Datasheet LT8309 (Analog Devices) - 7
| Hersteller | Analog Devices |
| Beschreibung | Secondary-Side Synchronous Rectifier Driver |
| Seiten / Seite | 14 / 7 — OPERATION. Figure 1. Drain and Gate Waveforms |
| Revision | B |
| Dateiformat / Größe | PDF / 604 Kb |
| Dokumentensprache | Englisch |
OPERATION. Figure 1. Drain and Gate Waveforms
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link to page 7 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
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 Typical Applications Package Description Revision History Typical Application Related Parts
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
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 Typical Applications Package Description Revision History Typical Application Related Parts