LTC3803-3 Datasheet, PDF (9/16 Pages) Linear Technology – Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT

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OPERATIO
Powering the LTC3803-3
In the simplest case, the LTC3803-3 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 9.5V as long as the V_CC pin is not forced
to sink more than 25mA. This shunt regulator is always
active, even when the LTC3803-3 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
LTC3803-3 has a wide UVLO hysteresis (1V 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.
LTC3803-3
The V_CC pin must be bypassed to ground immediately
adjacent to the IC pins with a minimum of a 1μF 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 LTC3803-3 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 approximately
linear and begins at zero current at 8% duty cycle, reach-
ing peak current at 80% duty cycle. Additional details are
provided in the Applications Information section.
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