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SP7651 Datasheet(PDF) 8 Page - Exar Corporation |
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SP7651 Datasheet(HTML) 8 Page - Exar Corporation |
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8 / 15 page  8 Rev J: 3/4/07 SP765 Wide Input Voltage Range 3A, 900kHz, Buck Regulator © Copyright 2007 Sipex Corporation Output Capacitor Selection The required ESR (Equivalent Series Re- sistance) and capacitance drive the selec- tion of the type and quantity of the output capacitors. The ESR must be small enough that both the resistive voltage deviation due to a step change in the load current and the output ripple voltage do not exceed the tolerance limits expected on the output voltage. During an output load transient, the output capacitor must supply all the ad- ditional current demanded by the load until the SP7651 adjusts the inductor current to the new value. In order to maintain VOUT, the capacitance must be large enough so that the output voltage is held up while the inductor current ramps up or down to the value correspond- ing to the new load current. Additionally, the ESR in the output capacitor causes a step in the output voltage equal to the current. Because of the fast transient response and inherent 100% to 0% duty cycle capability provided by the SP765 when exposed to an outputload transient,the outputcapacitor is typically chosen for ESR, not for capaci- tance value. The ESR of the output capacitor, combined with the inductor ripple current, is typically the main contributor to output voltage ripple. The maximum allowable ESR required to maintain a specified output voltage ripple can be calculated by: where: ∆VOUT = Peak-to-Peak Output Voltage Ripple IPK-PK = Peak-to-Peak Inductor Ripple Cur- rent The total output ripple is a combination of the ESR and the output capacitance value and can be calculated as follows: ∆VOUT = (IPP(–D))2 + (IPPRESR)2 COUTFS FS = Switching Frequency D = Duty Cycle COUT = Output Capacitance Value Input Capacitor Selection The input capacitor should be selected for ripple current rating, capacitance and volt- age rating. The input capacitor must meet the ripple current requirement imposed by the switching current. In continuous conduction mode, the source current of the high-side MOSFET is approximately a square wave of duty cycle VOUT/VIN. Most of this current is supplied by the input bypass capacitors. The RMS value of input capacitor current is determined at the maximum output current and under the assumption that the peak- to-peak inductor ripple current is low; it is given by: ICIN(rms) = IOUT(max) √D( - D) The worse case occurs when the duty cycle D is 50% and gives an RMS current value equal to IOUT/2. Select input capacitors with adequate ripple current rating to ensure reli- able operation. The power dissipated in the input capaci- tor is: This can become a significant part of power losses in a converter and hurt the overall energy transfer efficiency. The input volt- age ripple primarily depends on the input capacitor ESR and capacitance. Ignoring the inductor ripple current, the input voltage ripple can be determined by: The capacitor type suitable for the output capacitors can also be used for the input ca- pacitors. However, exercise extra caution APPLICATIONS INFORMATION ∆Vout Ipk-pk Pon= i2on(rms) resr(cin) ∆Vin=iout(max) resr(cin) + Iout(max) Vout (Vin-Vout) FscinVin2 Resr ≤ |
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