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AAT1218IWP-3.3-T1 Datasheet(PDF) 9 Page - Skyworks Solutions Inc. |
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AAT1218IWP-3.3-T1 Datasheet(HTML) 9 Page - Skyworks Solutions Inc. |
9 / 15 page 9 AAT1218 High Current, 1.2MHz Synchronous Boost Converterr DATA SHEET Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com 202051B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 19, 2013 Integrated Soft-Start During start-up, the AAT1218's integrated soft-start cir- cuitry gradually increases the peak inductor current until it reaches normal operating value. The inrush current during start-up can thus be minimized. Current Mode PWM Control The AAT1218 is based on a slope compensated current mode control topology. It operates at a fixed frequency of 1.2MHz. At the beginning of each clock cycle, the main switch (NMOS) is turned on and the inductor current starts to ramp up. After the maximum duty cycle or if the sense current signal is equal to the error amplifier (EA) output, the main switch is turned off and the synchro- nous switch (PMOS) is turned on. This control scheme has intrinsic cycle-by-cycle current limiting, which can prevent the main switch from overstress and prevent saturation of the external inductor. Power Saving Mode At very light load, the AAT1218 automatically switches into Power Saving Mode to improve efficiency. In this mode, the PWM control will skip some pulses to maintain regulation. If load increases and output voltage drops, the device will automatically switch back to regular PWM mode and maintain regulation. Anti-ringing Control Anti-ringing circuitry is included to remove the high fre- quency ringing that appears on the LX pin when the inductor current decreases to zero. In this case, ringing on the LX pin is caused by energy stored in parasitics of the MOSFETs and the inductor. The anti-ringing circuitry clamps the voltage to battery voltage internally and thus dampens the ringing. Device Enable When EN is set logic high, the AAT1218 begins operation. If EN is set logic low, the device is goes into shutdown mode and consumes less than 1μA current. After start- up, the internal circuitry is supplied by VOUT during normal operation. However, if shutdown mode is enabled, the internal circuitry will again be supplied by the battery. Application Information Setting the Output Voltage An external resistor divider is used to set the output volt- age. The output voltage of the switching regulator (VOUT) is determined by the following equation: ⎛⎞ ⎝⎠ VOUT = 1.20V · 1 + R1 R2 VOUT R1 (Ω) R2 (Ω) 3.3V 1.02M 576k 5.0V 1.02M 324k Table 1: Resistor Selection Example for Output Voltage Setting. Fixed output voltage devices are also available for 3.3V and 5V outputs. These devices integrate the feedback network into the die and can save two external resistors. Do not connect any component to the FB pin and leave it floating. Inductor Selection The AAT1218's high switching frequency of 1.2MHz allows for small surface mount inductors. For most designs, the AAT1218 operates with inductors of 2.2μH to 10μH depending on input/output voltage and load cur- rent. First determine the worst case VIN, VOUT, and IOUT. Then use the equation below to select the proper induc- tor value; assume the converter is operating in continu- ous current mode and is in steady state: D = 1 - VIN VOUT IPEAK = IRMS + ΔI 2 ΔI = VIN · D L · fSW η is the converter efficiency, IPEAK is the peak inductor current, ΔI is the peak-peak inductor ripple current, IRMS is the RMS current of the inductor current. fsw is the switching frequency, L is the inductance value and D is the steady state duty cycle. |
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