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S-8550AA-M5T1G Datasheet(PDF) 10 Page - Seiko Instruments Inc |
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S-8550AA-M5T1G Datasheet(HTML) 10 Page - Seiko Instruments Inc |
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10 / 31 page  STEP-DOWN, BUILT-IN FET, SYNCHRONOUS RECTIFICATION, PWM CONTROL SWITCHING REGULATORS S-8550 Series Rev.5.0_01 10 Operation Principle The S-8550 Series is a step-down synchronous rectification switching regulator based on constant PWM control. Figure 9 shows the basic circuit diagram. A step-down switching regulator starts current supply by the input voltage (VIN) when the Pch power MOS FET is turned on and holds energy in the inductor at the same time. When the Pch power MOS FET is turned off, the current held in the inductor is released. The released current flows in the smoothing circuit, with the energy loss held minimum, supplies the output voltage (VOUT) lower than VIN. VOUT is kept constant by controlling the switching frequency (fosc) and ON time (ton). With the PWM control method, VOUT is made constant by controlling the ON time with fOSC unchanged. L Control circuit Pch power MOS FET Nch power MOS FET VIN VOUT COUT I1 I2 Figure 9 Basic Circuit Drawing of Step-down Switching Regulator 1. Continuous mode The following explains how the current flows to the inductor when the step-down operation is constant and stable. When the Pch power MOS FET is turned on, current I1 flows in the direction shown by the arrow in Figure 9, and energy is stored in the inductor (L). When the output capacitor (COUT) is charged, supply of the output current (IOUT) is started at the same time. The inductor current (IL) gradually increases in proportion to the ON time (tON) of the Pch power MOS FET as shown in Figure 10 (changes from IL min. to IL max.). When the Pch power MOS FET is turned off, the Nch power MOS FET is turned on and IL tries to hold IL max. Consequently, current I2 flows in the direction shown by the arrow in Figure 9. As a result, IL gradually decreases and reaches IL min. when the OFF time (tOFF) has elapsed. When tOFF has elapsed, the Nch power MOS FET is turned off and the next cycle is entered. The above sequence is repeated. As explained in the above, the continuous mode refers to the operation in the current cycle in which IL linearly changes from IL min. to IL max. Even if IL min. is less than 0 A, IL min. keeps flowing (backflow current flows). ton toff T = 1/fOSC IL IL max. IL min. t Figure 10 Continuous Mode (Current Cycle of Inductor Current (IL)) |
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