Datasheet LT8303 (Analog Devices) - 7

LT8303
operaTion
The LT8303 is a current mode switching regulator IC conduction mode is a variable frequency, variable peak- designed specially for the isolated flyback topology. The current switching scheme. The power switch turns on key problem in isolated topologies is how to communicate and the transformer primary current increases until an the output voltage information from the isolated secondary internally controlled peak current limit. After the power side of the transformer to the primary side for regulation. switch turns off, the voltage on the SW pin rises to the Historically, opto-isolators or extra transformer windings output voltage multiplied by the primary-to-secondary communicate this information across the isolation bound- transformer turns ratio plus the input voltage. When the ary. Opto-isolator circuits waste output power, and the secondary current through the output diode falls to zero, extra components increase the cost and physical size of the SW pin voltage collapses and rings around VIN. A the power supply. Opto-isolators can also cause system boundary mode detector senses this event and turns the issues due to limited dynamic response, nonlinearity, unit- power switch back on. to-unit variation and aging over lifetime. Circuits employing Boundary conduction mode returns the secondary current extra transformer windings also exhibit deficiencies, as to zero every cycle, so parasitic resistive voltage drops using an extra winding adds to the transformer’s physical do not cause load regulation errors. Boundary conduc- size and cost, and dynamic response is often mediocre. tion mode also allows the use of smaller transformers The LT8303 samples the isolated output voltage through compared to continuous conduction mode and does not the primary-side flyback pulse waveform. In this manner, exhibit sub-harmonic oscillation. neither opto-isolator nor extra transformer winding is re- quired for regulation. Since the LT8303 operates in either
Discontinuous Conduction Mode Operation
boundary conduction mode or discontinuous conduction As the load gets lighter, boundary conduction mode in- mode, the output voltage is always sampled on the SW creases the switching frequency and decreases the switch pin when the secondary current is zero. This method im- peak current at the same ratio. Running at a higher switching proves load regulation without the need of external load frequency up to several MHz increases switching and gate compensation components. charge losses. To avoid this scenario, the LT8303 has an The LT8303 is a simple to use micropower isolated flyback additional internal oscillator, which clamps the maximum converter housed in a 5-lead TSOT-23 package. The output switching frequency to be less than 350kHz (typical). Once voltage is programmed with a single external resistor. By the switching frequency hits the internal frequency clamp, integrating the loop compensation and soft-start inside, the the part starts to delay the switch turn-on and operates in part further reduces the number of external components. discontinuous conduction mode. As shown in the Block Diagram, many of the blocks are similar to those found in traditional switching regulators
Low Ripple Burst Mode Operation
including reference, regulators, oscillator, logic, current Unlike traditional flyback converters, the LT8303 has to amplifier, current comparator, driver, and power switch. turn on and off at least for a minimum amount of time The novel sections include a flyback pulse sense circuit, and with a minimum frequency to allow accurate sampling a sample-and-hold error amplifier, and a boundary mode of the output voltage. The inherent minimum switch cur- detector, as well as the additional logic for boundary rent limit and minimum switch-off time are necessary to conduction mode, discontinuous conduction mode, and guarantee the correct operation of specific applications. low ripple Burst Mode operation. As the load gets very light, the LT8303 starts to fold back
Boundary Conduction Mode Operation
the switching frequency while keeping the minimum switch current limit. So the load current is able to decrease while The LT8303 features boundary conduction mode operation still allowing minimum switch-off time for the sample-and- at heavy load, where the chip turns on the primary power hold error amplifier. Meanwhile, the part switches between switch when the secondary current is zero. Boundary sleep mode and active mode, thereby reducing the effec- 8303fa For more information www.linear.com/LT8303 7 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts