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AAT1120IES-0.6-T1 Datasheet(PDF) 10 Page - Advanced Analogic Technologies |
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AAT1120IES-0.6-T1 Datasheet(HTML) 10 Page - Advanced Analogic Technologies |
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10 / 20 page  AAT1120 500mA Step-Down Converter 10 1120.2007.01.1.0 Control Loop The AAT1120 is a 500mA current mode step-down converter. The current through the P-channel MOSFET (high side) is sensed for current loop control, as well as short-circuit and overload pro- tection. A fixed slope compensation signal is added to the sensed current to maintain stability 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 compen- sation terminates the transconductance voltage error amplifier output. The error amplifier reference is fixed at 0.6V. Soft Start / Enable Soft start increases the inductor current limit point in discrete steps when the input voltage or enable input is applied. It limits the current surge seen at the input and eliminates output voltage overshoot. When pulled low, the enable input forces the AAT1120 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 lim- ited. As load impedance decreases and the output voltage falls closer to zero, more power is dissipated internally, raising the device temperature. Thermal protection completely disables switching when inter- nal dissipation becomes excessive, protecting the device from damage. The junction over-temperature threshold is 140°C with 15°C of hysteresis. Under-Voltage Lockout Internal bias of all circuits is controlled via the VIN power. Under-voltage lockout (UVLO) guarantees sufficient VIN bias and proper operation of all inter- nal circuits prior to activation. Applications Information Inductor Selection The step-down converter uses peak current mode control with slope compensation to maintain stability for duty cycles greater than 50%. The output induc- tor value must be selected so the inductor current down slope meets the internal slope compensation requirements. The internal slope compensation for the adjustable and low-voltage fixed versions of the AAT1120 is 0.45A/μsec. This equates to a slope compensation that is 75% of the inductor current down slope for a 1.8V output and 3.0μH inductor. This is the internal slope compensation for the AAT1120. When externally programming to 3.0V, the calculated inductance is 5.0μH. In this case, a standard 4.7μH value is selected. For most designs, the AAT1120 operates with an inductor value of 1μH to 4.7μH. Table 1 displays inductor values for the AAT1120 with different output voltage options. Manufacturer's specifications list both the inductor DC current rating, which is a thermal limitation, and the peak current rating, which is determined by the saturation characteristics. The inductor should not show any appreciable saturation under normal load conditions. Some inductors may meet the peak and average current ratings yet result in excessive losses due to a high DCR. Always consider the losses associated with the DCR and its effect on the total converter efficiency when selecting an inductor. 0.75 ⋅ VO L = = ≈ 1.67 ⋅ VO = 1.67 ⋅ 3.0V = 5.0µH m 0.75 ⋅ VO 0.45A µsec A µsec A A µsec 0.75 ⋅ VO m = = = 0.45 L 0.75 ⋅ 1.8V 3.0µH A µsec |
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