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ISL97653AIRZ-TK Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL97653AIRZ-TK Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 18 page  11 FN6367.1 February 21, 2008 Application Information AVDD Boost Converter The AVDD boost converter features a fully integrated 4.4A boost FET. The regulator uses a current mode PI control scheme which provides good line regulation and good transient response. It can operate in both discontinuous conduction mode (DCM) at light loads and continuous mode (CCM). In continuous current mode, current flows continuously in the inductor during the entire switching cycle in steady state operation. The voltage conversion ratio in continuous current mode is given by Equation 1: where D is the duty cycle of the switching MOSFET. The boost soft-start function is digitally controlled within a fixed 10ms time frame during which the current limit is increased in eight linear steps. The boost converter uses a summing amplifier architecture for voltage feedback, current feedback, and slope compensation. A comparator looks at the peak inductor current cycle by cycle and terminates the PWM cycle if the current limit is triggered. Since this comparison is cycle based, the PWM output will be released after the peak current goes below the current limit threshold. An external resistor divider is required to divide the output voltage down to the nominal reference voltage. Current drawn by the resistor network should be limited to maintain the overall converter efficiency. The maximum value of the resistor network is limited by the feedback input bias current and the potential for noise being coupled into the feedback pin. A resistor network in the order of 60k Ω is recommended. The boost converter output voltage is determined by Equation 2: where R3 and R4 are in the “” on page 5. Unless otherwise stated, component variables referred to in equations refer to the Typical Application Diagram. The current through the MOSFET is limited to 4.4A peak. This restricts the maximum output current (average) based on Equation 3: Where ΔIL is peak to peak inductor ripple current, and is set by Equation 4. fs is the switching frequency (680kHz). Table 1 gives typical values (worst case margins are considered 10%, 3%, 20%, 10% and 15% on VIN, VO, L, FSWand IOMAX): Boost Converter Input Capacitor An input capacitor is used to suppress the voltage ripple injected into the boost converter. A ceramic capacitor with capacitance larger than 10µF is recommended. The voltage rating of input capacitor should be larger than the maximum input voltage. Some capacitors are recommended in Table 2 for input capacitor. 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 recommended to match the internal slope compensation as well as to maintain a good transient response performance. The inductor must be able to handle the average and peak currents expressed in Equations 5 and 6: Some inductors are recommended in Table 3. V boost V IN ------------------ 1 1D – ------------- = (EQ. 1) A VDD R 3 R 4 + R 4 --------------------- V FBB × = (EQ. 2) I OMAX I LMT ΔI L 2 -------- – ⎝⎠ ⎛⎞ VIN V O --------- × = (EQ. 3) ΔI L V IN L --------- D f S ----- × = (EQ. 4) TABLE 1. MAXIMUM OUTPUT CURRENT CALCULATION VIN (V) VO (V) L (µH) IOMAX (mA) 5 9 6.8 2215 5 12 6.8 1673 5 15 6.8 1344 12 15 6.8 3254 12 18 6.8 2670 TABLE 2. BOOST CONVERTER INPUT CAPACITOR RECOMMENDATION CAPACITOR SIZE VENDOR PART NUMBER 10µF/25V 1210 TDK C3225X7R1E106M 10µF/25V 1210 Murata GRM32DR61E106K TABLE 3. BOOST INDUCTOR RECOMMENDATION INDUCTOR DIMENSIONS (mm) VENDOR PART NUMBER 10µH/ 5.1APEAK 13x13x4.5 TDK RLF12545T-100M5R1 5.9µH/ 6APEAK 12.9X12.9X4 Sumida CDEP12D38NP-5R9MB-120 I LAVG I O 1D – ------------- = (EQ. 5) I LPK I LAVG ΔI L 2 -------- + = (EQ. 6) ISL97653A |
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