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AAT1145 Datasheet(PDF) 8 Page - Advanced Analogic Technologies |
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AAT1145 Datasheet(HTML) 8 Page - Advanced Analogic Technologies |
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8 / 16 page  AAT1145 1.2A Step-Down Converter SwitchRegTM PRODUCT DATASHEET 8 1145.2007.11.1.1 www .analogictech.com until either the inductor current starts to reverse, as indicated by the current reversal comparator, IZERO, or the beginning of the next clock cycle. Control Loop The AAT1145 is a peak current mode step-down con- verter. The current through the P-channel MOSFET (high side) is sensed for current loop control, as well as short circuit and overload protection. A slope compensation signal is added to the sensed current to maintain stabil- ity for duty cycles greater than 50%. The peak current mode loop appears as a voltage-programmed current source in parallel with the output capacitor. The output of the voltage error amplifier programs the current mode loop for the necessary peak switch current to force a constant output voltage for all load and line conditions. Internal loop compensation terminates the transconduc- tance voltage error amplifier output. For fixed voltage versions, the error amplifier reference voltage is inter- nally set to program the converter output voltage. For the adjustable output, the error amplifier reference is fixed at 0.6V. Soft Start / Enable Soft start limits the current surge seen at the input and eliminates output voltage overshoot. The enable pin is active high. When pulled low, the enable input (EN) forces the AAT1145 into a low-power, non-switching state. The total input current during shutdown is less than 1μA. Current Limit and Over-Temperature Protection For overload conditions, the peak input current is limited to 2.5A. To minimize power dissipation and stresses under current limit and short-circuit conditions, switch- ing is terminated after entering current limit for a series of pulses. The termination lasts for seven consecutive clock cycles after a current limit has been sensed during a series of four consecutive clock cycles. Thermal protection completely disables switching when internal dissipation becomes excessive. The junction over-temperature threshold is 170°C with 10°C of hys- teresis. Once an over-temperature or over-current fault conditions is removed, the output voltage automatically recovers. Dropout Operation When the battery input voltage decreases near the value of the output voltage, the AAT1145 allows the main switch to remain on for more than one switching cycle and increases the duty cycle until it reaches 100%. The duty cycle D of a step-down converter is defined as: · 100% D = T ON · FOSC · 100% ≈ V OUT V IN Where TON is the main switch on time and FOSC is the oscillator frequency. The output voltage then is the input voltage minus the voltage drop across the main switch and the inductor. At low input supply voltage, the RDS(ON) of the P-channel MOSFET increases, and the efficiency of the converter decreases. Caution must be exercised to ensure the heat dissipated does not exceed the maxi- mum junction temperature of the IC. Maximum Load Current The AAT1145 will operate with an input supply voltage as low as 2.5V, however, the maximum load current decreases at lower input voltages due to a large IR drop on the main switch and synchronous rectifier. The slope compensation signal reduces the peak inductor current as a function of the duty cycle to prevent sub-harmonic oscillations at duty cycles greater than 50%. Conversely the current limit increases as the duty cycle decreases. |
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