Datasheet LTC1874 (Analog Devices) - 9

LTC1874
U U W U APPLICATIO S I FOR ATIO
short-circuited. Under this condition the diode must safely This formula has a maximum at VIN = 2VOUT, where IRMS handle IPEAK at close to 100% duty cycle. Therefore, it is = IOUT/2. This simple worst-case condition is commonly important to adequately specify the diode peak current and used for design because even significant deviations do not average power dissipation so as not to exceed the diode offer much relief. Several capacitors may be paralleled to ratings. meet the size or height requirements in the design. Due to the high operating frequency of the controller, ceramic Under normal load conditions, the average current con- capacitors can also be used for C ducted by the diode is: IN. Always consult the manufacturer if there is any question.  V V  The selection of C I IN OUT = − I OUT is driven by the required effective D OUT  V + V series resistance (ESR). Typically, once the ESR require- IN D   ment is satisfied, the capacitance is adequate for filtering. The allowable forward voltage drop in the diode is calcu- The output ripple (∆VOUT) is approximated by: lated from the maximum short-circuit current as:  1  ∆V ≈ I ESR + P OUT RIPPLE V D  4fC  F ≈ OUT ISC MAX ( ) where f is the operating frequency, COUT is the output where PD is the allowable power dissipation and will be capacitance and IRIPPLE is the ripple current in the induc- determined by efficiency and/or thermal requirements. tor. The output ripple is highest at maximum input voltage since ∆I Schottky diodes are a good choice for low forward drop L increases with input voltage. and fast switching times. Remember to keep lead length Once the ESR requirement for COUT has been met, the short and observe proper grounding (see Board Layout RMS current rating generally far exceeds the IRIPPLE(P-P) Checklist) to avoid ringing and increased dissipation. requirement. Multiple capacitors may have to be paral- leled to meet the ESR or RMS current handling require-
CIN and COUT Selection
ments of the application. Aluminum electrolytic and dry In continuous mode, the source current of the P-channel tantalum capacitors are both available in surface mount MOSFET is a square wave of duty cycle (V configurations. An excellent choice of tantalum capacitors OUT + VD)/ (V are the AVX TPS and KEMET T510 series of surface mount IN + VD). To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must tantalum capacitors. be used. The maximum RMS capacitor current is given by: / V V [ ( −V )]12 OUT IN OUT C Required I I IN RMS ≈ MAX VIN 9