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CS51031GDR8 Datasheet(PDF) 6 Page - ON Semiconductor |
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CS51031GDR8 Datasheet(HTML) 6 Page - ON Semiconductor |
6 / 10 page CS51031 http://onsemi.com 6 Buck Regulator Operation A block diagram of a typical buck regulator is shown in Figure 4. If we assume that the output transistor is initially off, and the system is in discontinuous operation, the inductor current IL is zero and the output voltage is at its nominal value. The current drawn by the load is supplied by the output capacitor CO. When the voltage across CO drops below the threshold established by the feedback resistors R1 and R2 and the reference voltage VREF, the power transistor Q1 switches on and current flows through the inductor to the output. The inductor current rises at a rate determined by (VIN − VOUT)/L. The duty cycle (or “on” time) for the CS51031 is limited to 80%. If output voltage remains higher than nominal during the entire COSC change time, the Q1 does not turn on, skipping the pulse. Figure 4. Buck Regulator Block Diagram R1 R2 CO RLOAD L D1 Feedback Control Q1 CIN VIN APPLICATIONS INFORMATION CS51031 DESIGN EXAMPLE Specifications 12 V to 5.0 V, 3.0 A Buck Controller • VIN = 12 V ±20% (i.e. 14.4 V max, 12 V nom, 9.6 V min) • VOUT = 5.0 V ±2% • IOUT = 0.3 A to 3.0 A • Output ripple voltage < 50 mV max • Efficiency > 80% • fSW = 200 kHz 1) Duty Cycle Estimates Since the maximum duty cycle D, of the CS51031 is limited to 80% min, it is necessary to estimate the duty cycle for the various input conditions over the complete operating range. The duty cycle for a buck regulator operating in a continuous conduction mode is given by: D + VOUT ) VF VIN * VSAT where: VSAT = RDS(ON) × IOUT max and RDS(ON) is the value at TJ 100°C. If VF = 0.60 V and VSAT = 0.60 V then the above equation becomes: DMAX + 5.6 9.0 + 0.62 DMIN + 5.6 13.8 + 0.40 2) Switching Frequency and On and Off Time Calculations Given that fSW = 200 kHz and DMAX = 0.80 T + 1.0 fSW + 5.0 ms TON(max) + T DMAX + 5.0 ms 0.62 ^ 3.0 ms TON(min) + T DMIN + 5.0 ms 0.40 ^ 2.0 ms TOFF(max) + TON(min) + 5.0 ms * 2.0 ms + 3.0 ms 3) Oscillator Capacitor Selection The switching frequency is set by COSC, whose value is given by: COSC in pF + 95 10)6 FSW 1 ) FSW 3 106 * 30 103 FSW 2 |
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