Datasheet

LT8302
9
Rev E
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OPERATION
The LT8302 is a current mode switching regulator IC
designed specially for the isolated flyback topology. The
key problem in isolated topologies is how to commu-
nicate the output voltage information from the isolated
secondary side of the transformer to the primary side
for regulation. Historically, opto-isolators or extra trans-
former windings communicate this information across
the isolation boundary. Opto-isolator circuits waste output
power, and the extra components increase the cost and
physical size of the power supply. Opto-isolators can also
cause system issues due to limited dynamic response,
nonlinearity, unit-to-unit variation and aging over life-
time. Circuits employing extra transformer windings also
exhibit deficiencies, as using an extra winding adds to
the transformer’s physical size and cost, and dynamic
response is often mediocre.
The LT8302 samples the isolated output voltage through
the primary-side flyback pulse waveform. In this man-
ner, neither opto-isolator nor extra transformer winding
is required for regulation. Since the LT8302 operates
in either boundary conduction mode or discontinuous
conduction mode, the output voltage is always sampled
on the SW pin when the secondary current is zero. This
method improves load regulation without the need of
external load compensation components.
The LT8302 is a simple to use micropower isolated fly
-
back converter housed in a thermally enhanced 8-lead
SO package. The output voltage is programmed with two
external resistors. An optional TC resistor provides easy
output diode temperature compensation. By integrating
the loop compensation and soft-start inside, the part
reduces the number of external components. As shown
in the Block Diagram, many of the blocks are similar to
those found in traditional switching regulators including
reference, regulators, oscillator, logic, current amplifier,
current comparator, driver, and power switch. The novel
sections include a flyback pulse sense circuit, a sample-
and-hold error amplifier, and a boundary mode detector,
as well as the additional logic for boundary conduction
mode, discontinuous conduction mode, and low ripple
Burst Mode operation.
Quasi-Resonant Boundary Mode Operation
The LT8302 features quasi-resonant boundary conduc
-
tion mode operation at heavy load, where the chip turns
on the primary power switch when the secondary current
is zero and the SW rings to its valley. Boundary conduc-
tion mode is a variable frequency, variable peak-current
switching scheme. The power switch turns on and the
transformer primary current increases until an internally
controlled peak current limit. After the power switch turns
off, the voltage on the SW pin rises to the output voltage
multiplied by the primary-to-secondary transformer turns
ratio plus the input voltage. When the secondary current
through the output diode falls to zero, the SW pin volt
-
age collapses and rings around V
IN
. A boundary mode
detector senses this event and turns the power switch
back on at its valley.
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