LTC3803 Datasheet, PDF (6/12 Pages) Linear Technology – Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT

LTC3803
U
OPERATIO
The LTC3803 is a constant frequency current mode con-
troller for flyback 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 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.
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 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 is 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
current comparator threshold and commands consider-
able influence 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 V_CC
supply pin voltage, and a forced shutdown whenever
external circuitry drives the I_TH/RUN pin low. The LTC3803
transitions into and out of shutdown according to the state
diagram (Figure 1).
V_CC < V_TURNOFF
(NOMINALLY 5.7V)
LTC3803
SHUT DOWN
V_ITH/RUN < V_ITHSHDN
(NOMINALLY 0.28V)
V_ITH/RUN > V_ITHSHDN
AND V_CC > V_TURNON
(NOMINALLY 8.7V)
LTC3803
ENABLED
3803 F01
Figure 1. Start-Up/Shutdown State Diagram
3803i
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