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LT8301 Datasheet(PDF) 8 Page - Linear Technology |
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LT8301 Datasheet(HTML) 8 Page - Linear Technology |
8 / 24 page LT8301 8 8301f For more information www.linear.com/LT8301 OPERATION The LT8301 is a simple to use micropower isolated flyback converterhousedina5-leadTSOT-23package.Theoutput voltage is programmed with a single external resistor. By integratingtheloopcompensationandsoft-startinside,the part further 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. Boundary Conduction Mode Operation TheLT8301featuresboundaryconductionmodeoperation at heavy load, where the chip turns on the primary power switch when the secondary current is zero. 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. Boundaryconductionmodereturnsthesecondarycurrent to zero every cycle, so parasitic resistive voltage drops do not cause load regulation errors. Boundary conduc- tion mode also allows the use of smaller transformers compared to continuous conduction mode and does not exhibit sub-harmonic oscillation. Discontinuous Conduction Mode Operation As the load gets lighter, boundary conduction mode in- creases the switching frequency and decreases the switch peakcurrentatthesameratio.Runningatahigherswitching frequency up to several MHz increases switching and gate charge losses. To avoid this scenario, the LT8301 has an additional internal oscillator, which clamps the maximum switching frequency to be less than 430kHz (typ). Once the switching frequency hits the internal frequency clamp, the part starts to delay the switch turn-on and operates in discontinuous conduction mode. Low Ripple Burst Mode Operation Unlike traditional flyback converters, the LT8301 has to turn on and off at least for a minimum amount of time and with a minimum frequency to allow accurate sampling of the output voltage. The inherent minimum switch cur- rent limit and minimum switch-off time are necessary to guarantee the correct operation of specific applications. As the load gets very light, the LT8301 starts to fold back theswitchingfrequencywhilekeepingtheminimumswitch current limit. So the load current is able to decrease while still allowing minimum switch-off time for the sample- and-hold error amplifier. Meanwhile, the part switches betweensleepmodeandactivemode,therebyreducingthe effective quiescent current to improve light load efficiency. In this condition, the LT8301 operates in low ripple Burst Mode. The 10kHz (typ) minimum switching frequency determines how often the output voltage is sampled and also the minimum load requirement. |
Similar Part No. - LT8301_15 |
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Similar Description - LT8301_15 |
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