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AOZ1017AI Datasheet(PDF) 8 Page - Alpha & Omega Semiconductors |
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AOZ1017AI Datasheet(HTML) 8 Page - Alpha & Omega Semiconductors |
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8 / 15 page  AOZ1017A Rev. 1.2 April 2009 www.aosmd.com Page 8 of 15 The combination of R1 and R2 should be large enough to avoid drawing excessive current from the output, which will cause power loss. Since the switch duty cycle can be as high as 100%, the maximum output voltage can be set as high as the input voltage minus the voltage drop on upper PMOS and inductor. Protection Features The AOZ1017A has multiple protection features to pre- vent system circuit damage under abnormal conditions. Over Current Protection (OCP) The sensed inductor current signal is also used for over current protection. Since the AOZ1017A employs peak current mode control, the COMP pin voltage is propor- tional to the peak inductor current. The COMP pin volt- age is limited to be between 0.4V and 2.5V internally. The peak inductor current is automatically limited cycle by cycle. The cycle by cycle current limit threshold is set between 4A and 5A. When the load current reaches the current limit threshold, the cycle by cycle current limit circuit turns off the high side switch immediately to terminate the current duty cycle. The inductor current stop rising. The cycle by cycle current limit protection directly limits inductor peak current. The average inductor current is also limited due to the limitation on peak inductor current. When cycle by cycle current limit circuit is triggered, the output voltage drops as the duty cycle is decreasing. The AOZ1017A has internal short circuit protection to protect itself from catastrophic failure under output short circuit conditions. The FB pin voltage is proportional to the output voltage. Whenever FB pin voltage is below 0.2V, the short circuit protection circuit is triggered. As a result, the converter is shut down and hiccups at a frequency equals to 1/8 of normal switching frequency. The converter will start up via a soft start once the short circuit condition is resolved. In short circuit protection mode, the inductor average current is greatly reduced because of the low hiccup frequency. Power-On Reset (POR) A power-on reset circuit monitors the input voltage. When the input voltage exceeds 4V, the converter starts opera- tion. When input voltage falls below 3.7V, the converter will be shut down. Output Over Voltage Protection (OVP) The AOZ1017A monitors the feedback voltage: when the feedback voltage is higher than 960mV, it immediately turns off the PMOS to protect the output voltage overshoot at fault condition. When feedback voltage is lower than 840mV, the PMOS is allowed to turn on in the next cycle. Thermal Protection An internal temperature sensor monitors the junction temperature. It shuts down the internal control circuit and high side PMOS if the junction temperature exceeds 150°C. Application Information The basic AOZ1017A application circuit is shown in Figure 1. Component selection is explained below. Input Capacitor The input capacitor must be connected to the VIN pin and PGND pin of the AOZ1017A to maintain steady input voltage and filter out the pulsing input current. The voltage rating of the input capacitor must be greater than the maximum input voltage plus the ripple voltage. The input ripple voltage can be approximated by the following equation: Since the input current is discontinuous in a buck converter, the current stress on the input capacitor is another concern when selecting the capacitor. For a buck circuit, the RMS value of input capacitor current can be calculated by: if let m equal the conversion ratio: The relation between the input capacitor RMS current and voltage conversion ratio is calculated and shown in Figure 2 on the next page. It can be seen that when VO is half of VIN, CIN is under the worst current stress. The worst current stress on CIN is 0.5 x IO. ΔVIN IO fCIN × ----------------- 1 VO VIN --------- – ⎝⎠ ⎜⎟ ⎛⎞ VO VIN --------- × × = ICIN_RMS IO VO VIN --------- 1 VO VIN --------- – ⎝⎠ ⎜⎟ ⎛⎞ × = VO VIN --------- m = |
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