S-8365AAABA-M5T1Y2 Datasheet, PDF (17/63 Pages) Seiko Instruments Inc – PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER

STEP-UP, SUPER-SMALL PACKAGE, 1.2 MHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
Rev.2.1_01
S-8365/8366 Series
 Operation Principles
The S-8365/8366 Series is a step-up switching regulator controller. Figure 14 shows the basic circuit diagram.
Step-up switching regulators start current supply by the input voltage (VIN) when the Nch power MOS FET is turned
on and holds energy in the inductor at the same time. When the Nch power MOS FET is turned off, the CONT pin
voltage is stepped up to discharge the energy held in the inductor and the current is discharged to VOUT through the
diode. When the discharged current is stored in CL, a voltage is generated, and the potential of VOUT increases until
the voltage of the FB pin reaches the same potential as the internal reference voltage.
For the PWM control method, the switching frequency (fOSC) is fixed and the VOUT voltage is held constant
according to the ratio of the ON time and OFF time (ON duty) of the Nch power MOS FET in each period.
For the PWM control method, the VOUT voltage is held constant by controlling the ON time.
In the S-8366 Series, the Nch power MOS FET is turned on when the fixed duty cycle is 28% for the PFM control
method. When energy is discharged to VOUT once and the VOUT potential exceeds the set value, the Nch power
MOS FET stays in the off status until VOUT decreases to the set value or less due to the load discharge. Time VOUT
decreases to the set value or less depends on the amount of load current, so, the switching frequency varies
depending on this current.
L
CONT ^SD
I₂
V_IN
I₁
V_OUT
I_OUT
Nch power
MOS FET
EXT
FB
VSS
C_OUT
R_L
Figure 14 Basic Circuit of Step-up Switching Regulator
The ON duty in the current continuous mode can be calculated by using the equation below. Use the S-8365/8366
Series in the range where the ON duty is less than the maximum duty. Note that the products with short-circuit
protection is set in the timer-latch status if the maximum duty lasts the short-circuit protection delay time (tPRO) or
more.
The maximum duty is 85% typ. for 1.2 MHz products and 90% for 600 kHz products.
( ) ^{ON duty =}
1−
VIN
VOUT + VD^*1
× 100 [%]
The ON time (tON) can be calculated by using the following equation :
tON = ¹ × ON duty
f OSC
( ) = 1 ×
f OSC
1−
VIN
VOUT + VD^*1
········ (1)
*1. VD : Forward voltage of diode
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