Datasheet LTC3900 (Analog Devices) - 7

LTC3900
applicaTions inForMaTion Overview
load through Q3, T1 and LO. In the next period, Q1 turns In a typical forward converter topology, a power trans- off, SG goes low and T2 generates a positive pulse at the former is used to provide the functions of input/output LTC3900 SYNC input. The LTC3900 forces FG to turn off isolation and voltage step-down to achieve the required and CG to turn on, Q4 conducts. Current continues to low output voltage. Schottky diodes are often used on flow to the load through Q4 and LO. Figure 2 shows the the secondary-side to provide rectification. Schottky LTC3900 synchronization waveforms. diodes, though easy to use, result in a loss of efficiency due to relatively high voltage drops. To improve efficiency,
External MOSFET Protection
synchronous output rectifiers utilizing N-channel MOSFETs A programmable timer and a differential input current sense can be used instead of Schottky diodes. The LTC3900 comparator are included in the LTC3900 for protection provides all of the necessary functions required to drive of the external MOSFET during power down and Burst the synchronous rectifier MOSFETs. Mode® operation. The chip also shuts off the MOSFETs Figure 1 shows a simplified forward converter application. if VCC < 4.1V. T1 is the power transformer; Q1 is the primary-side power When the primary controller is powering down, the primary transistor driven by the primary controller, LT1952 output controller shuts down first and the LTC3900 continues to (OUT). The pulse transformer T2 provides synchronization operate for a while by drawing power from the VCC bypass and is driven by LT1952 synchronization signal, SOUT or SG cap, CVCC. The SG signal stops switching and there is no from the primary controller. Q3 and Q4 are secondary-side SYNC pulse to the LTC3900. The LTC3900 keeps one of synchronous switches driven by the LTC3900’s FG and CG the drivers turned on depending on the polarity of the output. Inductor LO and capacitor COUT form the output last SYNC pulse. If the last SYNC pulse is positive, CG filter to provide a steady DC output voltage for the load. will remain high and the catch MOSFET, Q4 will stay on. Also shown in Figure 1 is the feedback path from VOUT The inductor current will start falling down to zero and through the optocoupler driver LT4430 and an optocoupler, continue going in the negative direction due to the voltage back to the primary controller to regulate VOUT. that is still present across the output capacitor (the current Each full cycle of the forward converter operation con- now flows from COUT back to LO). If Q4 is turned off while sists of two periods. In the first period, Q1 turns on and the inductor current is negative, the inductor current will the primary-side delivers power to the load through T1. produce high voltage across Q4, resulting in a MOSFET SG goes high and T2 generates a negative pulse at the avalanche. Depending on the amount of energy stored in LTC3900 SYNC input. The LTC3900 forces FG to turn on the inductor, this avalanche energy may damage Q4. and CG to turn off, Q3 conducts. Current flows to the GATE (OUT) SG (SOUT) SYNC FG CG 3900 F02
Figure 2. Synchronization Waveforms
3900fb 7 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts