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AAT1184 Datasheet(PDF) 11 Page - Skyworks Solutions Inc. |
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AAT1184 Datasheet(HTML) 11 Page - Skyworks Solutions Inc. |
11 / 18 page 11 AAT1184 High Voltage Step-Down Regulator DATA SHEET Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com 202000A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 29, 2012 Applications Information The high voltage DC/DC step-down converter provides an output voltage from 1.5V to 5.5V. The integrated high-side n-channel MOSFET device provides up to 1.2A output current1. Input voltage range is 6.0V to 24.0V. The step-down converter utilizes constant frequency (PWM-mode) voltage mode control to achieve high oper- ating efficiency while maintaining extremely low output noise across the operating range. High 490kHz (nominal) switching frequency allows small external filtering com- ponents, achieving minimum cost and solution size. External compensation allows the designer to optimize the transient response while achieving stability across the operating range. Output Voltage and Current The output voltage is set using an external resistor divider as shown in Table 1. Minimum output voltage is 1.5V and maximum output voltage is 5.5V. Typical max- imum duty cycle is 85%. VOUT(V) R5 = 6.04k R4(k) 1.5 9.09 1.8 12.1 1.85 12.4 2.0 14.0 2.5 19.1 3.0 24.3 3.3 27.4 5.0 44.2 Table 1: Feedback Resistor Values. Alternatively, the feedback resistor may be calculated using the following equation: (V OUT - 0.6) · R5 0.6 R 4 = R4 is rounded to the nearest 1% resistor value. Buck Regulator Output Capacitor Selection A 22μF ceramic output capacitor is required to filter the inductor current ripple and supply the load transient cur- rent for IOUT = 1.2A. The 1206 package with 10V mini- mum voltage rating is recommended for the output capacitors to maintain a minimum capacitance drop with DC bias. Output Inductor Selection The step-down converter utilizes constant frequency (PWM-mode) voltage mode control. A 4.7μH inductor value is selected to maintain the desired output current ripple and minimize the converter’s response time to load transients. The peak switch current should not exceed the inductor saturation current, the MOSFET or the external Schottky rectifier peak current ratings. Rectifier Selection When the high-side switch is on, the input voltage will be applied to the cathode of the Schottky diode. The recti- fier's rated reverse breakdown voltage must be chosen at least equal to the maximum input voltage of the step- down regulator. When the high-side switch is off, the current will flow from the power ground to the output through the Schottky diode and the inductor. The power dissipation of the Schottky diode during the time-off can be deter- mined by the following equation: V OUT P D = IOUT · VD · 1 - V IN Where VD is the voltage drop across the Schottky diode. Input Capacitor Selection For low cost applications, a 100μF/25V electrolytic capacitor is selected to control the voltage overshoot across the high side MOSFET. A small ceramic capacitor with voltage rating at least 1.05 times greater than the maximum input voltage is connected as close as possible to the input pin (Pin 14) for high frequency decoupling. Feedback and Compensation Networks The transfer function of the Error Amplifier is dominated by the DC Gain and the L COUT output filter of the regula- tor. This output filter and its equivalent series resistor (ESR) create a double pole at FLC and a zero at FESR in the following equations: Eq. 1: F LC = 1 2 · π · L · C OUT 1. Output current capability may vary and is dependent on package selection, maximum ambient temperature, airflow and PCB heatsinking. |
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