LTC3803-5_15 Datasheet, PDF (8/16 Pages) Linear Technology – Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT

LTC3803-5
OPERATION
The undervoltage lockout (UVLO) mechanism prevents
the LTC3803-5 from trying to drive a MOSFET with in-
sufficient V_GS. The voltage at the V_CC pin must exceed
V_TURNON (nominally 4.8V) at least momentarily to enable
LTC3803-5 operation. The V_CC voltage is then allowed to
fall to V_TURNOFF(nominally 4V) before undervoltage lockout
disables the LTC3803-5.
The I_TH/RUN pin can be driven below V_ITHSHDN (nominally
0.28V) to force the LTC3803-5 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_ITHSHDN, and V_CC exceeds V_TURNON, the LTC3803-5
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 represent current comparator thresholds from zero
to maximum.
Internal Soft-Start
An internal soft-start feature is enabled whenever the
LTC3803-5 comes out of shutdown. Specifically, the I_TH/
RUN voltage is clamped and is prevented from reaching
maximum until roughly 0.7ms has passed. This allows
the input and output currents of LTC3803-5-based power
supplies to rise in a smooth and controlled manner on
start-up.
Powering the LTC3803-5
In the simplest case, the LTC3803-5 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 8.1V as long as the V_CC pin is not
forced to sink more than 25mA. This shunt regulator is
always active, even when the LTC3803-5 is in shutdown,
since it serves the vital function of protecting the V_CC pin
from seeing too much voltage.
The V_CC pin must be bypassed to ground immediately ad-
jacent to the IC pins with a ceramic or tantalum capacitor.
Proper supply bypassing is necessary to supply the high
transient currents required by the MOSFET gate driver.
10μF is a good starting point.
Adjustable Slope Compensation
The LTC3803-5 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 6.5% duty cycle, reach-
ing peak current at 80% duty cycle. Additional details are
provided in the Applications Information section.
38035fd
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