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FAN5362UMP29X Arkusz danych(PDF) 9 Page - Fairchild Semiconductor |
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FAN5362UMP29X Arkusz danych(HTML) 9 Page - Fairchild Semiconductor |
9 / 14 page © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5362 • Rev. 1.0.1 9 Operation Description FAN5362 is a 500mA or 750mA, step-down switching voltage regulator that delivers a fixed output voltage from an input voltage supply up to 5.5V. Using a proprietary architecture with synchronous rectification, FAN5362 is capable of delivering a peak efficiency above 96%, while maintaining efficiency above 90% 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 inductor and 4.7µF for the output capacitor. High efficiency is maintained at light load with single-pulse PFM mode. Control Scheme The FAN5362 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 FAN5362 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 18mV at VOUT during the transition between DCM and CCM modes. Combined with exceptional transient response characteristics, the very low quiescent current of the controller (45µA) maintains high efficiency, even at very light loads, while preserving fast transient response for applications requiring tight output regulation. 100% Duty Cycle Operation When VIN approaches VOUT, the regulator increases its duty cycle until 100% duty cycle is reached. As the duty cycle approaches 100%, the switching frequency declines due to the minimum off-time (tOFF(MIN)) of about 35ns imposed by the control circuit. When 100% duty cycle is reached, VOUT follows VIN with a drop-out voltage (VDROPOUT) determined by the total resistance between VIN and VOUT: ( ) L ) ON ( DS LOAD DROPOUT DCR R PMOS I V + • = (1) To calculate the worst-case VDROPOUT, use the maximum PMOS RDS(ON) at high temperature from Figure 5. Enable and Soft Start When the EN pin is LOW, the IC is shut down and the part draws very little current. In addition, during shutdown, FB is actively discharged to ground through a 230 Ω path. Raising EN above its threshold voltage activates the part and starts the soft-start cycle. During soft-start, the internal reference is ramped using an exponential RC shape to prevent any overshoot of the output voltage. Current limiting minimizes inrush during soft-start. Synchronous rectification is inhibited during soft-start, allowing the IC to start into a pre-charged load. The IC may fail to start if heavy load is applied during startup and/or if excessive COUT is used. This is due to the current- limit fault response, which protects the IC in the event of an over-current condition present during soft-start. The current required to charge COUT during soft-start is commonly referred to as “displacement current” and given as: dt dV C I OUT DISP • = (2) where dt dV refers to the soft-start slew rate. To prevent shutdown during soft-start, the following condition must be met: ) DC ( MAX LOAD DISP I I I < + (3) where IMAX(DC) is the maximum load current the IC is guaranteed to support (500mA or 750mA). MODE Pin Logic 1 on this pin forces the IC to stay in PWM mode. A logic 0 allows the IC to automatically switch to PFM during light loads. If the MODE pin is toggled, the converter synchronizes its switching frequency to four times the frequency on the mode pin (fMODE). At startup, the mode pin must be held LOW or HIGH for at least 10 μs to ensure that the converter does not attempt to synchronize to this pin. Under-Voltage Lockout 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 high currents from causing damage. 16 consecutive PWM cycles in current limit causes the regulator to shut down and stay off for about 2900 μs before attempting a restart. In the event of a short circuit, the soft-start circuit attempts to restart at 240 μs, which results in a duty cycle of less than 10%, providing current into a short. The closed-loop peak-current limit, ILIM(PK), is not the same as 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. |
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