LTC3803-3 Datasheet, PDF (7/16 Pages) Linear Technology – Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT

U
OPERATIO
The LTC3803-3 is a constant frequency current mode
controller for flyback and DC/DC boost converter applica-
tions in a tiny ThinSOT package. The LTC3803-3 is de-
signed 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 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_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
decrease slightly, causing the V_FB pin voltage to fall below
800mV. The error amplifier 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 amplifier will
sink current away from the I_TH/RUN pin.
LTC3803-3
The voltage at the I_TH/RUN pin commands the pulse-width
modulator formed by the oscillator, current comparator
and RS latch. Specifically, the voltage at the I_TH/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 LTC3803-3 turns on the external power
MOSFET when the internal free-running 300kHz 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_TH/RUN voltage.
Since the I_TH/RUN voltage is increased by the error ampli-
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_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, bring-
ing 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 imped-
ance of this RC network converts the output current of the
error amplifier to the I_TH/RUN voltage which sets the cur-
rent comparator threshold and commands considerable
influence over the dynamics of the voltage regulation loop.
38033fa
7