5

Rev:001 Date:11/20/03

SP6649 Ultra-low Quiescent Current, High Efficiency Boost DC-DC Regulator

© Copyright 2002 Sipex Corporation

At light loads (plot A in Figure 3) the charge

would be:

The current built up in the coil during the charge

cycle gets fully discharged (discontinuous con-

duction mode, DCM) When the current in the

coil has reached zero the synchronous rectifier

switch is opened and the voltage across the coil

eliminate inductive ringing.

With increasing load (plot B in Figure 3) this

inductor damping time becomes shorter (be-

cause the output will drop quicker below its

regulation point due to the heavier load) up to

the point where it becomes zero. If the load

increases further the SP6649 enters continuous

conduction mode (CCM) where there is always

current in the inductor. The charge time is still

limit is not reached (plot C in Figure 3). the

inductor peak current limit can be programmed

ground where:

When the peak current limit is reached the

charge time is short-cycled.

In (plot D of Figure 3) the current reaches the

peak current limit during the charge cycle but

full load is still not reached becuse at the end of

its regulation point. Finally in plot E the maxi-

mum load is reached where the discharge time

has shrunk to its minimum allowed value

Inductor Current vs. Load

Ton Max.

Toff Min.

E. Iripple=Toff* (Vo - Vi)/L

llim

llim

llim

llim

llim

Ton Max.

Ton Max.

Ton Max.

Ton Max.

Toff Min.

Toff Min.

Toff Min.

Toff Min.

D. Toff*= (Vo - Vi)/L<Iripple<Ton*Vi/L

C. Iripple=Ton*Vi/L

B. Iripple=Ton*Vi/L

A. Iripple=Ton*Vi/L

Figure 3. Inductor Current vs. Load