ltc3803es6-5 Linear Technology Corporation, ltc3803es6-5 Datasheet - Page 7

• Manufacturer Part Number: ltc3803es6-5
• Description: Constant Frequency Current Mode Flyback Dc/dc Controller In Thinsot
• Manufacturer: Linear Technology Corporation

OPERATIO
The LTC3803-5 is a constant frequency current mode
controller for flyback, SEPIC and DC/DC boost converter
applications in a tiny ThinSOT package. The LTC3803-5 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 LTC3803-5’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 Applica-
tion on the front page of this data sheet. An external
resistive voltage divider presents a fraction of the output
voltage to the V
when the output is at the desired voltage, the V
voltage will equal the 800mV from the internal reference.
If the load current increases, the output voltage will
decrease slightly, causing the V
800mV. The error amplifier responds by feeding current
into the I
voltage will rise above 800mV and the error amplifier will
sink current away from the I
TH
/RUN pin. If the load current decreases, the V
FB
pin. The divider must be designed so that
U
TH
/RUN pin.
FB
pin voltage to fall below
FB
pin
FB
The voltage at the I
modulator formed by the oscillator, current comparator
and RS latch. Specifically, the voltage at the I
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 LTC3803-5 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 first. In this way, the peak
current levels through the flyback transformer’s primary
and secondary are controlled by the I
Since the I
fier 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 amplifier raises the
I
raising the current comparator threshold, thus increasing
the peak currents through the transformer primary and
secondary. This delivers more current to the load, bring-
ing the output voltage back up.
TH
/RUN voltage by sourcing current into the I
TH
/RUN voltage is increased by the error ampli-
TH
/RUN pin commands the pulse-width
LTC3803-5
TH
/RUN voltage.
TH
TH
/RUN pin,
/RUN pin
38035fb
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