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LT8302MPS8E

42VIN Micropower No-Opto Isolated Flyback Converter with 65V/3.6A Switch

Linear Technology 

LT8302

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 communicate

the output voltage information from the isolated secondary

side of the transformer to the primary side for regulation.

Historically, opto-isolators or extra transformer windings

communicate this information across the isolation bound-

ary. 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 lifetime. 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 manner,

neither opto-isolator nor extra transformer winding is re-

quired 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 im-

proves 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 conduction

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 conduction 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 voltage collapses

and rings around VIN. A boundary mode detector senses

this event and turns the power switch back on at its valley.

For more information www.linear.com/LT8302

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