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AAT2138 Datasheet(PDF) 14 Page - Skyworks Solutions Inc. |
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AAT2138 Datasheet(HTML) 14 Page - Skyworks Solutions Inc. |
14 / 18 page Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com 202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013 14 AAT2138 High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch DATA SHEET Considering 20% capacitance tolerance, the minimum capacitance should be 684μF. So select a 680μF tanta- lum capacitor as CREGIN, as well as an additional 10μF ceramic capacitor to closely filter the input voltage VREGIN of the step-down converter on the PCB board. Input Capacitor Selection Select a 1μF X7R or X5R ceramic capacitor for the sys- tem input. To estimate the required input capacitor size, determine the acceptable input ripple level and solve for CIN. The calculated value varies with input voltage and is a maximum when VIN is twice the output voltage. C IN = · V OUT V IN 1 - V OUT V IN f S · V PP I OUT - ESR for VIN = 2 · VOUT, = · V OUT V IN 1 - V OUT V IN 1 4 and C IN(MIN) = 1 4 · f S · V PP I OUT - ESR The input capacitor provides a low impedance loop for the edges of pulsed current drawn by the AAT2138. Low ESR/ESL X7R and X5R ceramic capacitors are ideal for this function due to their low ESR and ESL. To minimize stray parasitic inductance, place the capacitor as close as possible to the IC. This keeps the high frequency content of the input current localized, minimizing EMI and input voltage ripple. The proper placement of the input capacitor (C1) can be seen in the evaluation board layout in the Layout section of this datasheet (see Figure 4). A laboratory test set-up typically consists of two long wires running from the bench power supply to the eval- uation board input voltage pins. The inductance of these wires, along with the low-ESR ceramic input capacitor, can create a high Q network that may affect converter performance. This problem often becomes apparent in the form of excessive ringing in the output voltage dur- ing load transients. Errors in the loop phase and gain measurements can also result. Feedback Resistor Selection The output voltage on the AAT2138 is adjustable with external resistors R1 and R2. To limit the bias current required for the external feedback resistor string while maintaining good noise immunity, the minimum sug- gested value for R2 is 59kΩ. Although a larger value will further reduce quiescent current, it will also increase the impedance of the feedback node, making it more sensi- tive to external noise and interference. The maximum value of R1 should be below 1Mohm to keep reference voltage normal and avoid noise coupling. The external resistor R1, combined with an external capacitor up to 22pF feed-forward capacitor (C4 in Figure 3), delivers enhanced transient response for extreme pulsed load applications and reduces ripple in light load conditions. The addition of the feed forward capacitor typically requires a larger output capacitor C31-C32 for stability. The external resistors set the out- put voltage according to the following equation: R1 = - 1 · R2 = V OUT V REF - 1 · R2 V OUT 0.6V Table 2 shows the standard 1% metal film resistor examples for different step-down ouput voltages VOUT (V) R2 = 59kΩ, R1 (kΩ) 3.3 267 3.4 274 3.6 294 3.8 316 Table 2: Resistor Selections for Different Output Voltage Settings. |
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