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SP6642/6643APN/02
SP6642/6643 Application Note
Copyright 2000 Sipex Corporation
SP6642/3
Application Note:
High Current Output
Description
The SP6642/6643 devices are high-efficiency, low-power step-up DC-DC converters for +1V
inputs ideal for single alkaline cell applications such as pagers, remote controls, pointing
devices and other low-power portable products. This application note is for users like medical
devices, emergency lighting and others who need twice the output current than the datasheet
specified values for the SP6642/6643. Typically specified at 20mA for 3.3V out with a 1.2V
input, the SP6642 can actually output double the current if a lower value of inductor is selected.
s
40mA Output Current for the SP6642/3
Single Cell to 2V to 5V Converter
s
High Efficiency: 81%
s
Twice the Output Current as the
Pin-to-Pin Compatible MAX1642/3
s
0.75V Start-up Voltage
SP6642/6643 Applications Schematic 47
H Inductor
Single
Cell
47
H
SP6642/3
DC/DC Converter
2 to 5V
Pagers,
Remotes,
Medical
Devices
SP6642/3 40mA Output Application: Block Diagram
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SP6642/6643APN/02
SP6642/6643 Application Note
Copyright 2000 Sipex Corporation
Changing the inductor on the SP6642/3 from
100
H to 47
H 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
L
= Ldi/dt
where V
L
is set by Vin and Vout, dt is the charge
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
H inductor and you will see about double
the output current for the 47
H Vs the 100
H.
For example, for 5V out for Vin of 1.0 to 1.6V
(typical single cell voltages), the 47
H maximum
output current is 30 to 50mA, while the 100
H
maximum output is 15 to 25mA.
High Current Application
Table 1. Recommended Surface Mount Inductors
Comparing efficiencies, the 47
H is less efficient
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:
Ich = Imax*(1-e
-t/(L/R)
)
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
H to 47
H, 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
H, 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
(
H)
RESISTANCE
Isat
(mA)
47
Sumida CD54-470
0.37
720
100
Sumida CD54-101
0.7
520
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SP6642/6643APN/02
SP6642/6643 Application Note
Copyright 2000 Sipex Corporation
Model
Temperature Range
Package Types
SP6642EU
-40
C to +85
C
8-pin
SOIC
SP6643EU
-40
C to +85
C
8-pin
SOIC
SP6642UEB
SP6642 Evaluation Board
ORDERING INFORMATION
Please consult the factory for pricing and availability on a Tape-On-Reel option.
Corporation
SIGNAL PROCESSING EXCELLENCE
Sipex Corporation reserves the right to make changes to any products described herein. Sipex does not assume any liability arising out of the
application or use of any product or circuit described hereing; neither does it convey any license under its patent rights nor the rights of others.
Sipex Corporation
Headquarters and
Sales Office
22 Linnell Circle
Billerica, MA 01821
TEL: (978) 667-8700
FAX: (978) 670-9001
e-mail: sales@sipex.com
Sales Office
233 South Hillview Drive
Milpitas, CA 95035
TEL: (408) 934-7500
FAX: (408) 935-7600
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