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Date: 7/19/04

SP6648 Ultra-low Quiescent Current, High Efficiency Boost Regulator

© Copyright 2004 Sipex Corporation

Circuit Layout

Printed circuit board layout is a critical part of a

power supply design. Poor designs can result in

excessive EMI on the feedback paths and on the

ground planes with applications involving high

switching frequencies and large peak currents.

Excessive EMI can result in instability or regu-

lation 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 integrated into ground as a

pseudo-ground plane. On a multilayer PC board,

route the star ground using component-side cop-

per fill, then connect it to the internal ground

plane using vias. For the SP6648 devices, the

inductor and input and output filter capacitors

should be soldered with their ground pins as

close together as possible in a star-ground con-

rectly to ground as close to the SP6648 devices

as possible (within 0.2in or 5mm). The DC-DC

converter and any digital circuitry should be

placed on the opposite 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

copper to minimize EMI. See the SP6648EB

Evaluation Board Manual for PC Board Layout

design details.

Component Selection

Selection of capacitors for SP6648 power sup-

ply circuits can be made through the use of the

Component Selection Table. Capacitor equiva-

lent series resistance (ESR) in the range of 0.2

Ω

to 0.3

Ω is a requirement for obtaining sufficient

output voltage ripple for the SP6648 to properly

regulate under load. For ESR values in this

range, low ESR Tantalum capacitors are recom-

mended. For example, in the SP6648 applica-

tion circuit a 47

µF, 10V, low-ESR, surface-

mount tantalum output filter capacitor typically

provides 50mV output ripple when stepping up

from 2.6V to 3.3V at 200mA. Ceramic capaci-

tors have ESR too low to produce enough output

ripple for the SP6648 to regulate the output.

Designers should select input and output ca-

pacitors with a rating exceeding the inductor

current ripple, which is typically set by the

following relationship:

For the example of the 10

µH inductor the induc-

tor current ripple would be 330mA, while for the

22

µH inductor the inductor current ripple value

would be 150mA. Do not allow tantalum ca-

pacitors to exceed their ripple-current ratings.

An input filter capacitor can reduce peak cur-

rents drawn from the battery and improve effi-

ciency. For most applications, use the same

47

µF tantalum capacitor as used for the input.

Low-ESR aluminum electrolytic capacitors are

acceptable provided they meet the ESR require-

ment of 0.2

Ω to 0.3Ω, and we list an appropriate

100

µF aluminum electrolytic in the component

selection table, but standard aluminum electro-

lytic capacitors are not recommended.

In selecting an inductor, the saturation current

specified for the inductor needs to be greater

then the SP6648 peak current to avoid saturating

the inductor, which would result in a loss in

efficiency and could damage the inductor. The

SP6648 evaluation board uses a Sumida

CDRH5D28 10

of 1.3A and a DCR of 0.065

Ω, which easily

will deliver high efficiencies. Other inductors

APPLICATIONS INFORMATION