LTC3803 Datasheet, PDF (7/12 Pages) Linear Technology – Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT

U
OPERATIO
The undervoltage lockout (UVLO) mechanism prevents
the LTC3803 from trying to drive a MOSFET with insuffi-
cient V_GS. The voltage at the V_CC pin must exceed V_TURNON
(nominally 8.7V) at least momentarily to enable LTC3803
operation. The V_CC voltage is then allowed to fall to
V_TURNOFF (nominally 5.7V) before undervoltage lockout
disables the LTC3803. This wide UVLO hysteresis range
supports the use of a bias winding on the flyback trans-
former to power the LTC3803—see the section Powering
the LTC3803.
The I_TH/RUN pin can be driven below V_SHDN (nominally
0.28V) to force the LTC3803 into shutdown. An internal
0.3µA current source always tries to pull this pin towards
V_CC. When the I_TH/RUN pin voltage is allowed to exceed
V_SHDN, and V_CC exceeds V_TURNON, the LTC3803 begins to
operate and an internal clamp immediately pulls the
I_TH/RUN pin up to about 0.7V. In operation, the I_TH/RUN
pin voltage will vary from roughly 0.7V to 1.9V to repre-
sent current comparator thresholds from zero to maxi-
mum.
Internal Soft-Start
An internal soft-start feature is enabled whenever the
LTC3803 comes out of shutdown. Specifically, the
I_TH/RUN voltage is clamped and is prevented from reach-
ing maximum until roughly 1.4ms has passed. This
allows the input and output currents of LTC3803-based
power supplies to rise in a smooth and controlled manner
on start-up.
LTC3803
Powering the LTC3803
In the simplest case, the LTC3803 can be powered from a
high voltage supply through a resistor. A built-in shunt
regulator from the V_CC pin to GND will draw as much
current as needed through this resistor to regulate the V_CC
voltage to around 9.4V as long as the V_CC pin is not forced
to sink more than 25mA. This shunt regulator is always
active, even when the LTC3803 is in shutdown, since it
serves the vital function of protecting the V_CC pin from
seeing too much voltage.
For higher efficiency or for wide V_IN range applications,
flyback controllers are typically powered through a sepa-
rate bias winding on the flyback transformer. The LTC3803
has a wide UVLO hysteresis (1.5V min) and small V_CC
supply current draw (<90µA when V_CC < V_TURNON) that is
needed to support such bootstrapped hysteretic start-up
schemes.
The V_CC pin must be bypassed to ground immediately
adjacent to the IC pins with a minimum of a 10µF ceramic
or tantalum capacitor. Proper supply bypassing is neces-
sary to supply the high transient currents required by the
MOSFET gate driver.
Adjustable Slope Compensation
The LTC3803 injects a 5µA peak current ramp out through
its SENSE pin which can be used for slope compensation
in designs that require it. This current ramp is approxi-
mately linear and begins at zero current at 6% duty cycle,
reaching peak current at 80% duty cycle. Additional details
are provided in the Applications Information section.
3803i
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