Datasheet LT8302, LT8302-3 (Analog Devices) - 9

link to page 8 LT8302/LT8302-3
OPERATION
The LT8302/LT8302-3 is a current mode switching regula- provides easy output diode temperature compensation. tor IC designed specially for the isolated flyback topology. By integrating the loop compensation and soft-start The key problem in isolated topologies is how to commu- inside, the part reduces the number of external compo- nicate the output voltage information from the isolated nents. As shown in the Block Diagram, many of the blocks secondary side of the transformer to the primary side are similar to those found in traditional switching reg- for regulation. Historically, opto-isolators or extra trans- ulators including reference, regulators, oscillator, logic, former windings communicate this information across current amplifier, current comparator, driver, and power the isolation boundary. Opto-isolator circuits waste output switch. The novel sections include a flyback pulse sense power, and the extra components increase the cost and circuit, a sample-and-hold error amplifier, and a boundary physical size of the power supply. Opto-isolators can also mode detector, as wel as the additional logic for boundary cause system issues due to limited dynamic response, conduction mode, discontinuous conduction mode, and nonlinearity, unit-to-unit variation and aging over life- low ripple Burst Mode operation. time. Circuits employing extra transformer windings also exhibit deficiencies, as using an extra winding adds to
Quasi-Resonant Boundary Mode Operation
the transformer’s physical size and cost, and dynamic The LT8302/LT8302-3 features quasi-resonant bound- response is often mediocre. ary conduction mode operation at heavy load, where The LT8302/LT8302-3 samples the isolated output voltage the chip turns on the primary power switch when the through the primary-side flyback pulse waveform. In this secondary current is zero and the SW rings to its valley. manner, neither opto-isolator nor extra transformer wind- Boundary conduction mode is a variable frequency, vari- ing is required for regulation. Since the LT8302/LT8302-3 able peak-current switching scheme. The power switch operates in either boundary conduction mode or discon- turns on and the transformer primary current increases tinuous conduction mode, the output voltage is always until an internally controlled peak current limit. After the sampled on the SW pin when the secondary current is power switch turns off, the voltage on the SW pin rises to zero. This method improves load regulation without the the output voltage multiplied by the primary-to-secondary need of external load compensation components. transformer turns ratio plus the input voltage. When the secondary current through the output diode falls to zero, The LT8302/LT8302-3 is a simple to use micropower iso- the SW pin voltage collapses and rings around V lated flyback converter housed in a thermally enhanced IN. A boundary mode detector senses this event and turns the 8-lead SO package. The output voltage is programmed power switch back on at its valley. with two external resistors. An optional TC resistor Rev. G For more information www.analog.com 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Revision History Typical Application Related Parts