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AAT1141IGV-1.2-T1 Datasheet(PDF) 11 Page - Advanced Analogic Technologies |
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AAT1141IGV-1.2-T1 Datasheet(HTML) 11 Page - Advanced Analogic Technologies |
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11 / 20 page  AAT1141 Fast Transient 600mA Step-Down Converter SwitchRegTM PRODUCT DATASHEET 1141.2008.10.1.6 11 www .analogictech.com Applications Information Inductor Selection The step-down converter uses peak current mode con- trol with slope compensation to maintain stability for duty cycles greater than 50%. The output inductor value must be selected so the inductor current down slope meets the internal slope compensation require- ments. The internal slope compensation for the adjust- able and low-voltage fixed versions of the AAT1141 is 0.24A/μsec. This equates to a slope compensation that is 75% of the inductor current down slope for a 1.5V output and 4.7μH inductor. 0.75 · VO m = = = 0.24 L 0.75 · 1.5V 4.7μH A μs This is the internal slope compensation for the adjust- able (0.6V) version or low-voltage fixed versions. When externally programming the 0.6V version to 2.5V, the calculated inductance is 7.5μH. 0.75 · VO L = = ≈ 3 · VO = 3 · 2.5V = 7.5μH m 0.75 · VO 0.24 μs A μs A A μs In this case, a standard 6.8μH value is selected. For high-voltage fixed versions (≥2.5V), m = 0.48A/ μsec. Table 1 displays inductor values for the AAT1141 fixed and adjustable options. Manufacturer's specifications list both the inductor DC current rating, which is a thermal limitation, and the peak current rating, which is determined by the satura- tion characteristics. The inductor should not show any appreciable saturation under normal load conditions. Some inductors may meet the 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. Output Voltage (V) Inductor (μH) Output Capacitor (μF) 1, 1.2 2.2 10 1.5, 1.8 4.7 4.7 2.5, 3.3 6.8 4.7 Table 1: Inductor and Output Capacitor Values. The 4.7μH CDRH2D14 series inductor selected from Sumida has a 135mΩ typical DCR and a 1A DC current rating. At full load, the inductor DC loss is 48mW which gives a 4.5% loss in efficiency for a 600mA, 1.8V out- put. Input Capacitor Select a 4.7μF to 10μF X7R or X5R ceramic capacitor for the input. To estimate the required input capacitor size, determine the acceptable input ripple level (VPP) and solve for C. The calculated value varies with input volt- age and is a maximum when VIN is double the output voltage. ⎛⎞ · 1 - ⎝⎠ VO VIN CIN = VO VIN ⎛⎞ - ESR · FS ⎝⎠ VPP IO ⎛⎞ · 1 - = for VIN = 2 · VO ⎝⎠ VO VIN VO VIN 1 4 CIN(MIN) = 1 ⎛⎞ - ESR · 4 · FS ⎝⎠ VPP IO Always examine the ceramic capacitor DC voltage coef- ficient characteristics when selecting the proper value. For example, the capacitance of a 10μF, 6.3V, X5R ceramic capacitor with 5.0V DC applied is actually about 6μF. The maximum input capacitor RMS current is: ⎛⎞ IRMS = IO · · 1 - ⎝⎠ VO VIN VO VIN The input capacitor RMS ripple current varies with the input and output voltage and will always be less than or equal to half of the total DC load current. ⎛⎞ · 1 - = D · (1 - D) = 0.52 = ⎝⎠ VO VIN VO VIN 1 2 for VIN = 2 · VO IO RMS(MAX) I 2 = The term ⎛⎞ · 1 - ⎝⎠ VO VIN VO VIN appears in both the input voltage ripple and input capacitor RMS current equations and is a maximum when VO is twice VIN. This is why the input voltage ripple and the input capacitor RMS current ripple are a maximum at 50% duty cycle. |
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