Datasheet

LTC3803-3
8
38033fd
OPERATION
The undervoltage lockout (UVLO) mechanism prevents
the LTC3803-3 from trying to drive a MOSFET with in-
suffi cient V
GS
. The voltage at the V
CC
pin must exceed
V
TURNON
(nominally 8.7V) at least momentarily to enable
LTC3803-3 operation. The V
CC
voltage is then allowed
to fall to V
TURNOFF
(nominally 5.7V) before undervoltage
lockout disables the LTC3803-3. This wide UVLO hysteresis
range supports the use of a bias winding on the fl yback
transformer to power the LTC3803-3—see the section
Powering the LTC3803-3.
The I
TH
/RUN pin can be driven below the shutdown
threshold (V
ITHSHDN
) to force the LTC3803-3 into shut-
down. 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 LTC3803-3 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
LTC3803-3 comes out of shutdown. Specifi cally, the I
TH
/
RUN voltage is clamped and is prevented from reaching
maximum until roughly 1.4ms has passed. This allows
the input and output currents of LTC3803-3-based power
supplies to rise in a smooth and controlled manner on
start-up.
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 effi ciency or for wide V
IN
range applications,
fl yback controllers are typically powered through a separate
bias winding on the fl yback transformer. The LTC3803-3
has the 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.
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.