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ISL98012 Datasheet(PDF) 8 Page - Intersil Corporation |
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ISL98012 Datasheet(HTML) 8 Page - Intersil Corporation |
8 / 10 page 8 FN6654.1 December 8, 2010 completely dried out before the MOSFET is turned on again. The input voltage source, the inductor, and the MOSFET and output diode parasitic capacitors form a resonant circuit. Oscillation will occur in this period. This oscillation is normal and will not affect regulation. At very low load, the MOSFET will skip pulses sometimes; this is normal. Current Limit The MOSFET current limit is nominally 1.4A and guaranteed 1A. This restricts the maximum output current IOMAX based on Equation 1: where: • ΔIL is the inductor peak-to-peak current ripple and is decided by Equation 2: • D is the MOSFET turn-on ratio and is decided by Equation 3: •fS is the switching frequency Table1 gives typical values: Component Considerations It is recommended that CIN is larger than 10µF. Theoretically, the input capacitor has a ripple current of ΔIL. Due to high-frequency noise in the circuit, the input current ripple may exceed the theoretical value. A larger capacitor will reduce the ripple further. The inductor has peak and average current decided by Equations 4 and 5: The inductor should be chosen to handle this current. Furthermore, due to fixed internal compensation, it is recommended that maximum inductance of 10µH and 15µH be used in the 5V and 12V or higher output voltage, respectively. The output diode has an average current of IO and peak current is the same as the inductor's peak current. A Schottky diode is recommended and it should be able to handle those currents. The output voltage ripple can be calculated as Equation 6: Where: •CO is the output capacitance. • The ESR is the output capacitor ESR value. Low ESR capacitors should be used to minimize output voltage ripple. Multilayer ceramic capacitors (X5R and X7R) are preferred for output capacitors since they have a low ESR and small packages. Tantalum capacitors also can be used, but they take more board space and have higher ESR. A minimum of 22µF output capacitor is sufficient for high output current application. For lower output current, the output capacitor can be smaller, like 4.7µF. The capacitor should always have enough voltage rating. In addition to the voltage rating, the output capacitor should also be able to handle the RMS current, which is given by Equation 7: Output Voltage An external resistor divider is required to divide the output voltage down to the nominal reference voltage. The 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 less than 300k Ω is recommended. TABLE 1. MAX CONTINUOUS OUTPUT CURRENTS VIN (V) VO (V) L (µH) fS (kHz) IOMAX (mA) 2 5 10 750 360 2 9 10 750 190 2 12 10 750 140 3.3 5 10 750 600 3.3 9 10 750 310 3.3 12 10 750 230 5 9 10 750 470 5 12 10 750 340 9 12 10 750 630 12 15 10 750 670 IOMAX 1 ΔI L 2 -------- – ⎝⎠ ⎛⎞ VIN VO --------- × = (EQ. 1) ΔI L VIN L --------- D fS ----- × = (EQ. 2) D VO VIN – VO ------------------------ = (EQ. 3) ILPK ILAVG ΔI L 2 -------- + = (EQ. 4) ILAVG IO 1D – ------------- = (EQ. 5) ΔV O IO D × FS CO × ---------------------- ILPK ESR × + = (EQ. 6) ICORMS 1 ( D ) D ΔI L 2 ILAVG 2 -------------------- + ⎝ ⎜ ⎜ ⎛ × 1 12 ------ × ⎠ ⎟ ⎟ ⎞ – ILAVG × = (EQ. 7) ISL98012 |
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