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AAT1150IKS-3.3-T1 Datasheet(PDF) 11 Page - Advanced Analogic Technologies |
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AAT1150IKS-3.3-T1 Datasheet(HTML) 11 Page - Advanced Analogic Technologies |
11 / 17 page AAT1150 1MHz 1A Step-Down DC/DC Converter 1150.2006.09.1.5 11 Inductor The output inductor is selected to limit the ripple current to some predetermined value, typically 20% to 40% of the full load current at the maximum input voltage. Manufacturer's specifications list both the inductor DC current rating, which is a ther- mal limitation, and the peak current rating, which is determined by the saturation characteristics. The inductor should not show any appreciable satura- tion under normal load conditions. During overload and short-circuit conditions, the average current in the inductor can meet or exceed the ILIMIT point of the AAT1150 without affecting converter perform- ance. Some inductors may have sufficient peak and average current ratings yet result in excessive losses due to a high DCR. Always consider the losses associated with the DCR and its effect on the total converter efficiency when selecting an inductor. For a 1.0A load and the ripple set to 30% at the maximum input voltage, the maximum peak-to- peak ripple current is 300mA. The inductance value required is 3.9μH. The factor "k" is the fraction of full load selected for the ripple current at the maximum input voltage. The corresponding inductor RMS current is: IRMS = ⎛Io2 + Δ I2 ⎞ ≈ I o = 1.0A ⎝ 12 ⎠ ΔI is the peak-to-peak ripple current which is fixed by the inductor selection above. For a peak-to-peak current of 30% of the full load current, the peak cur- rent at full load will be 115% of the full load. The 4.1μH inductor selected from the Sumida CDRH5D18 series has a 57m Ω DCR and a 1.95A DC current rating. At full load, the inductor DC loss is 57mW which amounts to a 3.8% loss in efficiency. Input Capacitor The primary function of the input capacitor is to pro- vide a low impedance loop for the edges of pulsed current drawn by the AAT1150. A low ESR/ESL ceramic capacitor is ideal for this function. To mini- mize stray inductance, the capacitor should be placed as closely as possible to the IC. This keeps the high frequency content of the input current localized, minimizing radiated and conducted EMI while facilitating optimum performance of the AAT1150. Ceramic X5R or X7R capacitors are ideal for this function. The size required will vary depending on the load, output voltage, and input voltage source impedance characteristics. A typi- cal value is around 10μF. The input capacitor RMS V OUT V OUT 1.5 V 1.5V L = ⋅ 1 - L = 3.9 μH L = ⋅ 1 - I O ⋅ k ⋅ FS V IN 1.0A ⋅ 0.3 ⋅ 830kHz 4.2V ⎛ ⎝ ⎞ ⎠ ⎛ ⎝ ⎞ ⎠ Figure 3: Current Limit Characteristics. 0 0.5 1 1.5 2 2.5 3 3.5 0 0.5 1 1.5 2 2.5 Output Current (A) VCC = VP = 3.6V VO = 1.5V Figure 1 Schematic VCC = VP = 5.0V VO = 3.3V Figure 2 Schematic |
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