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ML4761ES データシート(PDF) 5 Page - Micro Linear Corporation |
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ML4761ES データシート(HTML) 5 Page - Micro Linear Corporation |
5 / 8 page 5 ML4761 FUNCTIONAL DESCRIPTION The ML4761 combines Pulse Frequency Modulation (PFM) and synchronous rectification to create a boost converter that is both highly efficient and simple to use. A PFM regulator charges a single inductor for a fixed period of time and then completely discharges before another cycle begins, simplifying the design by eliminating the need for conventional current limiting circuitry. Synchronous rectification is accomplished by replacing an external Schottky diode with an on-chip PMOS device, reducing switching losses and external component count. REGULATOR OPERATION A block diagram of the boost converter is shown in Figure 2. The circuit remains idle when VOUT is at or above the desired output voltage, drawing 45µA from VIN, and 8µA from VOUT through the feedback resistors R1 and R2. When VOUT drops below the desired output level, the output of amplifier A1 goes high, signaling the regulator to deliver charge to the output. Since the output of amplifier A2 is normally high, the flip-flop captures the A1 set signal and creates a pulse at the gate of the NMOS transistor Q1. The NMOS transistor will charge the inductor L1 for 10µs, resulting in a peak current given by: I TV L sV L L PEAK ON IN IN () = × ≈ × 1 10 1 µ (1) For reliable operation, L1 should be chosen so that IL(PEAK) does not exceed 2A. When the one-shot times out, the NMOS FET releases the VL pin, allowing the inductor to fly-back and momentarily charge the output through the body diode of PMOS transistor Q2. But, as the voltage across the PMOS transistor changes polarity, its gate will be driven low by the current sense amplifier A2, causing Q2 to short out its body diode. The inductor then discharges into the load through Q2. The output of A2 also serves to reset the flip- flop and one-shot in preparation for the next charging cycle. A2 releases the gate of Q2 when its current falls to zero. If VOUT is still low, the flip-flop will immediately initiate another pulse. The output capacitor (C1) filters the inductor current, limiting output voltage ripple. Inductor current and one-shot waveforms are shown in Figure 3. Q(ONE SHOT) Q1 ON Q1 ON Q2 ON Q2 ON INDUCTOR CURRENT Q1 & Q2 OFF Figure 3. PFM Inductor Current Waveforms and Timing. RESET COMPARATOR An additional comparator is provided to detect low VIN. The inverting input of the comparator is internally connected to VREF, while the non-inverting input is connected to the undervoltage lockout circuit. The output of the comparator is the RESET pin, which swings from VOUT to GND when an undervoltage condition is detected. DESIGN CONSIDERATIONS INDUCTOR Selecting the proper inductor for a specific application usually involves a trade-off between efficiency and maximum output current. Choosing too high a value will keep the regulator from delivering the required output current under worst case conditions. Choosing too low a value causes efficiency to suffer. It is necessary to know the maximum required output current and the input voltage range to select the proper inductor value. The maximum inductor value can be estimated using the following formula: L VT VI MAX IN MIN ON MIN OUT OUT MAX = ×× ×× () () () 2 2 η (2) where h is the efficiency, typically between 0.8 and 0.9. Note that this is the value of inductance that just barely delivers the required output current under worst case conditions. A lower value may be required to cover inductor tolerance, the effect of lower peak inductor currents caused by resistive losses, and minimum dead time between pulses. Another method of determining the appropriate inductor value is to make an estimate based on the typical performance curves given in Figures 4 and 5. Figure 4 shows maximum output current as a function of input voltage for several inductor values. These are typical performance curves and leave no margin for inductance and ON-time variations. To accommodate worst case conditions, it is necessary to derate these curves by at least 10% in addition to inductor tolerance. Interpolation between the different curves will give a reasonable starting point for an inductor value. |
同様の部品番号 - ML4761ES |
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同様の説明 - ML4761ES |
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