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RV4391N Datasheet(PDF) 9 Page - Fairchild Semiconductor |
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RV4391N Datasheet(HTML) 9 Page - Fairchild Semiconductor |
9 / 22 page RC4391 PRODUCT SPECIFICATION 9 The oscillator creates a squarewave using a method similar to the 555 timer IC, with a current steering flip-flop con- trolled by two voltage sensing comparators. The oscillator frequency is set by the timing capacitor (CX) 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 3. 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 efficiency. Inverting Design Procedure 1. Select an operating frequency and timing capacitor value as shown above (frequencies from 10kHz to 50kHz are typical). 2. Find the maximum on time TON (add 3 mS for the turn off base recombination delay of Q1): 3. Calculate the peak inductor current IMAX (if this value is greater than 375mA then an external power transistor must be used in place of Q1): Where: VS = Supply Voltage VSW = Saturation Voltage of Q1 (typically 0.5V) VD = Diode Forward Voltage (typically 0.7V) IL = DC Load Current 4. Find an inductance value for LX: The inductor chosen must exhibit this value of inductance and have a current rating equal to IMAX. F O Hz () 4.1x10 6 – C x pF () ----------------------- = T ON 1 2F O ----------3 mS + = I MAX V OUT V D + ()2I L F O () T ON () V S V SW – () ---------------------------------------------------------- = L X Henries () V S V SW – I MAX ------------------------- èø æö T ON () = Figure 12. Step-Down Regulator Waveforms 65-2474 A B C D E F V L V - V L BAT X OUT BAT X 1.78V 0.62V (Internal) I 0 mA +V (Internal) +V - 0.7V I 0 mA +V - V V V ( -0.7V) C OSC I V I V L S MAX S SW OUT S X LOAD BEQ1 LX LX |
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