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SP6123A Datasheet(PDF) 10 Page - Sipex Corporation |
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SP6123A Datasheet(HTML) 10 Page - Sipex Corporation |
10 / 18 page 10 Date: 5/25/04 SP6123 Low Voltage, Synchronous Step Down PWM Controller © Copyright 2004 Sipex Corporation 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 ESR value multiplied by the change in load current. Because of the fast transient response provided by the SP6123 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 con- tributor 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 ∆ ≤ where: ∆VOUT = 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: ∆VOUT = (IPP (1–D))2+(IPPRESR)2 COUTFS where: D = duty cycle equal to VOUT/VIN COUT = output capacitance value Recommended capacitors that can be used ef- fectively in SP6123 applications are: low-ESR aluminum electrolytic capacitors, OS-CON ca- pacitors that provide a very high performance/ size ratio for electrolytic capacitors and low- ESR tantalum capacitors. AVX TPS series and Kemet T510 surface mount capacitors are popu- lar tantalum capacitors that work well in SP6123 applications. POSCAP from Sanyo is a solid electrolytic chip capacitor that has low ESR and high capacitance. For the same ESR value, POSCAP has lower profile compared with tan- talum capacitor. Panasonic offers the SP series of specialty poly- mer aluminum electrolytic surface mount ca- pacitors. These capacitors have a lower ESR than tantalum capacitors, reducing the total num- ber of capacitance required for a given transient response. 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. 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(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 opera- tion. 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. Ignor- ing the inductor ripple current, the input voltage ripple can be determined by: 2 ) ( ) ( (max) ) ( IN IN S OUT IN OUT MAX OUT CIN ESR out IN V C F V V V I R I V − + = ∆ The capacitor type suitable for the output ca- pacitors can also be used for the input capaci- tors. However, exercise extra caution when tan- talum capacitors are considered. Tantalum ca- pacitors 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 APPLICATIONS INFORMATION |
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