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FAN5350UCX 데이터시트(PDF) 6 Page - Fairchild Semiconductor |
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FAN5350UCX 데이터시트(HTML) 6 Page - Fairchild Semiconductor |
6 / 15 page © 2007 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5350 Rev. 1.0.3 6 Operation Description The FAN5350 is a step-down switching voltage regulator that delivers a fixed 1.82V from an input voltage supply of 2.7V to 5.5V. Using a proprietary architecture with synchronous rectification, the FAN5350 is capable of delivering 600mA at over 90% efficiency, while maintaining a light load efficiency of over 80% at load currents as low as 1mA. The regulator operates at a nominal frequency of 3MHz at full load, which reduces the value of the external components to 1µH for the output inductor and 4.7µF for the output capacitor. Control Scheme The FAN5350 uses a proprietary non-linear, fixed- frequency PWM modulator to deliver a fast load transient response, while maintaining a constant switching frequency over a wide range of operating conditions. The regulator performance is independent of the output capacitor ESR, allowing for the use of ceramic output capacitors. Although this type of operation normally results in a switching frequency that varies with input voltage and load current, an internal frequency loop holds the switching frequency constant over a large range of input voltages and load currents. For very light loads, the FAN5350 operates in discontinuous current (DCM) single-pulse PFM mode, which produces low output ripple compared with other PFM architectures. Transition between PWM and PFM is seamless, with a glitch of less than 14mV at VOUT during the transition between DCM and CCM modes. Combined with exceptional transient response characteristics, the very low quiescent current of the controller (<16µA) maintains high efficiency, even at very light loads, while preserving fast transient response for applications requiring very tight output regulation. Enable and Soft Start Maintaining the EN pin LOW keeps the FAN5350 in non-switching mode in which all circuits are off and the part draws ~50nA of current. Increasing EN above its threshold voltage activates the part and starts the soft- start cycle. During soft start, the current limit is increased in discrete steps so that the inductor current is increased in a controlled manner. This minimizes any large surge currents on the input and prevents any overshoot of the output voltage. Under-Voltage Lockout When EN is high, the under-voltage lock-out keeps the part from operating until the input supply voltage rises high enough to properly operate. This ensures no misbehavior of the regulator during start-up or shutdown. Current Limiting A heavy load or short circuit on the output causes the current in the inductor to increase until a maximum current threshold is reached in the high-side switch. Upon reaching this point, the high-side switch turns off, preventing high currents from causing damage. The peak current limit shown in Figure 16, ILIM(PK) is slightly higher than the open-loop tested current limit, ILIM(OL), in the Electrical Characteristics table. This is primarily due to the effect of propagation delays of the IC current limit comparator. Thermal Shutdown When the die temperature increases, due to a high load condition and/or a high ambient temperature, the output switching is disabled until the temperature on the die has fallen sufficiently. The junction temperature at which the thermal shutdown activates is nominally 150°C with a 20°C hysteresis. |
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