MLP2012S datasheet(16/50 Pages) SII

16 / 50 page

STEP-UP, SUPER-SMALL PACKAGE, 1.2 MHz PWM/PFM SWITCHABLE SWITCHING REGULATOR

S-8363 Series

Rev.2.0_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, as

shown in Figure 15.

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

× 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 ........................ (1)

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) + Δ

= IIN(DC) +

VOUT

− VIN

× L

× tOFF

= IIN(DC) +

VIN

× L ×

tON ............... (2)

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 (1) and (2), the direct current component (IIN(DC)) depends on IOUT.

IOUT(0) when IIN(DC) reaches 0 (critical point) :

IOUT(0) =

tON

× VIN2

× L × VOUT

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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