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AAT1151 Datasheet(PDF) 9 Page - Advanced Analogic Technologies |
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AAT1151 Datasheet(HTML) 9 Page - Advanced Analogic Technologies |
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9 / 16 page  AAT1151 850kHz 700mA Synchronous Buck DC/DC Converter 1151.2007.01.1.6 9 Current Limit and Over-Temperature Protection For overload conditions, the peak input current is limited. As load impedance decreases and the output voltage falls closer to zero, more power is dissipated internally, raising the device tempera- ture. Thermal protection completely disables switching when internal dissipation becomes excessive, protecting the device from damage. The junction over-temperature threshold is 140°C with 10°C of hysteresis. Inductor The output inductor is selected to limit the ripple cur- rent 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 thermal limita- tion, and the peak current rating, which is deter- mined by the saturation characteristics. The inductor should not show any appreciable saturation under normal load conditions. During overload and tran- sient conditions, the average current in the inductor can meet or exceed the current limit point of the AAT1151. These conditions can tolerate greater sat- uration in the inductor without degradation in con- verter performance. Some inductors may meet the peak and average current ratings yet result in exces- sive 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 40% at the maximum input voltage, the maximum peak-to- peak ripple current is 280mA. The inductance value required is 2.84μH. The factor "k" is the fraction of full load selected for the ripple current at the maximum input voltage. For ripple current at 40% of the full load current, the peak current will be 120% of full load. Selecting a standard value of 3.0μH gives 38% ripple current. A 3.0μH inductor selected from the Sumida CDRH5D28 series has a 24m Ω DCR and a 2.4A DC current rating. At full load, the inductor DC loss is 24mW, which amounts to a 1.6% loss in efficiency. L = • 1 - V OUT V OUT I O • k • FS V IN = • 1 - 1.5V 1.5V 1A • 0.4 • 850kHz 4.2V ⎛⎞ ⎝⎠ ⎛⎞ ⎛ ⎞ ⎝⎠ ⎝ ⎠ = 2.84 μH Figure 1: AAT1151 Evaluation Board. Efficiency vs. Load Current (VOUT = 1.8V) 50 60 70 80 90 100 1 10 100 1000 Load Current (mA) 2.7V 3.6V 4.2V L1 3.3 μH C3, C4 2x22 μF C1 10 μF R1 100 C2 0.1 μF C1 Murata 10 μF 6.3V X5R GRM42-6X5R106K6.3 R2 100k C3-C4 MuRata 22 μF 6.3V GRM21BR60J226ME39L X5R 0805 1.8V 2.7V-4.2V OUT 4 GND 5 EN 7 VCC 9 VP 10 LX 13 LX 14 PGND 3 VP 11 GND 6 PGND 2 LX 15 N/C 16 PGND 1 N/C 8 VP 12 U1 AAT1151-QFN R6 100k L1 Sumida CDRH3D16-4R7NC or CDRH3D16-3R3NC |
Similar Part No. - AAT1151_07 |
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Similar Description - AAT1151_07 |
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