SP6648 Datasheet, PDF (9/16 Pages) Sipex Corporation – Ultra-low Quiescent Current, High Efficiency Boost Regulator

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-
figuration. The V_OUT pin must be bypassed di-
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
V_OUT node and separated from it using grounded
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 0.2Ω
to 0.3Ω 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 47µF, 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
APPLICATIONS INFORMATION
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
inductor value and the K_ON value as given in the
following relationship:
I_L(RIPPLE) = K_ON/L
For the example of the 10µH inductor the induc-
tor current ripple would be 330mA, while for the
22µH 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
47µF tantalum capacitor as used for the input.
Low-ESR aluminum electrolytic capacitors are
acceptable provided they meet the ESR require-
ment of 0.2Ω to 0.3Ω, and we list an appropriate
100µF 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 10µH inductor with an I_SAT value
of 1.3A and a DCR of 0.065Ω, which easily
handles the I_PEAK of 0.85A of the SP6648 and
will deliver high efficiencies. Other inductors
could be selected provided their I_SAT is greater
than the I_PEAK of the SP6648.
Date: 7/19/04
SP6648 Ultra-low Quiescent Current, High Efficiency Boost Regulator
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© Copyright 2004 Sipex Corporation