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RM4391D Datasheet(PDF) 7 Page - Fairchild Semiconductor |
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RM4391D Datasheet(HTML) 7 Page - Fairchild Semiconductor |
7 / 22 page RC4391 PRODUCT SPECIFICATION 7 Figure 9. Inverting Regulator Waveforms B A C D E F G 1.78V 0.62V (Internal) I 0 mA +V (Internal) +V - 0.7V Max 0 mA I 0 mA +V - V -V - V Ground C O I V I I V X SC LOAD BEQ1 LX D LX L S S S MAX SW OUT D 65-2472 BAT OUT X X V L V L This feedback system will vary the duration of the on time in response to changes in load current or battery voltage (see Figure 9). If the load current increases (waveform C), then the transistor will remain on (waveform D) for a longer por- tion of the oscillator cycle, (waveform B) to build up to a higher peak value. The duty cycle of the switch transistor varies in response to changes in load and line. Step-Down Regulator The step-down circuit function is similar to inversion; it uses the same components (switch, inductor, diode, filter capaci- tor), and charges and discharges the inductor by closing and opening the switch. The great difference is that the inductor is in series with the load; therefore, both the charging current and the discharge current flow into the load. In the inverting circuit only the discharge current flows into the load. Refer to Figure 10. When the switch S is closed, current flows from the battery, through the inductor, and through the load resistor to ground. After the switch is opened, stored energy in the inductor causes current to keep flowing through the load, the circuit being completed by the catch diode D. Since current flows to the load during charge and discharge, the average load cur- rent will be greater than in an inverting circuit. The signifi- cance of that is that for equal load currents the step-down circuit will require less peak inductor current than an invert- ing circuit. Therefore, the inductor will not require as large of a core, and the switch transistor will not be stressed as heavily for equal load currents. Figure 11 depicts a complete schematic for a step-down cir- cuit using the RC4391. Observe that the ground lead of the 4391 is not connected to circuit ground; instead, it is tied to the output voltage. It is by this rearrangement that the feed- back system, which senses voltages more negative than the ground lead, can be used to regulate a non-negative output voltage. Figure 10. Simple Step-Down Regulator D (+) (-) VOUT S C S L +V 65-2473 RL |
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