Datasheet LTC1772B (Analog Devices) - 9

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