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SP7650 Datasheet(PDF) 8 Page - Exar Corporation |
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SP7650 Datasheet(HTML) 8 Page - Exar Corporation |
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8 / 16 page  Date: /5/06 SP7650 Wide Input Voltage Range 3A, 300kHz, Buck Regulator © Copyright 2006 Sipex Corporation 8 Date: 2/16/06 SP7650 Wide Input Voltage Range 3A, 300kHz, Buck Regulator © Copyright 2006 Sipex Corporation Output Capacitor Selection The required ESR (Equivalent Series Resis- tance) and capacitance drive the selection of the type and quantity of the output capacitors. The ESR must be small enough that both the resis- tive 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 addi- tional current demanded by the load until the SP7650 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 to the value corresponding to the new load current. Addi- tionally, the ESR in the output capacitor causes a step in the output voltage equal to the current. Because of the fast transient response and inher- ent 100%/0% duty cycle capability provided by the SP7650 when exposed to output load tran- sients, 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 maxi- mum allowable ESR required to maintain a specified output voltage ripple can be calculated by: RESR VOUT IPK-PK where: VOUT = Peak to Peak Output Voltage Ripple IPK-PK = 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: VOUT = (IPP(1–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 voltage rating. The input capacitor must meet the ripple current requirement imposed by the switching current. In continuous conduction mode, the source cur- rent of the high-side MOSFET is approximately a square wave of duty cycle VOUT/VIN. More accurately the current wave form is trapezoidal, given a finite turn-on and turn-off, switch tran- sition slope. Most of this current is supplied by the input bypass capacitors. The RMS current handling capability of the input capacitors is determined at maximum output current and under the assumption that the peak to peak inductor ripple current is low, it is given by: I CIN(RMS) = IOUT(max) D(1 - 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 reliable operation. The power dissipated in the input capacitor is: ) ( 2 ) ( CIN ESR rms CIN CIN R I P = 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 deter- mined by: 2 ) ( ) ( (max) ) ( IN IN S OUT IN OUT MAX OUT CIN E SR out IN V C F V V V I R I V + = APPLICATIONS INFORMATION |
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