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FAN5358S713X Arkusz danych(PDF) 9 Page - Fairchild Semiconductor |
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FAN5358S713X Arkusz danych(HTML) 9 Page - Fairchild Semiconductor |
9 / 13 page © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5358 • Rev. 1.0.2 9 Operation Description The FAN5358 is a step-down switching voltage regulator that delivers a fixed output from an input voltage supply of 2.7V to 5.5V. Using a proprietary architecture with synchronous rectification, the device is capable of delivering 500mA and maintaining a very high efficiency of over 80% at load currents as low as 1mA. The regulator operates at a nominal frequency of 2MHz, which reduces the value of the external components to as low as 2.2 μH for the output inductor and 4.7µF for the output capacitor. Control Scheme The FAN5358 uses a proprietary, non-linear, quasi fixed- frequency PWM modulator to deliver a fast load transient response, while maintaining a nominal switching frequency over a wide range of load conditions. The regulator performance is independent of the output capacitor ESR, allowing the use of ceramic output capacitors. For very light loads, the device 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 near seamless, exhibiting very little VOUT glitch. Combined with exceptional transient response characteristics, the very low quiescent current of the controller (25µA) 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 FAN5358 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 output is ramped using a slow RC time constant. This minimizes any large surge currents on the input and prevents any overshoot of the output voltage. Current limit is enforced in case the output cannot keep pace with the reference or in case of a shorted output. The current-limit fault response protects the IC in the event of an over-current condition present during soft-start. This protection can cause the IC to fail to start if heavy load is applied during startup or if excessive COUT is used. Table 2 shows combinations of COUT that allow the IC to start successfully with the minimum RLOAD that can be supported. Table 2. Minimum RLOAD Values for Soft-Start with Various COUT Values COUT Minimum RLOAD 4.7 μF No restriction 10 μF VOUT / 0.40 Under-Voltage Lockout (UVLO) When EN is HIGH, the under-voltage lockout keeps the part from operating until the input supply voltage rises high enough to properly operate. This ensures no misbehavior of the regulator during startup 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 current from increasing further. After 12 consecutive PWM cycles that terminate in current limit, the IC shuts down. About 275 μs after shutting down, the IC attempts to restart. If the fault has not cleared, the IC continues to shut down, then attempts to restart as shown in Figure 16. Thermal Shutdown When the die temperature increases, due to a heavy load condition and/or high ambient temperature, output switching is disabled until the die temperature falls sufficiently. The junction temperature at which the thermal shutdown activates is nominally 150°C with a 20°C hysteresis. Upon cooling, the output is enabled and goes through the regular soft start. |
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