LTC3803ES6#TRMPBF Linear Technology, LTC3803ES6#TRMPBF Datasheet - Page 7

• Manufacturer Part Number: LTC3803ES6#TRMPBF
• Description: IC FLYBACK SYNC ADJ 1A TSOT23-6
• Manufacturer: Linear Technology
• Type: Flybackr

Specifications of LTC3803ES6#TRMPBF

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adjustable
• Current - Output: 1A
• Frequency - Switching: 200kHz
• Voltage - Input: Adjustable
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: TSOT-23-6, TSOT-6
• Dc To Dc Converter Type: FLYBACK CONTROLLER
• Pin Count: 6
• Output Current: 1A
• Package Type: TSOT-23
• Mounting: Surface Mount
• Operating Temperature Classification: Automotive
• Operating Temperature (min): -40C
• Operating Temperature (max): 125C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Compliant
• Other names: LTC3803ES6#PBF; LTC3803ES6#PBF

OPERATION
The LTC3803 is a constant frequency current mode control-
ler for fl 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 V
that 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 de-
crease slightly, causing the V
800mV. The error amplifi er responds by feeding current
into the I
V
will sink current away from the I
The voltage at the I
modulator formed by the oscillator, current comparator
and RS latch. Specifi cally, 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 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 fi rst. In this way, the peak
current levels through the fl yback transformer’s primary
and secondary are controlled by the I
Since the I
fi er whenever the output voltage is below nominal, and
decreased whenever output voltage exceeds nominal, the
FB
voltage will rise above 800mV and the error amplifi er
TH
TH
/RUN pin. If the load current decreases, the
/RUN voltage is increased by the error ampli-
FB
TH
pin. The divider must be designed so
/RUN pin commands the pulse-width
FB
pin voltage to fall below
TH
/RUN pin.
TH
/RUN voltage.
TH
/RUN pin
FB
pin
voltage regulation loop is closed. For example, whenever
the load current increases, output voltage will decrease
slightly, and sensing this, the error amplifi er 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, bringing
the output voltage back up.
The I
the control loop. Typically, an external series RC network
is connected from I
optimal response to load and line transients. The impedance
of this RC network converts the output current of the error
amplifi er to the I
comparator threshold and commands considerable infl u-
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
V
external circuitry drives the I
transitions into and out of shutdown according to the state
diagram (Figure 1).
(NOMINALLY 5.7V)
TH
CC
V
/RUN voltage by sourcing current into the I
CC
supply pin voltage, and a forced shutdown whenever
TH
< V
TURNOFF
/RUN pin serves as the compensation point for
Figure 1. Start-Up/Shutdown State Diagram
TH
/RUN voltage which sets the current
TH
/RUN to ground and is chosen for
SHUT DOWN
ENABLED
LTC3803
LTC3803
TH
V
(NOMINALLY 0.28V)
ITH/RUN
/RUN pin low. The LTC3803
3803 F01
< V
ITHSHDN
LTC3803
V
AND V
(NOMINALLY 8.7V)
ITH/RUN
TH
CC
/RUN pin,
> V
> V
ITHSHDN
TURNON
3803fc
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