LT8302/LT8302-3 OPERATION Boundary conduction mode returns the secondary current minimum switch current limit and minimum switch-off to zero every cycle, so parasitic resistive voltage drops time are necessary to guarantee the correct operation of do not cause load regulation errors. Boundary conduc- specific applications. tion mode also allows the use of smaller transformers As the load gets very light, the LT8302/LT8302-3 starts to compared to continuous conduction mode and does not fold back the switching frequency while keeping the min- exhibit subharmonic oscillation. imum switch current limit. So the load current is able to Discontinuous Conduction Mode Operation decrease while still allowing minimum switch-off time for the sample-and-hold error amplifier. Meanwhile, the part As the load gets lighter, boundary conduction mode switches between sleep mode and active mode, thereby increases the switching frequency and decreases the reducing the effective quiescent current to improve light switch peak current at the same ratio. Running at a higher load efficiency. In this condition, the LT8302/LT8302-3 switching frequency up to several MHz increases switch- runs in low ripple Burst Mode operation. The typical ing and gate charge losses. To avoid this scenario, the 12kHz minimum switching frequency determines how LT8302/LT8302-3 has an additional internal oscillator, often the output voltage is sampled and also the mini- which clamps the maximum switching frequency to be mum load requirement. less than 380kHz. Once the switching frequency hits the internal frequency clamp, the part starts to delay the switch Difference Between LT8302 and LT8302-3 turn-on and operates in discontinuous conduction mode. The difference between LT8302 and LT8302-3 is Low Ripple Burst Mode Operation the boundary detection method. The LT8302 is using the dv/dt slope on RREF pin, while the LT8302-3 is using the Unlike traditional flyback converters, the LT8302/ voltage level on RREF pin. For good transformers with LT8302-3 has to turn on and off at least for a minimum low leakage inductance, both the LT8302 and LT8302-3 amount of time and with a minimum frequency to allow are behaving the same. The LT8302-3 is recommended accurate sampling of the output voltage. The inherent for multiple-winding output applications due to its lower sensitivity to the noise on RREF pin. Rev. G 10 For more information www.analog.com 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
