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NX2113A Datasheet(PDF) 10 Page - Microsemi Corporation |
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NX2113A Datasheet(HTML) 10 Page - Microsemi Corporation |
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10 / 19 page  NX2113/2113A 10 Rev. 2.0 11/18/05 pacitors, both terms in equation (3) need to be evaluated to determine the overall ripple. Usually when this type of capacitors are selected, the amount of capacitance per single unit is not sufficient to meet the transient specifi- cation, which results in parallel configuration of multiple capacitors . For example, one 100uF, X5R ceramic capacitor with 2m Ω ESR is used. The amount of output ripple is ∆ = Ω ×+ ×× = += RIPPLE 3A V 2m 3A 8 600kHz 100uF 6mV 6.2mV 12.2mV Although this meets DC ripple spec, however it needs to be studied for transient requirement. Based On Transient Requirement Typically, the output voltage droop during transient is specified as: DROOP TRAN V <V ∆∆ @ step load DI STEP During the transient, the voltage droop during the transient is composed of two sections. One Section is dependent on the ESR of capacitor, the other section is a function of the inductor, output capacitance as well as input, output voltage. For example, for the overshoot, when load from high load to light load with a DI STEP transient load, if assuming the bandwidth of system is high enough, the overshoot can be estimated as the fol- lowing equation. 2 OUT overshoot step OUT V V ESRI 2LC ∆ = × ∆ + ×τ ×× ...(6) where τ is the a function of capacitor, etc. crit step OUT crit OUT 0 ifLL LI ESR C ifLL V ≤ ×∆ τ= − ×≥ ...(7) where OUT OUT EE OUT crit step step ESR C V ESRCV L II × × ×× == ∆∆ ...(8) where ESR E and CE represents ESR and capaci- tance of each capacitor if multiple capacitors are used in parallel. The above equation shows that if the selected out- put inductor is smaller than the critical inductance, the voltage droop or overshoot is only dependent on the ESR of output capacitor. For low frequency capacitor such as electrolytic capacitor, the product of ESR and ca- pacitance is high and crit LL ≤ is true. In that case, the transient spec is dependent on the ESR of capacitor. In most cases, the output capacitors are multiple capacitors in parallel. The number of capacitors can be calculated by the following E step 2 OUT tran E tran ESRI V N V 2L CV ×∆ = + ×τ ∆ × × ×∆ ...(9) where crit step E E crit OUT 0 ifLL LI ESR C ifLL V ≤ ×∆ τ= − ×≥ ...(10) For example, assume voltage droop during tran- sient is 100mV for 10A load step. If the POSCAP 2R5TPE220MC(220uF, 12m Ω ) is used, the critical inductance is given as: EE OUT crit step ESRCV L I 12m 220 F 1.6V 0.42H 10A ×× == ∆ Ω× µ× =µ The selected inductor is 0.78uH which is bigger than critical inductance. In that case, the output voltage transient not only dependent on the ESR, but also ca- pacitance. number of capacitors is step EE OUT LI ESRC V 0.78 H 10A 12m 220 F 2.24us 1.6V ×∆ τ = −× µ× = − Ω× µ= E step 2 OUT tran E tran 2 ESRI V N V 2L CV 12m 10A 80mV 1.6V (2.24us) 2 1.5 H 220 F 80mV 1.7 ×∆ = + ×τ ∆ × × ×∆ Ω× =+ × × µ × µ× ≈ The number of capacitors has to satisfy both ripple and transient requirement. Overall, we can choose N=3. |
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