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AOZ7111 Datasheet(PDF) 10 Page - Alpha & Omega Semiconductors |
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AOZ7111 Datasheet(HTML) 10 Page - Alpha & Omega Semiconductors |
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10 / 15 page  AOZ7111 Rev. 1.0 January 2013 www.aosmd.com Page 10 of 15 Figure 4. Switching and Current Sense Operation Over-Current Protection The over-current detection protective circuit detects the inductor current and protects the power MOSFET by turning off the output driver when it becomes higher than the set current level. With the over-current detection, the voltage across the current detection resistance RCS connected to the GND is fed to the CS pin. If the CS pin voltage compared by the over-current detection comparator becomes lower than -0.7V, it is then regarded as over-current state; therefore, the feed-forward of the output driver is reset to turn off the power MOSFET. Restart Timer AOZ7111 utilizes self-oscillation instead of the oscillator with fixed frequency. In steady operation, it turns on the MOSFET with a signal from the zero current detector. In startup or light load conditions, a trigger signal is required for starting up or stable operation. When the output of the IC continues turn-off (150µs or more), the restart trigger signal is automatically generated. Maximum Switching Frequency Limit Because the MOSFET turn-on depends on the CS input, switching frequency may increase to higher than several MHz due to the mis-triggering or noise on the nearby CS pin. If the switching frequency is higher than needed for critical conduction mode, it will enter CCM. In CCM, inductor current can be raised very high, which may exceed the current rating of the power switch or diode. This can seriously damage the power switch and burn it down. To avoid this, the maximum switching frequency limitation is embedded. If the ZCD signal is applied again within 2.9µs after the previous edge of gate signal, this signal is ignored and AOZ7111 waits for another ZCD signal. Overvoltage Protection (OVP) It is critical that over voltage protection (OVP) prevents the output voltage exceeding the ratings of PFC stage components. Over-voltage protection (OVP) is embedded by the information at the INV pin. That information comes from the output through the voltage dividing resistors. AOZ7111 has dynamic OVP function to narrow the on time when the INV voltage is higher than 2.575V. When the voltage further rises and exceeds the comparator reference voltage of static OVP (2.685V), the OVP comparator shuts down the output drive pulse. The OVP logic includes hysteresis to ensure that output voltage has sufficient time to discharge before the AOZ7111 driver recovery and also to ensure noise immunity. Additional OVP detection Over-voltage protection (OVP) is embedded by the information at the INV pin. That information comes from the output through the voltage dividing resistors. When the upper divider resistor gets damaged and resistance becomes too high the output electrolytic capacitor may explode. To prevent such a catastrophe the additional OVP pin is assigned to double check output voltage. When the second OVP triggers, switching can be recovered only when the VCC supply voltage falls below VSTOP and builds up higher than UVLO again. -15mV ZCD CS t OUT t Ramp+ Offset t IL t VCOMP |
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Similar Description - AOZ7111 |
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