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LTC1871-1 Datasheet(PDF) 28 Page - Linear Technology |
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LTC1871-1 Datasheet(HTML) 28 Page - Linear Technology |
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28 / 36 page  LTC1871-1 28 18711fb APPLICATIONS INFORMATION choice is AVX TPS series of surface mount tantalum. Also, ceramic capacitors are now available with extremely low ESR, ESL and high ripple current ratings. SEPIC Converter: Input Capacitor Selection The input capacitor of a SEPIC converter is less critical than the output capacitor due to the fact that an inductor is in series with the input and the input current waveform is triangular in shape. The input voltage source impedance determines the size of the input capacitor which is typi- cally in the range of 10μF to 100μF. A low ESR capacitor is recommended, although it is not as critical as for the output capacitor. The RMS input capacitor ripple current for a SEPIC con- verter is: IRMS(CIN) = 1 12 • IL Please note that the input capacitor can see a very high surge current when a battery is suddenly connected to the input of the converter and solid tantalum capacitors can fail catastrophically under these conditions. Be sure to specify surge-tested capacitors! SEPIC Converter: Selecting the DC Coupling Capacitor The coupling capacitor C1 in Figure 16 sees nearly a rectan- gular current waveform as shown in Figure 17. During the switch off-time the current through C1 is IO(VO/VIN) while approximately – IO flows during the on-time. This current waveform creates a triangular ripple voltage on C1: VC1(P P) = IO(MAX) C1• f • VO VIN + VO + VD The maximum voltage on C1 is then: VC1(MAX) = VIN + VC1(P P) 2 which is typically close to VIN(MAX). The ripple current through C1 is: IRMS(C1) =IO(MAX) • VO + VD VIN(MIN) The value chosen for the DC coupling capacitor normally starts with the minimum value that will satisfy 1) the RMS current requirement and 2) the peak voltage requirement (typically close to VIN). Low ESR ceramic and tantalum capacitors work well here. SEPIC Converter Design Example The design example given here will be for the circuit shown in Figure 18. The input voltage is 5V to 15V and the output is 12V at a maximum load current of 1.5A (2A peak). 1. The duty cycle range is: D = VO + VD VIN + VO + VD = 45.5% to 71.4% 2. The operating mode chosen is pulse skipping, so the MODE/SYNC pin is shorted to INTVCC. 3. The operating frequency is chosen to be 300kHz to reduce the size of the inductors; the resistor from the FREQ pin to ground is 80k. 4. An inductor ripple current of 40% is chosen, so the peak input current (which is also the minimum saturation current) is: IL1(PEAK) = 1+ 2 •IO(MAX) • VO + VD VIN(MIN) = 1+ 0.4 2 • 1.5 • 12 + 0.5 5 = 4.5A The inductor ripple current is: IL = •IO(MAX) • DMAX 1– DMAX = 0.4 •1.5 • 0.714 1– 0.714 = 1.5A |
Similar Part No. - LTC1871-1_15 |
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Similar Description - LTC1871-1_15 |
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