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LTC3822EMSE Datasheet(PDF) 11 Page - Linear Technology |
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LTC3822EMSE Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LTC3822 3822f JUNCTION TEMPERATURE ( °C) –50 1.0 1.5 150 3822 F02 0.5 0 0 50 100 2.0 The power dissipated in the MOSFETs strongly depends on their respective duty cycles and load current. When the LTC3822 is operating in continuous mode, the duty cycles for the MOSFETs are: Top MOSFET Duty Cycle = Bottom MOSFET Duty Cycle = V V VV V OUT IN IN OUT IN – The MOSFET power dissipations at maximum output current are: P V V IR V IC f P VV V IR TOP OUT IN OUT MAX T DS ON IN OUT MAX RSS BOT IN OUT IN OUT MAX T DS ON =+ = •• • • •• • – •• • () ( ) () () ( ) 22 2 2 ρ ρ Both MOSFETs have I2R losses and the PTOP equation includes an additional term for transition losses, which are largest at high input voltages. The bottom MOSFET losses are greatest at high input voltage or during a short-circuit when the bottom duty cycle is 100%. The LTC3822 utilizes a non-overlapping, anti-shoot- through gate drive control scheme to ensure that the MOSFETs are not turned on at the same time. To function properly, the control scheme requires that the MOSFETs used are intended for DC/DC switching applications. Many power MOSFETs are intended to be used as static switches and therefore are slow to turn on or off. Reasonable starting criteria for selecting the MOSFETs are that they must typically have a gate charge (QG) less than 30nC (at 2.5VGS) and a turn-off delay (tD(OFF)) of less than approximately 140ns. However, due to differences in test and specification methods of various MOSFET manufac- turers, and in the variations in QG and tD(OFF) with gate drive (VIN) voltage, the MOSFETs ultimately should be evaluated in the actual LTC3822 application circuit to ensure proper operation. Shoot-through between the MOSFETs can most easily be spotted by monitoring the input supply current. As the input supply voltage increases, if the input supply current increases dramatically, then the likely cause is shoot- through. Operating Frequency The choice of operating frequency, fOSC, is a trade-off between efficiency and component size. Low frequency operation improves efficiency by reducing MOSFET switch- ing losses, both gate charge loss and transition loss. However, lower frequency operation requires more induc- tance for a given amount of ripple current. Figure 2. RDS(ON) vs Temperature APPLICATIO S I FOR ATIO |
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