LT8309 operaTion The LT8309 is a synchronous rectifier designed for An external resistor connects the part’s DRAIN pin to the boundary conduction mode/critical conduction mode drain of the MOSFET. This resistor sets the trip point of and discontinuous mode flyback converters. Existing the comparator with a precise internal current source. To solutions use a pulse signal from the primary side to set the trip point to –10mV, a 3000Ω resistor is needed. control the synchronous rectifier, but the LT8309 senses The trip point decreases by 1mV for every 100Ω added the drain-source voltage of the MOSFET to control the to this drain resistor. This trip point will be referred to as synchronous rectifier. This technique not only elimi- VOFFSET. Figure 1 shows the drain waveform on top and nates a transformer, but allows it to work with Linear the gate waveform on the bottom. The gate node goes high Technology’s line of no-opto critical conduction flyback when the drain node goes 74mV below the VOFFSET. The converters. Synchronous rectification improves ef- comparator’s output is ignored for a minimum on-time to ficiency and more importantly increases the maximum eliminate the chance that ringing triggers the comparator. output current when compared to nonsynchronous After the minimum on-time, the comparator is ready to designs. trigger at the VOFFSET voltage. Once the drain voltage goes The LT8309 imitates the behavior of a diode but re- above VOFFSET, the gate turns off after a very fast propaga- places the forward voltage with the R tion delay, tD(OFF). The body diode begins to conduct again DS(ON) of an external N-channel MOSFET. The LT8309 includes an LDO, a very before the current reaches 0A. The drain voltage needs to fast comparator and a powerful gate driver. go above 1.21V and then wait for the minimum off-time before the comparator is re-enabled. GND VOFFSET 80mV tMIN(ON) tMIN(OFF) tD(ON) tD(OFF) 8309 F01 Figure 1. Drain and Gate Waveforms 8309fa For more information www.linear.com/LT8309 7
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