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ISL97645AIRZ-T Datasheet(PDF) 9 Page - Intersil Corporation |
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ISL97645AIRZ-T Datasheet(HTML) 9 Page - Intersil Corporation |
9 / 15 page 9 FN6353.0 July 2, 2007 Operation The boost converter is a current mode PWM converter operating at either a 650kHz or 1.2MHz. It can operate in both discontinuous conduction mode (DCM) at light load 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. Figure 12 shows the block diagram of the boost regulator. It uses a summing amplifier architecture consisting of gm stages 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 reached. 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: The current through the MOSFET is limited to 2.6APEAK. This restricts the maximum output current (average) based on Equation 3: Where ΔI L is peak to peak inductor ripple current, and is set by Equation 4: where fS is the switching frequency (650kHz or 1.2MHz). Table 2 gives typical values (margins are considered 10%, 3%, 20%, 10% and 15% on VIN, VO, L, fS and IOMAX). Capacitor An input capacitor is used to suppress the voltage ripple injected into the boost converter. The 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 1 for input capacitor. V Boost V IN ------------------- 1 1D – ------------- = (EQ. 1) V Boost R 1 R 2 + R 2 --------------------- V FB × = (EQ. 2) TABLE 1. BOOST CONVERTER INPUT CAPACITOR RECOMMENDATION CAPACITOR SIZE MFG PART NUMBER 10µF/16V 1206 TDK C3216X7R1C106M 10µF/10V 0805 Murata GRM21BR61A106K 22µF/10V 1210 Murata GRB32ER61A226K I OMAX I LMT ΔI L 2 -------- – ⎝⎠ ⎛⎞ VIN V O --------- × = (EQ. 3) ΔI L V IN L --------- D f s ---- × = (EQ. 4) TABLE 2. MAXIMUM OUTPUT CURRENT CALCULATION VIN (V) VO (V) L (µH) FS (MHz) IOMAX (mA) 3 9 10 0.65 636 3 12 10 0.65 419 3 15 10 0.65 289 5 9 10 0.65 1060 5 12 10 0.65 699 5 15 10 0.65 482 5 18 10 0.65 338 3 9 10 1.2 742 3 12 10 1.2 525 3 15 10 1.2 395 5 9 10 1.2 1236 5 12 10 1.2 875 5 15 10 1.2 658 5 18 10 1.2 514 ISL97645A |
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