S8550 datasheet(10/31 Pages) SII

10 / 31 page

STEP-DOWN, BUILT-IN FET, SYNCHRONOUS RECTIFICATION, PWM CONTROL SWITCHING REGULATORS

S-8550 Series

Rev.5.0_01



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.

Control

circuit

Pch power MOS FET

Nch power MOS FET

VIN

VOUT

COUT

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 max.

IL min.

Figure 10 Continuous Mode (Current Cycle of Inductor Current (IL))

S8550 Datasheet(PDF) 10 Page - Seiko Instruments Inc