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ISL78010 Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL78010 Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 19 page  11 FN6501.1 May 3, 2011 Table 2 gives typical values (margins are considered 10%, 3%, 20%, 10%, and 15%) on VIN, VO, L, fS, and IOMAX. Input Capacitor An input capacitor is used to supply the peak charging current to the converter. It is recommended that CIN be larger than 10µF. The reflected ripple voltage will be smaller with larger CIN. The voltage rating of the input capacitor should be larger than the maximum input voltage. Boost Inductor The boost inductor is a critical part which influences the output voltage ripple, transient response, and efficiency. Values of 3.3µH to 10µH are to match the internal slope compensation. The inductor must be able to handle the following average (Equation 5) and peak (Equation 6) current: Rectifier Diode A high-speed diode is necessary due to the high switching frequency. Schottky diodes are recommended because of their fast recovery time and low forward voltage. The rectifier diode must meet the output current and peak inductor current requirements. Output Capacitor The output capacitor supplies the load directly and reduces the ripple voltage at the output. Output ripple voltage consists of two components: the voltage drop due to the inductor ripple current flowing through the ESR of output capacitor, and the charging and discharging of the output capacitor (Equation 7). For low ESR ceramic capacitors, the output ripple is dominated by the charging and discharging of the output capacitor. The voltage rating of the output capacitor should be greater than the maximum output voltage. NOTE: Capacitors have a voltage coefficient that makes their effective capacitance drop as the voltage across them increases. COUT in Equation 7 assumes the effective value of the capacitor at a particular voltage and not the manufacturer’s stated value, measured at zero volts. Compensation The ISL78010 can operate in either P-mode or PI-mode. P-mode may be preferred in applications where excellent transient load performance is required but regulation is not critical. Connecting the CINT pin directly to VIN will enable P-mode; for better load regulation, use PI-mode with a 4.7nF capacitor in series with a 10k resistor between CINT and ground. This value may be reduced to improve transient performance; however, very low values will reduce loop stability. Figures 5 through 10 show a comparison of P-mode vs PI-mode performance. Boost Feedback Resistors As the boost output voltage, AVDD, is reduced below 12V, the effective voltage feedback in the IC increases the ratio of voltage to current feedback at the summing comparator because R2 decreases relative to R1. To maintain stable operation over the complete current range of the IC, the voltage feedback to the FBB pin should be reduced proportionally, as AVDD is reduced. This can be accomplished by means of a series resistor-capacitor network (R7 and C7; Equations 8 and 9) in parallel with R1, with a pole frequency (fp) set to approximately 10kHz for C2 (effective) = 10µF and 4kHz for C2 (effective) = 30µF. PI-Mode CINT (C23) and RINT (R10) The IC is designed to operate with a minimum C23 capacitor of 4.7nF and a minimum C2 (effective) = 10µF. Note that, for high voltage AVDD, the voltage coefficient of ceramic capacitors (C2) reduces their effective capacitance greatly; a 16V, 10µF ceramic can drop to around 3µF at 15V. To improve the transient load response of AVDD in PI-mode, a resistor may be added in series with the C23 capacitor. The larger the resistor, the lower the overshoot, but at the expense of stability of the converter loop, especially at high currents. With L = 10µH, AVDD = 15V, and C23 = 4.7nF, C2 (effective) should have a capacitance of greater than 10µF. RINT (R7) can have values up to 5k Ω for C2 (effective) up to 20µF and up to 10k for C2 (effective) up to 30µF. TABLE 2. TYPICAL VIN, VO, L, fS, AND IOMAX VALUES VIN (V) VO (V) L (µH) fS (MHz) IOMAX (A) 3.3 9 6.8 1 0.490686 3.3 12 6.8 1 0.307353 3.3 15 6.8 1 0.197353 5 9 6.8 1 0.743464 5 12 6.8 1 0.465686 5 15 6.8 1 0.29902 ILAVG IO 1D – ------------- = (EQ. 5) ILPK ILAVG ΔI L 2 -------- + = (EQ. 6) VRIPPLE ILPK ESR VO VIN – VO ------------------------ IO COUT ---------------- 1 fS ----- × × + × = (EQ. 7) R7 1 0.1 R2 × ---------------------- ⎝⎠ ⎛⎞ 1 R1 ------- – ⎝⎠ ⎛⎞ 1 – = (EQ. 8) C7 1 2 3.142 fp R7 × × × ------------------------------------------------- = (EQ. 9) ISL78010 |
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