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Date: 7/19/04

SP6648 Ultra-low Quiescent Current, High Efficiency Boost Regulator

© Copyright 2004 Sipex Corporation

At light loads (as shown in plot A in Figure 1)

the charge cycle will last the maximum value for

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 (as shown in plot B in

Figure 1) this inductor damping time becomes

shorter, because the output will quickly drop

below its regulation point due to heavier load. If

the load current increases further the SP6648

enters continuous conduction mode (CCM)

where there is always current flowing in the

inductor. The charge time remains at maximum

not reached as shown in plot C in Figure 1. 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 1, the switch current reaches

the peak current limit during the charge period

which ends the charge cycle and starts the dis-

charge cycle. However, full load is not yet

achieved because at the end of the minimum

discharge time the output was still within regu-

lation.

Maximum load is reached when this discharge

time has shrunk to the 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

E

D

C

B

A

Figure 1. Inductor Current vs. Load

THEORY OF OPERATION: Continued