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SP6134CU-L/TR Datasheet(PDF) 9 Page - Exar Corporation |
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SP6134CU-L/TR Datasheet(HTML) 9 Page - Exar Corporation |
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9 / 15 page  S SP P6 61 13 34 4 6 60 00 0k kH Hz z S Sy yn nc ch hrro on no ou us s P PW WM M S Stte ep p D Do ow wn n C Co on nttrro olllle err © 2008 Exar Corporation 9/15 Rev. 2.0.0 exceed the tolerance limits expected on the output voltage. During an output load transient, the output capacitor must supply all the additional current demanded by the load until the SP6134CU adjusts the inductor current to the new value. Therefore the capacitance must be large enough so that the output voltage is help up while the inductor current ramps up or down to the value corresponding 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% and 0% duty cycle capability provided by the SP6134CU when exposed to output load transient, the output capacitor is typically chosen for ESR, not for capacitance value. The output capacitor’s ESR, 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: PP () OUT ESR I V R Δ ≤ ΔVOUT = Peak to Peak Output Voltage Ripple IPP = Peak to Peak Inductor Ripple Current The total output ripple is a combination of the ESR and the output capacitance value and can be calculated as follows: ()2 2 1 ESR PP S OUT PP OUT R I F C D I V + ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − = Δ () ( ) () Where: 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 voltage 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: D D I I OUT rms CIN − = 1 max () () CIN ESR rms CIN CIN R I P 2 = () ( ) () 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 reliable operation. The power dissipated in the input capacitor is: This can become a significant part of power losses in a converter and hurt the overall energy transfer efficiency. The input voltage ripple primarily depends on the input capacitor ESR and capacitance. Ignoring the inductor ripple current, the input voltage ripple can be determined by: ( ) 2 max max IN IN S OUT IN OUT OUT CIN ESR OUT IN V C F V V V I R I V − + = Δ The capacitor type suitable for the output capacitors can also be used for the input capacitors. However, exercise extra caution when tantalum capacitors are considered. Tantalum capacitors are known for catastrophic failure when exposed to surge current, and input capacitors are prone to such surge current when power supplies are connected “live” to low impedance power sources. |
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