Datasheet LT8301 (Analog Devices) - 7

LT8301
OPERATION
The LT8301 is a current mode switching regulator IC
Boundary Conduction Mode Operation
designed specially for the isolated flyback topology. The The LT8301 features boundary conduction mode opera- key problem in isolated topologies is how to commu- tion at heavy load, where the chip turns on the primary nicate the output voltage information from the isolated power switch when the secondary current is zero. secondary side of the transformer to the primary side Boundary conduction mode is a variable frequency, vari- for regulation. Historically, opto-isolators or extra trans- able peak-current switching scheme. The power switch former windings communicate this information across turns on and the transformer primary current increases the isolation boundary. Opto-isolator circuits waste output until an internally controlled peak current limit. After the power, and the extra components increase the cost and power switch turns off, the voltage on the SW pin rises to physical size of the power supply. Opto-isolators can also the output voltage multiplied by the primary-to-secondary cause system issues due to limited dynamic response, transformer turns ratio plus the input voltage. When the nonlinearity, unit-to-unit variation and aging over life- secondary current through the output diode falls to zero, time. Circuits employing extra transformer windings also the SW pin voltage collapses and rings around V exhibit deficiencies, as using an extra winding adds to IN. A boundary mode detector senses this event and turns the the transformer’s physical size and cost, and dynamic power switch back on. response is often mediocre. Boundary conduction mode returns the secondary current The LT8301 samples the isolated output voltage through to zero every cycle, so parasitic resistive voltage drops the primary-side flyback pulse waveform. In this man- do not cause load regulation errors. Boundary conduc- ner, neither opto-isolator nor extra transformer winding tion mode also allows the use of smaller transformers is required for regulation. Since the LT8301 operates compared to continuous conduction mode and does not in either boundary conduction mode or discontinuous exhibit sub-harmonic oscillation. conduction mode, the output voltage is always sampled on the SW pin when the secondary current is zero. This
Discontinuous Conduction Mode Operation
method improves load regulation without the need of external load compensation components. As the load gets lighter, boundary conduction mode increases the switching frequency and decreases the The LT8301 is a simple to use micropower isolated fly- switch peak current at the same ratio. Running at a higher back converter housed in a 5-lead TSOT-23 package. The switching frequency up to several MHz increases switch- output voltage is programmed with a single external resis- ing and gate charge losses. To avoid this scenario, the tor. By integrating the loop compensation and soft-start LT8301 has an additional internal oscillator, which clamps inside, the part further reduces the number of external the maximum switching frequency to be less than 430kHz components. As shown in the Block Diagram, many of (typ). Once the switching frequency hits the internal fre- the blocks are similar to those found in traditional switch- quency clamp, the part starts to delay the switch turn-on ing regulators including reference, regulators, oscillator, and operates in discontinuous conduction mode. logic, current amplifier, current comparator, driver, and power switch. The novel sections include a flyback pulse
Low Ripple Burst Mode Operation
sense circuit, a sample-and-hold error amplifier, and a boundary mode detector, as wel as the additional logic for Unlike traditional flyback converters, the LT8301 has to boundary conduction mode, discontinuous conduction turn on and off at least for a minimum amount of time mode, and low ripple Burst Mode operation. and with a minimum frequency to allow accurate sampling Rev. B For more information www.analog.com 7 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 Typical Applications Package Description Revision History Typical Application Related Parts