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AOZ1210 Datasheet(PDF) 8 Page - Alpha & Omega Semiconductors |
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AOZ1210 Datasheet(HTML) 8 Page - Alpha & Omega Semiconductors |
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8 / 14 page  AOZ1210 Rev. 1.3 December 2007 www.aosmd.com Page 8 of 14 The cycle-by-cycle current limit threshold is internally set. When the load current reaches the current limit thresh- old, the cycle-by-cycle current limit circuit turns off the high side switch immediately to terminate the current duty cycle. The inductor current stops 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 the peak inductor current. When cycle-by-cycle current limit circuit is triggered, the output voltage drops as the duty cycle decreases. The AOZ1210 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 the FB pin voltage is below 0.2V, the short circuit protection circuit is triggered. To prevent current limit running away when the comp pin voltage is higher than 2.1V, the short circuit protection is also triggered. As a result, the converter is shut down and hiccups at a frequency equals to 1/16 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 4.3V, the converter starts operation. When input voltage falls below 4.1V, the converter will stop switching. Thermal Protection An internal temperature sensor monitors the junction temperature. It shuts down the internal control circuit and high side NMOS if the junction temperature exceeds 145°C. The regulator will restart automatically under the control of soft-start circuit when the junction temperature decreases to 100°C. Application Information The basic AOZ1210 application circuit is shown in Figure 1. Component selection is explained below. Input Capacitor The input capacitor (C1 in Figure 1) must be connected to the VIN pin and GND pin of the AOZ1210 to maintain steady input voltage and filter out the pulsing input current. The voltage rating of the input capacitor must be greater than maximum input voltage + ripple voltage. The input ripple voltage can be approximated by equation below: 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 relationship between the input capacitor RMS current and voltage conversion ratio is calculated and shown in Figure 2. 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. Figure 2. ICIN vs. Voltage Conversion Ratio For reliable operation and best performance, the input capacitors must have a current rating higher than ICIN_RMS at the worst operating conditions. Ceramic capacitors are preferred for input capacitors because of their low ESR and high ripple current rating. Depending on the application circuits, other low ESR tantalum capacitor or aluminum electrolytic capacitor may also be used. When selecting ceramic capacitors, X5R or X7R type dielectric ceramic capacitors are preferred for their better temperature and voltage characteristics. Note that the ripple current rating from capacitor manufactures is based on certain amount of life time. Further de-rating may be necessary for practical design requirement. Inductor The inductor is used to supply constant current to output when it is driven by a switching voltage. For given input and output voltage, inductance and switching frequency together decide the inductor ripple current, which is, ∆V IN I O fC IN × ------------------- 1 V O V IN ---------- – V O V IN ---------- × × = I CIN _RMS I O V O V IN ---------- 1 V O V IN ---------- – × = V O V IN ---------- m = 0 0.1 0.2 0.3 0.4 0.5 0 0.5 1 m I CIN_RMS(m) I O ∆I L V O fL × ------------ 1 V O V IN ---------- – × = |
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Similar Description - AOZ1210 |
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