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RM4190D883B Datasheet(PDF) 9 Page - Raytheon Company |
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RM4190D883B Datasheet(HTML) 9 Page - Raytheon Company |
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9 / 25 page  RC4190 PRODUCT SPECIFICATION 9 Figure 12. Step-Down Regulator Greater Than 30V VBAT 65-2677 4190 7 4 3 5 6 Lx D1 VOUT 2N2907 2 Cx R2 R3 1N914 CF R1 R4 1 R5 R6 R7 Z1 R6 R7 50 I 10 V I L L S ~ ~ ~ ~ +VS LX VFB GND LBR IC CX R4 = R5 = VS - 1.31V 5 µA 260K Design Equations The inductor value and timing capacitor (CX) value must be carefully tailored to the input voltage, input voltage range, output voltage, and load current requirements of the applica- tion. The key to the problem is to select the correct inductor value for a given oscillator frequency, such that the inductor current rises to a high enough peak value (IMAX) to meet the average load current drain. The selection of this inductor value must take into account the variation of oscillator frequency from unit to unit and the drift of frequency over temperature. Use ±20% as a maximum change from the nominal oscillator frequency. The worst-case conditions for calculating ability to supply load current are found at the minimum supply voltage; use +VS (min) to calculate the inductor value. Worst-case condi- tions for ripple are at +VS (max). The value of the timing capacitor is set according to the following equation: The squarewave output of the oscillator is internal and cannot be directly measured, but is equal in frequency to the triangle waveform measurable at pin 4. The switch transistor is normally on when the triangle waveform is ramping up and off when ramping down. Capacitor selection depends on the application; higher operating frequencies will reduce the output voltage ripple and will allow the use of an inductor with a physically smaller inductor core, but excessively high frequencies will reduce load driving capability and effi- ciency. f O Hz ( ) 2.4 10 6 × C X pF ( ) ---------------------- = Find a value for the start-up resistor R1: Find a value for the feedback resistors R2 and R3: Where IA is the feedback divider current (recommended value is between 50 µA and 100 µA). Step-Up Design Procedure 1. Select an operating frequency and timing capacitor as shown above (10 kHz to 40kHz is typical). 2. Find the maximum on time (add 5 µS for the turn-off base recombination delay of Q1): 3. Calculate the peak inductor current IMAX (if this value is greater than 375 mA, then an external power transis- tor must be used in place of Q1): where: VS = supply voltage VD = diode forward voltage IL = dc load current VSW = saturation voltage of Q1 (typ 0.5V) R1 V S 1.2V – 5 µA ------------------------- = R2 V OUT 1.31V – I A ------------------------------------ = R3 1.31V I A --------------- = T ON 1 2F O ---------- 5 µs + = I MAX V OUT V D V S – + F O ( )T ON VS V SW – [ ] ----------------------------------------------------- 2I L = |
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