S-8550AA-M5T1X Datasheet, PDF (10/31 Pages) Seiko Instruments Inc – RECTIFICATION, PWM CONTROL SWITCHING REGULATORS

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.
I₁
Pch power MOS FET
L
VIN
Control
circuit
I₂
Nch power MOS FET
C_OUT
VOUT
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).
I_L
I_L max.
I_L min.
t
t_on
t_off
T = 1/f_OSC
Figure 10 Continuous Mode (Current Cycle of Inductor Current (IL))
10