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XRP6124HVES0.5-F Datasheet(PDF) 9 Page - Exar Corporation |
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XRP6124HVES0.5-F Datasheet(HTML) 9 Page - Exar Corporation |
9 / 12 page X XR RP P6 61 12 24 4 N No on n--S Sy yn nc ch hrro on no ou us s P PF FE ET T S Stte ep p--D Do ow wn n C Co on nttrro olllle err © 2011 Exar Corporation 9/12 Rev. 1.1.0 OUTPUT INDUCTOR Select the output inductor L1 for inductance L, DC current rating IDC and saturation current rating ISAT. IDC should be larger than regulator output current. ISAT, as a rule of thumb, should be 50% higher than the regulator output current. Calculate the inductance from: () ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ × × Δ − = IN L OUT OUT IN V fs I V V V L Where: ΔIL is peak-to-peak inductor current ripple nominally set to 30% of IOUT fS is nominal switching frequency from table 2 OUTPUT CAPACITOR COUT Select the output capacitor for voltage rating, capacitance COUT and Equivalent Series Resistance ESR. The voltage rating, as a rule of thumb, should be twice the output voltage. When calculating the required capacitance, usually the overriding requirement is current load-step transient. If the unloading transient requirement (i.e., when IOUT transitions from a high to a low current) is met, then usually the loading transient requirement (when IOUT transitions from a low to a high current) is met as well. Therefore calculate the COUT capacitance based on the unloading transient requirement from: () ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − + − × = 2 2 2 2 OUT transient OUT LOW High OUT V V V I I L C Where: L is the inductance calculated in the preceding step IHigh is the value of IOUT prior to unloading. This is nominally set equal to regulator current rating. ILow is the value of IOUT after unloading. This is nominally set equal to 50% of regulator current rating. Vtransient is the maximum permissible voltage transient corresponding to the load step mentioned above. Vtransient is typically specified from 3% to 5% of VOUT. ESR of the capacitor has to be selected such that the output voltage ripple requirement VOUT(ripple), nominally 1% of VOUT, is met. Voltage ripple VOUT(ripple) is composed mainly of two components: the resistive ripple due to ESR and capacitive ripple due to COUT charge transfer. For applications requiring low voltage ripple, ceramic capacitors are recommended because of their low ESR which is typically in the range of 5mΩ. Therefore VOUT(ripple) is mainly capacitive. For ceramic capacitors calculate the VOUT(ripple) from: fs C I OUT L × × Δ = 8 V ) OUT(ripple Where: COUT is the value calculated above If tantalum or electrolytic capacitors are used then VOUT(ripple) is essentially a function of ESR: ESR I L × Δ = V ) OUT(ripple INPUT CAPACITOR CIN Select the input capacitor for voltage rating, RMS current rating and capacitance. The voltage rating, as a rule of thumb, should be 50% higher than the regulator’s maximum input voltage. Calculate the capacitor’s current rating from: () D D I OUT − × × = 1 I RMS CIN, Where: IOUT is regulator’s maximum current D is duty cycle (D=VOUT/VIN) Calculate the CIN capacitance from: () IN IN OUT IN OUT OUT V V fs V V V I Δ × × − × × = 2 IN C Where: ΔVIN is the permissible input voltage ripple, nominally set to 1% of VIN. |
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