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AOZ1022DI-01 Datasheet(PDF) 8 Page - Alpha & Omega Semiconductors |
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AOZ1022DI-01 Datasheet(HTML) 8 Page - Alpha & Omega Semiconductors |
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8 / 15 page  AOZ1022DI-01 Rev.3.1 November 2012 www.aosmd.com Page 8 of 15 Some standard value of R1, R2 and most used output voltage values are listed in Table 1. 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 AOZ1022DI-01 has multiple protection features to prevent system circuit damage under abnormal conditions. Over Current Protection (OCP) The sensed inductor current signal is also used for over current protection. Since the AOZ1022DI-01 employs peak current mode control, the COMP pin voltage is proportional to the peak inductor current. The COMP pin voltage is limited to be between 0.4V and 2.5V internally. The peak inductor current is automatically limited cycle by cycle. When the output is shorted to ground under fault conditions, the inductor current decays very slow during a switching cycle because of VO = 0V. To prevent catastrophic failure, a secondary current limit is designed inside the AOZ1022DI-01. The measured inductor current is compared against a preset voltage which represents the current limit, between 3.5A and 5.0A. When the output current is more than current limit, the high side switch will be turned off. The converter will initiate a soft start once the over-current condition is resolved. Power-On Reset (POR) A power-on reset circuit monitors the input voltage. When the input voltage exceeds 4.1V, the converter starts operation. When input voltage falls below 3.7V, the converter shuts down. 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. The regulator will restart automatically under the control of soft-start circuit when the junction temperature decreases to 100°C. Application Information The basic AOZ1022DI-01 application circuit is show in Figure 1. Component selection is explained below. Input Capacitor The input capacitor must be connected to the VIN pin and PGND pin of AOZ1022DI-01 to maintain steady input voltage and filter out the pulsing input current. The voltage rating of input capacitor must be greater than maximum input voltage plus 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 we 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. For reliable operation and best performance, the input capacitors must have current rating higher than ICIN_RMS at worst operating conditions. Ceramic capacitors are preferred for input capacitors because of their low ESR and high current rating. Depending on the application circuits, other low ESR tantalum capacitor may also be used. When selecting ceramic capacitors, X5R or X7R type dielectric ceramic capacitors should be used for their better temperature and voltage characteristics. VO (V) R1 (kΩ) R2 (kΩ) 0.8 1.0 open 1.2 4.99 10 1.5 10 11.5 1.8 12.7 10.2 2.5 21.5 10 3.3 31.1 10 5.0 52.3 10 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 = |
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