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Date: 7/20/05

SP7648 Ultra-low Quiescent Current, High Efficiency Boost Regulator

© Copyright 2005 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

t

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

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 mini-

mum discharge time the output was still

within regulation. Maximum load is reached

when this discharge time has shrunk to the

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. Capaci-

tor equivalent series resistance (ESR) in the

range of 0.2 to 0.3Ω is a requirement for

obtaining sufficient output voltage ripple for

the SP7648 to properly regulate under its

load. For example, in the SP7648 applica-

tion circuit a 10µF, 10V, X5R, surface mount

ceramic output filter capacitor is used.

Ceramic capacitors have an ESR too low to

produce enough output ripple for the SP7648

to regulate the output; therefore, a 0.33Ω

resistor is added in series with the 10µF

select input and output capacitors with a

rating exceeding the inductor current ripple,

which is typically set by the inductor value

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