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LTC1772B Datasheet(PDF) 9 Page - Linear Technology |
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LTC1772B Datasheet(HTML) 9 Page - Linear Technology |
9 / 16 page LTC1772B 9 1772bfa APPLICATIONS INFORMATION Since the LTC1772B is designed for operation down to low input voltages, a logic level threshold MOSFET (RDS(ON) guaranteed at VGS = 2.5V) is required for applications that work close to this voltage. When these MOSFETs are used, make sure that the input supply to the LTC1772B is less than the absolute maximum VGS rating, typically 8V. The required minimum RDS(ON) of the MOSFET is governed by its allowable power dissipation. For applications that may operate the LTC1772B in dropout, i.e., 100% duty cycle, at its worst case the required RDS(ON) is given by: RDS(ON) DC=100% = PP IOUT(MAX) () 2 1+ p ( ) where PP is the allowable power dissipation and δp is the temperature dependency of RDS(ON). (1 + δp) is generally given for a MOSFET in the form of a normalized RDS(ON) vs temperature curve, but δp = 0.005/°C can be used as an approximation for low voltage MOSFETs. In applications where the maximum duty cycle is less than 100% and the LTC1772B is in continuous mode, the RDS(ON) is governed by: RDS(ON) PP DC ()IOUT 2 1+ p ( ) where DC is the maximum operating duty cycle of the LTC1772B. Output Diode Selection The catch diode carries load current during the off-time. The average diode current is therefore dependent on the P-channel switch duty cycle. At high input voltages the diode conducts most of the time. As VIN approaches VOUT the diode conducts only a small fraction of the time. The most stressful condition for the diode is when the output is short-circuited. Under this condition the diode must safely handle IPEAK at close to 100% duty cycle. Therefore, it is important to adequately specify the diode peak cur- rent and average power dissipation so as not to exceed the diode ratings. Under normal load conditions, the average current con- ducted by the diode is: ID = VIN VOUT VIN+ VD IOUT The allowable forward voltage drop in the diode is calculated from the maximum short-circuit current as: V P I F D SC MAX ≈ () where PD is the allowable power dissipation and will be determined by efficiency and/or thermal requirements. A fast switching diode must also be used to optimize efficiency. Schottky diodes are a good choice for low forward drop and fast switching times. Remember to keep lead length short and observe proper grounding (see Board Layout Checklist) to avoid ringing and increased dissipation. CIN and COUT Selection In continuous mode, the source current of the P-chan- nel MOSFET is a square wave of duty cycle (VOUT + VD)/ (VIN + VD). To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: CI VV V V IN MAX OUT IN OUT IN Required IRMS ≈ − () [] 12 / This formula has a maximum value at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is com- monly used for design because even significant deviations do not offer much relief. Note that capacitor manufacturer’s ripple current ratings are often based on 2000 hours of life. This makes it advisable to further derate the capacitor, or to choose a capacitor rated at a higher temperature than required. Several capacitors may be paralleled to meet the |
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