Datasheet LTC3801, LTC3801B (Analog Devices) - 9

LTC3801/LTC3801B
U U W U APPLICATIO S I FOR ATIO Output Diode Selection
input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: 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 V [ (V −V )]12 OUT IN OUT C I Required ≈ I diode conducts most of the time. As V IN RMS MAX IN approaches VOUT VIN the diode conducts only a small fraction of the time. The most stressful condition for the diode is when the output This formula has a maximum value at VIN = 2VOUT, where is short-circuited. Under this condition the diode must IRMS = IOUT/2. This simple worst-case condition is com- safely handle I monly used for design because even significant deviations PEAK at close to 100% duty cycle. Therefore, it is important to adequately specify the diode peak current do not offer much relief. Note that capacitor manufacturer’s and average power dissipation so as not to exceed the ripple current ratings are often based on 2000 hours of life. diode ratings. This makes it advisable to further derate the capacitor, or to choose a capacitor rated at a higher temperature than Under normal load conditions, the average current con- required. Several capacitors may be paralleled to meet the ducted by the diode is: size or height requirements in the design. Due to the high  operating frequency of the LTC3801/LTC3801B, ceramic V V  I IN OUT = − I D OUT capacitors can also be used for C  IN. Always consult the V + V IN D   manufacturer if there is any question. The allowable forward voltage drop in the diode is calcu- The selection of COUT is driven by the required effective lated from the maximum short-circuit current as: series resistance (ESR). Typically, once the ESR require- ment is satisfied, the capacitance is adequate for filtering. P V D The output ripple (∆VOUT) is approximated by: F ≈ ISC MAX ( ) ∆ ≈  + 1  where P V I ESR OUT RIPPLE D is the allowable power dissipation and will be  8fCOUT  determined by efficiency and/or thermal requirements. where f is the operating frequency, C A fast switching diode must also be used to optimize OUT is the output capacitance and I efficiency. Schottky diodes are a good choice for low RIPPLE is the ripple current in the induc- tor. The output ripple is highest at maximum input voltage forward drop and fast switching times. Remember to keep since ∆I lead length short and observe proper grounding to avoid L increases with input voltage. ringing and increased dissipation. Manufacturers such as Nichicon, United Chemicon and Sanyo should be considered for high performance through- An additional consideration in applications where low no- hole capacitors. The OS-CON semiconductor dielectric load quiescent current is critical is the reverse leakage capacitor available from Sanyo has the lowest ESR (size) current of the diode at the regulated output voltage. A product of any aluminum electrolytic at a somewhat leakage greater than several microamperes can represent higher price. Once the ESR requirement for C a significant percentage of the total input current. OUT has been met, the RMS current rating generally far exceeds the
C
I
IN and COUT Selection
RIPPLE(P-P) requirement. In continuous mode, the source current of the P-channel In surface mount applications, multiple capacitors may MOSFET is a square wave of duty cycle (V have to be paralleled to meet the ESR or RMS current OUT + VD)/ (V handling requirements of the application. Aluminum elec- IN + VD). To prevent large voltage transients, a low ESR trolytic and dry tantalum capacitors are both available in sn3801 3801fs 9