8

Rev. 5/26/05,

*Patent Pending

SP6641A/6641B 500mA Alkaline DC/DC Boost Regulator in SOT-23

© Copyright 2002 Sipex Corporation

APPLICATION INFORMATION

Circuit Layout

Printed circuit board layout is a critical part of a

power supply design. Poor designs can result in

excessive EMI on the voltage gradients and

feedback paths on the ground planes with appli-

cations involving high switching frequencies

and large peak currents. Excessive EMI can

result in instability or regulation errors. All

power components should be placed on the PC

board as closely as possible with the traces kept

short, direct, and wide (>50mils or 1.25mm).

Extra copper on the PC board should be inte-

grated into ground as a pseudo-ground plane.

On a multilayer PC board, route the star ground

using component-side copper fill, then connect

it to the internal ground plane using vias. For the

SP6641A/6641B devices, input and output fil-

ter capacitors should be soldered with their

ground pins as close together as possible in a

star-ground configuration. The VOUT pin must

be bypassed directly to ground as close to the

SP6641A/6641B devices as possible (within

0.2in or 5mm). The DC-DC converter and any

digital circuitry should be placed on the oppo-

site corner of the PC board as far away from

sensitive RF and analog input stages. Noisy

traces, such as from the LX pin, should be kept

away from the voltage-feedback VOUT node

and separated from it using grounded copper to

minimize EMI. See the SP6641A/6641B Evalu-

ation Board Manual for PC Board Layout de-

sign details.

Component Selection

Selection of capacitors, inductors and schottky

diodes for SP6641A and SP6641B power sup-

ply circuits can be made through the use of

Table 1 component selection. Capacitor equiva-

lent series resistance is a major contributor to

output ripple, usually greater than 60%. Low

ESR capacitors are recommended. Ceramic ca-

pacitors have the lowest ESR. Low-ESR tanta-

lum capacitors may be a more acceptable solu-

tion having both a low ESR and lower cost than

large ceramic capacitors. Designers should se-

lect input and output capacitors with a rating

exceeding the peak inductor current. Do not

allow tantalum capacitors to exceed their ripple-

current ratings. For example, in the SP6641A a

22

µF, 6V, low-ESR, surface-mount tantalum

output filter capacitor typically provides 60mV

output ripple when stepping up from 1.3V to

3.3V at 20mA. An input filter capacitor can

reduce peak currents drawn from the battery and

improve efficiency. Low-ESR aluminum elec-

trolytic capacitors are acceptable in some appli-

cations but standard aluminum electrolytic ca-

pacitors are not recommended.

In selecting an inductor, the saturation current

specified for the inductor needs to be greater

then the SP6641A/B peak current to avoid satu-

rating the inductor, which would result in a loss

in efficiency and could damage the inductor.

The SP6641A evaluation board uses a Sumida

CDRH5D28 22

µH inductor with an Isat value

of 0.9A and a DCR of 0.095

Ω, which easily

handles the Ipeak of 0.33A of the SP6641A and

will deliver high efficiencies. The SP6641B

evaluation board uses a Sumida CDRH5D28

10

µH inductor with an Isat value of 1.3A and a

DCR of 0.065

Ω, which easily handles the Ipeak

of 1.0A of the SP6641B and will deliver high

efficiencies. Other inductors could be selected

provided their Isat is greater than the Ipeak of

the SP6641A/SP6641B.

Output Filter or LDO Regulator

Designers could add LC pi filters, linear post-

regulators, or shielding in applications necessary

to address excessive noise, voltage ripple, or EMI

concerns.TheLCpifilter’scutofffrequencyshould

be at least a decade or two below the DC-DC

converters’ switching frequency for the specified

loadandinputvoltage.TheSP6201,asmallSOT23-

5pin 200mA Low Drop Out linear regulator can be

used at the SP6641A/6641B output to reduce

output noise and ripple. The schematic in figure 15

illustrates this circuit on the SP6641A Evaluation

Board with the SP6641 3.3V output followed by

the Sipex SP6201 3.0V output Low Drop Out

linear regulator.