Datasheet SD8942 (Shouding) - 6

SD8942 APPLICATION INFORMATION
Setting the Output Volta ge Selecting the Output Capacitor
The external resistor divi der is used to set the output The output capacitor (C2) is required to maintain the voltage (see Typical Application on page 1). The DC output voltage. C eramic, tantalum, or low ESR feedback resis tor R1 also sets the feedback loop electrolytic c apacitors are recommended. Low ESR bandwidth wit h the internal compens ation capacitor. capacitor s are pref erred to keep the output voltage Choos e R1 to be around 100kΩ for optimal transient ripple low. T response. R2 is then given by: he output voltage ripple can be estimated by: R1 V R =  V   1  ∆V = OUT × 1− OUT × R + 2 OUT fS    ESR  V /V −1 × L  VIN   8× f × C S 2  out FB Where L is the inductor value and RESR is the equivalent series resistance (ESR) value of the output capacitor. In the case of ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance. The output voltage ripple is mainly caused by the capacitance. For simplification, the output voltage ripple can be
Selecting the Inductor
estimated A 4.7μH to 22μH inductor with a DC current rating of at least 25% percent higher than the maximum load V  V  current is recommended for most applications. For ∆V = OUT × 1− OUT by: OUT 2   8× f × L × highest efficiency, the inductor DC resistance should C V S 2  IN  be less than 15mΩ. For most designs, the inductance value can be derived from the following In the case of tantalum or electrolytic capacitors, the equation. ESR dominates the impedance at the switching frequency. For simplification, the output ripple can V × V ( −V out in out be approximated to: L = ) V × I ∆ × f in L OSC   V V OUT OUT V ∆ = × 1− × R Where ΔIL is the inductor ripple current. Choose OUT   ESR × f L V inductor ripple current to be approximately 30% if S  IN  the maximum load current, 2A. The maximum inductor peak current is: The characteristics of the output capacitor also affect the stability of the regulation system. The SD8942 can be optimized for a wide range of capacitance I ∆ L I = I + and ESR values. L( MAX ) LOAD 2 Under light load conditions below 100mA, larger inductance is recommended for improved ef iciency. www.shouding.net 6