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AIC2359 Datasheet(PDF) 9 Page - Analog Intergrations Corporation |
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AIC2359 Datasheet(HTML) 9 Page - Analog Intergrations Corporation |
9 / 11 page AIC2359 9 APPLICATION INFORMATION Operation The AIC2359 is a low-noise step-down DC/DC converter with current-mode PWM/PSM control architecture. It features an internal synchronous rectifier, which eliminates the external Schottky diode and increases efficiency. During normal operation, the AIC2359 can regulate its output voltage through a feedback control circuit, which is composed of an error amplifier; a current comparator and several control signal generators. By comparing the feedback voltage to the reference voltage of 0.6V, the error amplifier varies its output voltage. The output voltage of the error amplifier is compared with the summing signal of current sensing signal and slope compensation signal to determine the duty cycle of internal main power switch (P-channel MOSFET). While the main power switch is turned on, the synchronous power switch (N-channel MOSFET) will be turned off through anti-short-through block. Similarly, when the main power switch is turned off, the synchronous power switch will be turned on until the inductor current starts to reverse or the beginning of the next switching cycle. In order to achieve better efficiency and prevent overcharging the output capacitor, AIC2359 will enter pulse-skipping- modulated mode (PSM) operation while working at light load conditions. Current Limitation The AIC2359 provides current limit function by using an in- ternal sensing resistor. When the main power switch turns on, current follows through the internal sensing resistor. And current amplifier senses the voltage, which crosses the re- sistor, and amplifies it. While the sensed voltage gets higher than reference voltage, the current limitation function is acti- vated. While the current limitation function is activated, the duty cycle will be reduced to limit the output power to protect the internal power switches. Short Circuit Protection While the output is shorted to ground, the switching frequency of AIC2359 will be reduced to one third of the normal switching frequency. This lower switching frequency ensures the inductor current has more time to discharge, thereby preventing inductor current runaway. The switching frequency will automatically return to its designed value while short circuit condition is released. Shutdown By connecting the EN pin to GND, the AIC2359 can be shut down to reduce the supply current to 0.1 A (typical). At this operation mode, the output voltage of step-down converter is equal to 0V. 100% Duty Cycle Operation When the input voltage approaches the output voltage, the AIC2359 smoothly transits to 100% duty cycle operation. This allows AIC2359 to regulate the output voltage until AIC2359 completely enters 100% duty cycle operation. In 100% duty cycle mode, the output voltage is equal to the input voltage minus the voltage, which is the drop across the main power switch. The AIC2359 achieves 100% duty cycle operation by ex- tending the turn-on time of the main power switch. If the summing signal of current sensing signal and slope com- pensation signal does not reach the output voltage level of the error amplifier at the end of 90% switching period, the main power switch is continuously turned on and the oscilla- tor remains off until the summing signal of current sensing signal and slope compensation signal reaches the output voltage level of the error amplifier. After the summing signal of current sensing signal and slope compensation signal reaches the output voltage level of the error amplifier, the main power switch is turned off and the synchronous power switch is turned on for a constant off time. At the end of the constant off time, the next switching cycle is begun. While the input voltage approaches the output voltage, the switching frequency decreases gradually to smoothly transit to 100% duty cycle operation. |
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