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AAT1231ITP-1-T1 Datasheet(PDF) 10 Page - Advanced Analogic Technologies |
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AAT1231ITP-1-T1 Datasheet(HTML) 10 Page - Advanced Analogic Technologies |
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10 / 27 page  AAT1231 Step-Up DC/DC Converters for White LED Backlight Applications SwitchRegTM PRODUCT DATASHEET 10 1231.2008.06.1.5 www .analogictech.com pin can be programmed from 3.0x to 1.5x. In both prod- ucts, the step size is determined by the programmed voltage at the FB pin where the internal default setting is 3.0x in the AAT1231 and 1.5x in the AAT1231-1. Control Loop The AAT1231/1231-1 provide the benefits of current mode control with a simple hysteretic output current loop providing exceptional stability and fast response with minimal design effort. The device maintains excep- tional constant current regulation, transient response, and cycle-by-cycle current limit without additional com- pensation components. The AAT1231/1231-1 modulate the power MOSFET switching current to maintain the programmed FB volt- age. This allows the FB voltage loop to directly program the required inductor current in order to maintain the desired LED current. The switching cycle initiates when the N-channel MOSFET is turned ON and current ramps up in the inductor. The ON interval is terminated when the inductor current reaches the programmed peak current level. During the OFF interval, the input current decays until the lower threshold, or zero inductor current, is reached. The lower current is equal to the peak current minus a preset hys- teresis threshold, which determines the inductor ripple current. The peak current is adjusted by the controller until the LED output current requirement is met. The magnitude of the feedback error signal determines the average input current. Therefore, the AAT1231/1231-1 controller implements a programmed current source connected to the output capacitor, parallel with the LED string and ballast resistor. There is no right-half plane zero, and loop stability is achieved with no additional compensation components. An increase in the feedback voltage (VFB) results in an increased error signal sensed across the ballast resistor (R1). The controller responds by increasing the peak inductor current, resulting in higher average current in the inductor and LED string(s). Alternatively, when the VFB is reduced, the controller responds by decreasing the peak inductor current, resulting in lower average current in the inductor and LED string(s). Under light load conditions, the inductor OFF interval current goes below zero and the boost converter enters discontinuous mode operation. Further reduction in the load current results in a corresponding reduction in the switching frequency. The AAT1231/1231-1 provide pulsed frequency operation which reduces switching losses and maintains high efficiency under light load conditions. Operating frequency varies with changes in the input volt- age, output voltage, and inductor size. Once the boost converter has reached continuous mode, further increases in the LED current will not significantly change the operat- ing frequency. A small 2.2μH (±20%) inductor is selected to maintain high frequency switching (up to 2MHz) and high efficiency operation for outputs up to 24V. Soft Start / Enable The input disconnect switch is activated when a valid input voltage is present and the EN/SET pin is pulled high. The slew rate control on the P-channel MOSFET ensures minimal inrush current as the output voltage is charged to the input voltage, prior to switching of the N-channel power MOSFET. Monotonic turn-on is guaran- teed by the built-in soft-start circuitry. Soft start elimi- nates output current overshoot across the full input volt- age range and all loading conditions. After the soft start sequence has terminated, the initial LED current is determined by the internal, default FB voltage across the external ballast resistor at the FB pin. Additionally, the AAT1231 and the AAT1231-1 have been designed to offer the system designer two choices for the default FB voltage based on the state of the SEL pin. Changing the LED current from its initial default setting is easy by using the S2Cwire single wire serial interface; the FB voltage can be increased (as in the AAT1231; see Table 2) or decreased (as in the AAT1231-1; see Table 3) relative to the default FB voltage. Some applications may require the output to be active when a valid input voltage is present. In these cases, add a 10kΩ resistor between the VIN, VP, and EN/SET pins to avoid startup issues. |
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