S-8365AAABA-M5T1Y2 Datasheet, PDF (18/63 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
S-8365/8366 Series
Rev.2.1_01
1. Continuous current mode
The following explains the current that flows into the inductor when the step-up operation stabilizes in a certain
status and IOUT is sufficiently large.
When the Nch power MOS FET is turned on, current (I1) flows in the direction shown in Figure 14. The inductor
current (IL) at this time gradually increases in proportion with the ON time (tON) of the Nch power MOS FET.
Current change of inductor within tON :
ΔIL(ON) = IL max. − IL min.
=
VIN
L
× tON
When the Nch power MOS FET is turned off, the voltage of the CONT pin is stepped up to VOUT + VD and the
voltage on both ends of the inductor becomes VOUT + VD − VIN. However, it is assumed here that VOUT >> VD and VD
is ignored.
Current change of inductor within tOFF :
ΔIL(OFF) =
VOUT − VIN
L
× tOFF
The input power equals the output power in an ideal situation where there is no loss by components.
IIN(AV) :
PIN = POUT
IIN(AV) × VIN = IOUT × VOUT
∴IIN(AV) =
VOUT
VIN
× IOUT....................
(2)
The current that flows in the inductor consists of a ripple current that changes due to variation over time and a
direct current.
From Figure 15 :
IIN(AV) :
IIN(AV)
= IIN(DC) +
ΔIL
2
= IIN(DC) +
VOUT − VIN
2×L
× tOFF
= IIN(DC) +
VIN
2×L
× tON...........
(3)
Above, the continuous mode is the operation mode when IIN(DC) > 0 as shown in Figure 15 and the inductor current
continuously flows.
While the output current (IOUT) continues to decrease, IIN(DC) reaches 0 as shown in Figure 16. This point is the
critical point of the continuous mode.
As shown in equations (2) and (3), the direct current component (IIN(DC)) depends on IOUT.
IOUT(0) when IIN(DC) reaches 0 (critical point) :
IOUT(0) =
tON × VIN2
2 × L × VOUT
tON can be calculated using equation (1).
When the output current decreases below IOUT(0), the current flowing in the inductor stops flowing in the tOFF period
as shown in Figure 17. This is the discontinuous mode.
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