. It turns off the MOSFET when the cur-
rent 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 I TH /RUN voltage.
Since the I TH /RUN voltage is increased by the error ampli-
LTC3803-3
ENABLED
38033 F01
Figure 1. Start-Up/Shutdown State Diagram
?er whenever the output voltage is below nominal, and
decreased whenever output voltage exceeds nominal, the
38033fd
7
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