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AOZ1057AI Datasheet(PDF) 8 Page - Alpha & Omega Semiconductors |
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AOZ1057AI Datasheet(HTML) 8 Page - Alpha & Omega Semiconductors |
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8 / 15 page  AOZ1057 Rev. 1.3 November 2009 www.aosmd.com Page 8 of 15 Output Voltage Programming Output voltage can be set by feeding back the output to the FB pin with a resistor divider network. In the application circuit shown in Figure 1. The resistor divider network includes R1 and R2. Usually, a design is started by picking a fixed R2 value and calculating the required R1 with equation below. Some standard values of R1 and R2 for most commonly used output voltage values are listed in Table 1. 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 AOZ1057 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 AOZ1057 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. The cycle-by-cycle current limit threshold is set between 3.5A 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 cur- rent. When cycle-by-cycle current limit circuit is triggered, the output voltage drops as the duty cycle decreases. The AOZ1057 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. The converter will start up via a soft start once the short circuit condition disappears. In short circuit protection mode, the inductor average current is greatly reduced. UVLO A UVLO circuit monitors the input voltage. When the input voltage exceeds 4V, the converter starts operation. When input voltage falls below 3.7V, the converter will stop switching. Output Over Voltage Protection (OVP) The AOZ1057 monitors the feedback voltage: when the feedback voltage is higher than 960mV, it immediately turns-off the PMOS to protect the output voltage over- shoot 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. The regulator will restart automatically under the control of soft-start circuit when the junction temperature decreases to 100°C. Application Information The basic AOZ1057 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 AOZ1057 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 + ripple voltage. The input ripple voltage can be approximated by equation below: 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 31.6 10 5.0 52.3 10 VO 0.8 1 R1 R2 ------- + ⎝⎠ ⎜⎟ ⎛⎞ × = ΔVIN IO fCIN × ----------------- 1 VO VIN --------- – ⎝⎠ ⎜⎟ ⎛⎞ VO VIN --------- × × = |
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