LTC3803-5 Datasheet, PDF (8/12 Pages) Linear Technology – Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT

LTC3803-5
U
OPERATIO
The undervoltage lockout (UVLO) mechanism prevents
the LTC3803-5 from trying to drive a MOSFET with insuf-
ficient 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 lock-
out disables the LTC3803-5.
The I_TH/RUN pin can be driven below V_SHDN (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_SHDN, 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 repre-
sent current comparator thresholds from zero to maxi-
mum.
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 reach-
ing 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 8V 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
adjacent to the IC pins with a ceramic or tantalum capaci-
tor. 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, reaching peak current at 80% duty cycle.
Additional details are provided in the Applications Infor-
mation section.
38035f
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