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RV4391N Datasheet(PDF) 10 Page - Fairchild Semiconductor |
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RV4391N Datasheet(HTML) 10 Page - Fairchild Semiconductor |
10 / 22 page PRODUCT SPECIFICATION RC4391 10 Step-Down Design Procedure 1. Select an operating frequency. 2. Determine the maximum on time TON as in the inverting design procedure. 3. Calculate IMAX: 4. Calculate LX: Alternate Design Procedure The design equations above will not work for certain input/ output voltage ratios, and for these circuits another method of defining component values must be used. If the slope of the current discharge waveform is much less than the slope of the current charging waveform, then the inductor current will become continuous (never discharging completely), and the equations will become extremely complex. So, if the voltage applied across the inductor during the charge time is greater than during the discharge time, use the design proce- dure below. For example, a step-down circuit with 20V input and 5V output will have approximately 15V across the inductor when charging, and approximately 5V when dis- charging. So in this example the inductor current will be con- tinuous and the alternate procedure will be necessary. The alternate procedure may also be used for discontinuous cir- cuits. 1. Select an operating frequency based on efficiency and component size requirements (a value between 10kHz and 50kHz is typical). 2. Build the circuit and apply the worst case conditions to it, i.e., the lowest battery voltage and the highest load current at the desired output voltage. 3. Adjust the inductor value down until the desired output voltage is achieved, then decrease its value by 30% to cover manufacturing tolerances. 4. Check the output voltage with an oscilloscope for ripple, at high supply voltages, at voltages as high as are expected. Also check for efficiency by monitoring supply and output voltages and currents: 5. If the efficiency is poor, go back to Step 1 and start over. If the ripple is excessive, then increase the output filter capacitor value or start over. I MAX 2I L F O () T ON () V S V OUT – () V OUT V D – () --------------------------------- 1 + ----------------------------------------------------------------------------- = L X(Henries) V S V SW – I MAX ------------------------- èø æö T ON () = eff V OUT () I OUT () +V S () I SY ()x100 ------------------------------------------ = èø æö Compensation When large values (> 50 k W) are used for the voltage setting resistors (R1 and R2 of Figure 8) stray capacitance at the VFB input can add lag to the feedback response, destabiliz- ing the regulator, increasing low frequency ripple, and lower- ing efficiency. This can often be avoided by minimizing the stray capacitance at the VFB node. It can also be remedied by adding a lead compensation capacitor of 100 pF to 10 nF. In inverting applications, the capacitor connects between -VOUT and VFB; for step-down circuits it connects between ground and VFB. Most applications do not require this capacitor. Inductors Efficiency and load regulation will improve if a quality high Q inductor is used. A ferrite pot core is recommended; the wind-yourself type with an air gap adjustable by washers or spacers is very useful for bread-boarding prototypes. Care must be taken to choose a core with enough permeability to handle the magnetic flux produced at IMAX. If the core satu- rates, then efficiency and output current capability are severely degraded and excessive current will flow through the switch transistor. A pot core inductor design section is provided later in this datasheet. An isolated AC current probe for an oscilloscope (example: Tektronix P6042) is an excellent tool for saturation prob- lems; with it the inductor current can be monitored for non- linearity at the peaks (a sign of saturation). Low Battery Detector An open collector signal transistor Q2 with comparator C2 provides the designer with a method of signaling a display or computer whenever the battery voltage falls below a pro- grammed level (see Figure 13). This level is determined by the +1.25V reference level and by the selection of two exter- nal resistors according to the equation: When the battery drops below this threshold Q2 will turn on and sink typically 600 mA. The low battery detection circuit can also be used for other less conventional applications such as the voltage dependent oscillator circuit of Figure 18. Figure 13. Low Battery Detector V TH V REF R4 R5 -------1 + èø æö = +Vs R4 R5 1 C2 V 1.25V REF Q2 2 I LBD 65-1651A LBR LBD |
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