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AAT1152IKS-1.5-T1 Datasheet(PDF) 11 Page - Advanced Analogic Technologies |
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AAT1152IKS-1.5-T1 Datasheet(HTML) 11 Page - Advanced Analogic Technologies |
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11 / 18 page  AAT1152 850kHz 1A Synchronous Buck DC/DC Converter 1152.2003.01.0.9 11 Power Good The AAT 1152 features an integrated Power Good (POK) comparator and open-drain output signal. The POK pin goes low when the converter’s output is 12% or more below its nominal regulation volt- age or when the device is in shutdown. Connect a pull-up resistor from POK to the converter’s input or output. Typical resistor pull-up values range from 100k to 10k. Inductor The output inductor is selected to limit the ripple current to some predetermined value, typically 20- 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 limi- tation, and the peak current rating, which is deter- mined by the saturation characteristics. The induc- tor should not show any appreciable saturation under all normal load conditions. During over load and short circuit conditions, the average current in the inductor can meet or exceed the ILIMIT point of the AAT1152 without effecting the converter per- formance. Some inductors may have sufficient peak and average current ratings yet result in excessive losses due to a high DCR. Always con- sider the losses associated with the DCR and its effect on the total converter efficiency when select- ing an inductor. For a 1 Amp load and the ripple set to 30% at the maximum input voltage, the maximum peak to peak ripple current is 300 mA. 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 + ∆I2 ≈ I o = 1.0A 12 ∆I is the peak to peak ripple current which is fixed by the inductor selection above. For a peak to peak cur- rent of 30% of the full load current the peak current at full load will be 115% of the full load. The 4.1µH inductor selected from the Sumida CDRH5D18 series has a 57 m Ω DCR and a 1.95 Amp DC cur- rent 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 AAT1152. A low ESR/ESL ceramic capacitor is ideal for this function. To mini- mize the stray inductance the capacitor should be placed as close 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 AAT1152. 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 current varies with the input voltage and the output voltage. The equation for the maximum RMS cur- rent in the input capacitor is: The input capacitor RMS ripple current reaches a maximum when VIN is two times the output voltage where it is approximately one half of the load cur- rent. Losses associated with the input ceramic capacitor are typically minimal and not an issue. The proper placement of the input capacitor can be seen in the reference design layout in figures 5 and 6. V O V O I RMS = IO ⋅ ⋅ 1 - V IN V IN V OUT V OUT 1.5 V 1.5V L = ⋅ 1 - L = 3.9 µH L = ⋅ 1 - I O ⋅ k ⋅ F V IN 1.0A ⋅ 0.3 ⋅ 830kHz 4.2V |
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