Datasheet LTC1772 (Analog Devices) - 8

LTC1772
U U W U APPLICATIONS INFORMATION Output Diode Selection
This formula has a maximum value at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is com- The catch diode carries load current during the off-time. monly used for design because even significant deviations The average diode current is therefore dependent on the do not offer much relief. Note that capacitor manufacturer’s P-channel switch duty cycle. At high input voltages the ripple current ratings are often based on 2000 hours of life. diode conducts most of the time. As VIN approaches VOUT This makes it advisable to further derate the capacitor, or the diode conducts only a small fraction of the time. The to choose a capacitor rated at a higher temperature than most stressful condition for the diode is when the output required. Several capacitors may be paralleled to meet the is short-circuited. Under this condition the diode must size or height requirements in the design. Due to the high safely handle IPEAK at close to 100% duty cycle. Therefore, operating frequency of the LTC1772, ceramic capacitors it is important to adequately specify the diode peak current can also be used for CIN. Always consult the manufacturer and average power dissipation so as not to exceed the if there is any question. diode ratings. The selection of COUT is driven by the required effective Under normal load conditions, the average current con- series resistance (ESR). Typically, once the ESR require- ducted by the diode is: ment is satisfied, the capacitance is adequate for filtering. The output ripple ( ⎛ V V ⎞ ∆VOUT) is approximated by: I IN OUT = − I D OUT ⎝⎜ V + V IN D ⎠ ⎟ ∆ ⎛ 1 ⎞ V ≈ I ESR OUT RIPPLE + The allowable forward voltage drop in the diode is calcu- ⎝⎜ 4fCOUT ⎠⎟ lated from the maximum short-circuit current as: where f is the operating frequency, COUT is the output P capacitance and IRIPPLE is the ripple current in the induc- V D F ≈ tor. The output ripple is highest at maximum input voltage ISC MAX ( ) since ∆IL increases with input voltage. where PD is the allowable power dissipation and will be Manufacturers such as Nichicon, United Chemicon and determined by efficiency and/or thermal requirements. Sanyo should be considered for high performance through- A fast switching diode must also be used to optimize hole capacitors. The OS-CON semiconductor dielectric efficiency. Schottky diodes are a good choice for low capacitor available from Sanyo has the lowest ESR (size) forward drop and fast switching times. Remember to keep product of any aluminum electrolytic at a somewhat lead length short and observe proper grounding (see higher price. Once the ESR requirement for COUT has been Board Layout Checklist) to avoid ringing and increased met, the RMS current rating generally far exceeds the dissipation. IRIPPLE(P-P) requirement. In surface mount applications, multiple capacitors may
CIN and COUT Selection
have to be paralleled to meet the ESR or RMS current In continuous mode, the source current of the P-channel handling requirements of the application. Aluminum elec- MOSFET is a square wave of duty cycle (VOUT + VD)/ trolytic and dry tantalum capacitors are both available in (VIN + VD). To prevent large voltage transients, a low ESR surface mount configurations. In the case of tantalum, it is input capacitor sized for the maximum RMS current must critical that the capacitors are surge tested for use in be used. The maximum RMS capacitor current is given by: switching power supplies. An excellent choice is the AVX TPS, AVX TPSV and KEMET T510 series of surface mount / V [ (V −V )]1 2 OUT IN OUT tantalum, available in case heights ranging from 2mm to C I Required ≈ I IN RMS MAX 4mm. Other capacitor types include Sanyo OS-CON, VIN Nichicon PL series and Panasonic SP. 1772fb 8