Datasheet LTC1779 (Analog Devices) - 9

LTC1779
applicaTions inForMaTion
Under normal load conditions, the average current con- The selection of COUT is driven by the required effective ducted by the diode is: series resistance (ESR). Typically, once the ESR require- ⎛ V ⎞ ment is satisfied, the capacitance is adequate for filtering. I IN− VOUT D =⎜ ⎟IOUT The output ripple (∆VOUT) is approximated by: ⎝ VIN+ VD ⎠ ⎛ 1 ⎞ The allowable forward voltage drop in the diode is calculated ΔVOUT ≈IRIPPLE ES ⎜ R+ ⎟ ⎝ 8fC from the maximum short-circuit current as: OUT ⎠ P where f is the operating frequency, COUT is the output V D F ≈ I capacitance and IRIPPLE is the ripple current in the induc- CL(MAX) tor. The output ripple is highest at maximum input voltage where PD is the allowable power dissipation and will be since ∆IL increases with input voltage. determined by efficiency and/or thermal requirements. Manufacturers such as Nichicon, United Chemicon and A fast switching diode must also be used to optimize Sanyo should be considered for high performance through- efficiency. Schottky diodes are a good choice for low hole capacitors. The OS-CON semiconductor dielectric forward drop and fast switching times. Remember to capacitor available from Sanyo has the lowest ESR (size) keep lead length short and observe proper grounding (see product of any aluminum electrolytic at a somewhat higher Board Layout Checklist) to avoid ringing and increased price. Once the ESR requirement for COUT has been met, dissipation. the RMS current rating generally far exceeds the IRIPPLE(P- P) requirement.
CIN and COUT Selection
In surface mount applications, multiple capacitors may In continuous mode, the source current of the internal have to be paralleled to meet the ESR or RMS current P-channel MOSFET is a square wave of duty cycle handling requirements of the application. Aluminum elec- (VOUT + VD)/(VIN + VD). To prevent large voltage transients, trolytic and dry tantalum capacitors are both available in a low ESR input capacitor sized for the maximum RMS surface mount configurations. In the case of tantalum, it current must be used. The maximum RMS capacitor is critical that the capacitors are surge tested for use in current is given by: switching power supplies. An excellent choice is the AVX TPS, AVX TPSV and KEMET T510 series of surface mount V ⎡ 1/2 ⎣ ( )⎤ C OUT VIN− VOUT ⎦ tantalum, available in case heights ranging from 2mm IN Required IRMS ≈IMAX VIN to 4mm. Other capacitor types include Sanyo OS-CON, This formula has a maximum at V Nichicon PL series and Panasonic SP. IN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly
Low Supply Operation
used for design because even significant deviations do not offer much relief. Note that capacitor manufacturer’s Although the LTC1779 can function down to approxi- ripple current ratings are often based on 2000 hours of life. mately 2.0V, the maximum allowable output current is This makes it advisable to further derate the capacitor, or reduced when VIN decreases below 3V. Figure 3 shows to choose a capacitor rated at a higher temperature than the amount of change as the supply is reduced down to required. Several capacitors may be paralleled to meet the 2V. Also shown in Figure 4 is the effect of VIN on VREF as size or height requirements in the design. Due to the high VIN goes below 2.3V. operating frequency of the LTC1779, ceramic capacitors can also be used for CIN. Always consult the manufacturer if there is any question. 1779fa For more information www.linear.com/LTC1779 9