6

Mar16-06 Rev B

SP7648 Ultra-low Quiescent Current, High Efficiency Boost Regulator

© 2006 Sipex Corporation

THeORY OF OPeRaTiOn

Figure 1. Inductor Current vs. Load

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

the charge cycle will last the maximum value

for

1.17µS. The current built up in the coil during

the charge cycle gets fully discharged in the

discontinuous conduction mode (DCM).

When the current in the coil has reached

zero, the synchronous rectifier switch is

opened and the voltage across the coil (from

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 regula-

tion point due to heavier load. If the load

current increases further, the SP7648 en-

ters continuous conduction mode (CCM)

where there is always current flowing in the

inductor. The charge time remains at maxi-

current limit is not reached as shown in plot

C in Figure 1. The inductor peak current

limit can be programmed by tying a resistor

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 discharge cycle. However, full load is not

yet achieved because at the end of the

minimum discharge time the output was still

within regulation. Maximum load is reached

when this discharge time has shrunk to

in Plot E of Figure 1.

Component Selection

Selection of capacitors for SP7648 power

supply circuits can be made through the use

of the Component Selection Table. Capa-

citor equivalent series resistance (ESR) in

the range of 0.2 to 0.3 is a requirement for

obtaining sufficient output voltage ripple for

theSP7648toproperlyregulateunderitsload.

Forexample,intheSP7648applicationcircuit

a 0µF, 10V, X5R, surface mount ceramic

outputfiltercapacitorisused.Ceramiccapa-

citorshaveanESRtoolowtoproduceenough

output ripple for the SP7648 to regulate the

output;therefore,a0.33

resistorisadded

pin.Designersshouldselectinputandoutput

capacitorswitharatingexceedingtheinductor

current ripple, which is typically set by the

the following relationship:

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