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ADP1822 Datasheet(PDF) 14 Page  Analog Devices 

ADP1822 Datasheet(HTML) 14 Page  Analog Devices 
14 / 24 page ADP1822 Rev. B  Page 14 of 24 APPLICATION INFORMATION SELECTING THE INPUT CAPACITOR The output ripple voltage is a function of the inductor ripple current and the capacitor impedance at the switching frequency. For high ESR capacitors, the impedance is dominated by the ESR, while for low ESR capacitors the impedance is dominated by the capacitance. Determine if the capacitor is high ESR or low ESR by comparing the zero frequency formed by the capacitance and the ESR to the switching frequency: The input capacitor absorbs the switched input current of the dctodc converter, allowing the input source to deliver smooth dc current. Choose an input capacitor whose impedance at the switching frequency is lower than the input source impedance. Use low equivalent series resistance (ESR) capacitors, such as low ESR tantalum, ceramic, or organic electrolyte (such as Sanyo OSCON) types. For all types of capacitors, make sure that the current rating of the capacitor is greater than the input rms ripple current, which is approximately 1 − × × ≅ OUT IN IN OUT LOAD IN_RMS V V V V I I (1) OUTPUT LC FILTER The output LC filter smoothes the switched voltage at SW, making the output an almost dc voltage. Choose the output LC filter to achieve the desired output ripple voltage. Since the output LC filter is part of the regulator negativefeedback control loop, the choice of the output LC filter components affects the regulation controlloop stability. Choose an inductor value such that the inductor ripple current is approximately 1/3 of the maximum dc output load current. Using a larger value inductor results in a physical size larger than required, and using a smaller value results in increased losses in the inductor and/or MOSFET switches. Choose the inductor value by ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ − = IN OUT OUT L SW V V V I f L 1 ) Δ )( ( 1 (2) where: L is the inductor value. fSW is the switching frequency. VOUT is the output voltage. VIN is the input voltage. ΔI L is the inductor ripple current, typically 1/3 of the maximum dc load current. Choose the output capacitor to set the desired output voltage ripple. The ADP1822 functions with output capacitors that have both high and low ESR. For high ESR capacitors, such as tantalum or electrolytic types, many parallel connected capacitors may be required to achieve the desired output ripple voltage. When choosing an output capacitor, consider ripple current rating, capacitance, and ESR. Make sure that the ripple current rating is higher than the maximum inductor ripple current (ΔIL). ()( ) ESR C f OUT ESRZ π 2 1 = (3) where: fESRZ is the frequency of the output capacitor ESR zero. COUT is the output capacitance. ESR is the equivalent series resistance of the capacitor. If fESRZ is much less than the switching frequency, then the capacitor is high ESR, and the ESR dominates the impedance at the switching frequency. If fESRZ is much greater than the switching frequency, the capacitor is low ESR, and the impedance is dominated by the capacitance at the switching frequency. When using capacitors whose impedance is dominated by ESR at the switching frequency (such as tantalum or aluminum electrolytic capacitors), approximate the output voltage ripple current by ) ( Δ Δ ESR I V L OUT ≅ (4) where: ΔV OUT is the output ripple voltage. ΔI L is the inductor ripple current. ESR is the total equivalent series resistance of the output capacitor (or the parallel combination of ESR of all parallel connected output capacitors). Make sure that the ripple current rating of the output capacitor(s) is greater than the maximum inductor ripple current. For output capacitors whose ESR is much lower than the capacitive impedance at the switching frequency, the capacitive impedance dominates the output ripple current. In this case, determine the ripple voltage by ()( ) SW OUT L OUT f C I V 8 Δ Δ ≅ (5) where: fSW is the switching frequency. C is the output capacitance. OUT 
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