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SP6642/6643APN/02

SP6642/6643 Application Note

© Copyright 2000 Sipex Corporation

Changing the inductor on the SP6642/3 from

100

µH to 47µH will increase the peak inductor

current by 2 times from about 250mA to about

500mA. This can be seen from the relationship

of inductor current in the equation

V

where V

time set by the On-Time Constant, K as specified

in the SP6642/3 datasheet. Since L is the only

factor changing, for a decrease of L by 1/2, the

peak current, di, will double. With a doubling of

peak current, the output current the SP6642/3

can provide will double. The saturation current

specified for the inductor needs to be greater then

the peak current to avoid saturating the inductor,

which would result in a loss in efficiency and

could damage the inductor. Table 1 below lists

the inductors recommended for their low DC

resistance and sufficient saturation current rating.

High Current Performance

The curves in figures 1-6 illustrate SP6642

efficiency Vs output current using the Sumida

47uH inductor CD54-470 and the Sumida 100uH

inductor CD54-101 for 3 different output

voltages (2.4V, 3.3V, 5.0V) and 4 different input

voltages (0.85V, 1.0V, 1.2V, 1.6V). These figures

cover the full range of input and output voltages

and currents for a single cell step-up converter

for 2-5V output. Comparing figures 1,3,5 for

the 47uH inductor to figures 2,4,6 for the

100

µH inductor and you will see about double

the output current for the 47

µH Vs the 100µH.

For example, for 5V out for Vin of 1.0 to 1.6V

(typical single cell voltages), the 47

µHmaximum

output current is 30 to 50mA, while the 100

µH

maximum output is 15 to 25mA.

High Current Application

Table 1. Recommended Surface Mount Inductors

Comparing efficiencies, the 47

µHislessefficient

than the 100uH at the maximum output current

by about 4%. This difference can be attributed to

the difference in time constant of charge for the

inductor (L/R) in the equation for charging

current:

where Imax is the peak inductor current, t is

the charging time, L/R is the time constant for

charge in the inductor. Since L changes by 1/2

from 100

µH to 47µH, the DC series resistance

R needs to decrease by 1/2 for L/R to remain the

same. But, the DC resistance is composed of

SP6642/3 switch resistance and the DC resistance

of the inductor. Even though inductor resistance

reduces by about 1/2 (table 1) the SP6642/3

switch resistance remains the same and the total

DC resistance R will not reduce by 1/2. From this

you can see that to decrease inductor values to be

even less than 47

µH, the efficiency will continue

to decrease and peak current could rise to

unacceptable levels.

In summary, the SP6642/3 can obtain higher

output currents by using a properly selected

inductor value. The efficiency curves show double

the output current allowing the SP6642/3 to be

used in a wider variety of portable products

including medical devices and emergency

lighting.

INDUCTANCE VENDOR/PART NO.

INDUCTOR SPECIFICATION

(

µH)

RESISTANCE

Isat

(mA)

47

Sumida CD54-470

0.37

720

100

Sumida CD54-101

0.7

520