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7 / 12 page MC33466 7 MOTOROLA ANALOG IC DEVICE DATA DEFINITIONS Quiescent Bias Current – Current which is used to operate the switching regulator chip and is not delivered to the load. Leakage Current – Current drawn through a transistor junction, under a specified collector voltage, when the transistor is off. FUNCTIONAL DESCRIPTION Introduction The MC33466 series are monolithic power switching regulators optimized for dc–to–dc converter applications where power drain must be minimized. The combination of features in this series allows the system designer to directly implement step–up, step–down or flyback converters with a small number of external components. Potential applications include low power consumer products and battery powered portable products. Typical application circuits are shown in Figures 17 through 21. Operating Description The MC33466 series converters operate as a fixed frequency voltage mode regulator. Operation is intended to be in the discontinuous mode, where the inductor current ramps up to a peak value which is greater than or equal to twice the value of the dc input current during the on–time of the transistor switch. During the off–time of the transistor switch, the inductor current ramps down to zero and remains at zero until another switching cycle begins. Because the output voltage pin is also used as the supply voltage for powering internal circuitry, an external startup circuit is needed in step–down converter and flyback designs to provide initial power to the integrated circuit to begin switching. The startup circuit needed can be three discrete components, as shown in Figure 19, or a micropower undervoltage sensor, as shown in Figure 20. Oscillator The oscillator frequency, is internally programmed to 50 kHz. The timing capacitor (CT) discharge to charge ratio of the oscillator is designed for a maximum duty cycle of 80% at the Lx or EXT output. During the charge of CT, the oscillator generates an internal blanking pulse that holds the PWM control off, disabling the output transistor drive. The oscillator peak and valley thresholds are 0.5 V and ground, respectively. Pulse Width Modulator The Pulse Width Modulator consists of a comparator with the oscillator ramp voltage applied to the inverting input, while the error amplifier output is applied to the noninverting input. Output switch conduction is initiated when the timing capacitor is charged to its peak voltage value. When the timing capacitor ramp discharges to a voltage below the error amplifier output, the comparator resets a latch terminating output transistor drive for the duration of the oscillator ramp period. Error Amplifier and Reference An Error Amplifier is provided which has a nominal 80 dB of voltage gain at dc. Internal compensation components provide poles at 0.25 Hz, 30 kHz and 33 kHz. Two zeros are provided at 1.0 kHz and at 2.5 kHz. The output voltage value is set by the internal voltage divider and a 0.7 V reference which is trimmed to an accuracy of ±2.5%. Because the loop compensation components are located within the IC, discontinuous mode operation is recommended for most applications. Driver and Output Switch To aid in system design flexibility and conversion efficiency, two output driver options are provided. The MC33466H–XXJT1 converters have an internal drive transistor which is capable of sinking currents greater than 60 mA into the Lx pin. An internal VLx limiter circuit senses if the Lx pin voltage exceeds 1.0 V during ton and turns off the drive transistor. The MC33466H–XXJT1 provides output drive for an external transistor. Applications The following converter applications show the simplicity and flexibility of the converter architecture. Three main converter topologies are demonstrated in Figures 17 through 21. Figure 17. MC33466H–50KT1 Typical Step–Up Application MBRD520LT1 Vin 3 VO Lx Gnd MC33466H–50KT1 22 µF 120 µH 1 VO 2 22 µF Figure 18. MC33466H–50LT1 Typical Step–Up Application MBRD520LT1 Vin 3 MMBT2222ALT1 300 VO EXT Gnd MC33466H–50LT1 100 µF 28 µH 0.01 µF 1 VO 5.0 V 2 22 µF |
Số phần tương tự - MC33466H-30LT1 |
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Mô tả tương tự - MC33466H-30LT1 |
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