Datasheet LT8300 (Analog Devices) - 7

link to page 6 LT8300
OPERATION
The LT8300 is a current mode switching regulator IC Boundary conduction mode is a variable frequency, vari- designed special y for the isolated flyback topology. The able peak-current switching scheme. The power switch key problem in isolated topologies is how to communicate turns on and the transformer primary current increases the output voltage information from the isolated secondary until an internally controlled peak current limit. After the side of the transformer to the primary side for regulation. power switch turns off, the voltage on the SW pin rises to Historically, opto-isolators or extra transformer windings the 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 extra secondary current through the output diode falls to zero, components increase the cost and physical size of the the SW pin voltage collapses and rings around VIN. A power supply. Opto-isolators can also cause system issues boundary mode detector senses this event and turns the due to limited dynamic response, nonlinearity, unit-to-unit power switch back on. variation and aging over lifetime. Circuits employing extra Boundary conduction mode returns the secondary current transformer windings also exhibit deficiencies, as using an to zero every cycle, so parasitic resistive voltage drops extra winding adds to the transformer’s physical size and do not cause load regulation errors. Boundary conduc- cost, and dynamic response is often mediocre. tion mode also allows the use of smaller transformers The LT8300 samples the isolated output voltage through compared to continuous conduction mode and does not the primary-side flyback pulse waveform. In this man- exhibit subharmonic oscillation. ner, neither opto-isolator nor extra transformer winding is required for regulation. Since the LT8300 operates
Discontinuous Conduction Mode Operation
in either boundary conduction mode or discontinuous As the load gets lighter, boundary conduction mode conduction mode, the output voltage is always sampled increases the switching frequency and decreases the on the SW pin when the secondary current is zero. This switch peak current at the same ratio. Running at a higher method improves load regulation without the need of switching frequency up to several MHz increases switch- external load compensation components. ing and gate charge losses. To avoid this scenario, the The LT8300 is a simple to use micropower isolated fly- LT8300 has an additional internal oscillator, which clamps back converter housed in a 5-lead TSOT-23 package. The the maximum switching frequency to be less than 750kHz. output voltage is programmed with a single external resis- Once the switching frequency hits the internal frequency tor. By integrating the loop compensation and soft-start clamp, the part starts to delay the switch turn-on and inside, the part further reduces the number of external operates in discontinuous conduction mode. components. As shown in the Block Diagram, many of the blocks are similar to those found in traditional switch-
Low Ripple Burst Mode Operation
ing regulators including reference, regulators, oscillator, Unlike traditional flyback converters, the LT8300 has to logic, current amplifier, current comparator, driver, and turn on and off at least for a minimum amount of time power switch. The novel sections include a flyback pulse and with a minimum frequency to allow accurate sampling sense circuit, a sample-and-hold error amplifier, and a of the output voltage. The inherent minimum switch cur- boundary mode detector, as wel as the additional logic for rent limit and minimum switch-off time are necessary to boundary conduction mode, discontinuous conduction guarantee the correct operation of specific applications. mode, and low ripple Burst Mode operation. As the load gets very light, the LT8300 starts to fold
Boundary Conduction Mode Operation
back the switching frequency while keeping the mini- mum switch current limit. So the load current is able to The LT8300 features boundary conduction mode opera- decrease while still allowing minimum switch-off time for tion at heavy load, where the chip turns on the primary the sample-and-hold error amplifier. Meanwhile, the part power switch when the secondary current is zero. switches between sleep mode and active mode, thereby Rev. A 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 Operation Applications Information Typical Applications Package Description Typical Application Related Parts