LM3668SD-2833 NSC [National Semiconductor], LM3668SD-2833 Datasheet - Page 15

• Manufacturer Part Number: LM3668SD-2833
• Description: 1A, High Efficiency Dual Mode Single Inductor Buck-Boost DC/DC Converter
• Manufacturer: NSC [National Semiconductor]

Dissipation Rating Table
Inductor Selection
There are two main considerations when choosing an induc-
tor: the inductor should not saturate, and the inductor current
ripple should be small enough to achieve the desired output
voltage ripple. Different saturation current rating specifica-
tions are followed by different manufacturers so attention
must be given to details. Saturation current ratings are typi-
cally specified at 25°C. However, ratings at the maximum
ambient temperature of application should be requested from
the manufacturer. Shielded inductors radiate less noise and
should be preferred.
In the case of the LM3668, there are two modes (Buck &
Boost) of operation that must be consider when selecting an
inductor with appropriate saturation current. The saturation
current should be greater than the sum of the maximum load
current and the worst case average to peak inductor current.
The first equation shows the buck mode operation for worst
case conditions and the second equation for boost condition.
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Example using above equations:
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34°C/W ( 4
layers board
per JEDEC
standard)
I
I
V
L : Min inductor value including worst case tolerances
(30% drop can be considered)
f : Minimum switching frequency
V
D: Duty Cycle for CCM Operation
V
V
V
V
I
L = 2.2µH
F = 2MHz
Buck: I
Boost: I
RIPPLE
OUTMAX
OUT
IN
OUT
OUT
IN
IN
OUT
θ
: Maximum input voltage in application
: Input Voltage
= 2.8V to 4V
JA
= 500mA
: Output voltage
: Output Voltage
= 3.3V
: Peak inductor current
SAT
: Maximum load current
SAT
= 567mA
= 638mA
T
2941mW
A
≤
25°C
T
1912mW
A
≤
60°C
T
1176mW
A
≤
85°C
15
As a result, the inductor should be selected according to the
highest of the two I
A more conservative and recommended approach is to
choose an inductor that has a saturation current rating greater
than the maximum current limit of 2.05A.
A 2.2µH inductor with a saturation current rating of at least
2.05A is recommended for most applications. The inductor’s
resistance should be less than 100mΩ for good efficiency. For
low-cost applications, an unshielded bobbin inductor could be
considered. For noise critical applications, a toroidal or shield-
ed-bobbin inductor should be used. A good practice is to lay
out the board with overlapping footprints of both types for de-
sign flexibility. This allows substitution of a low-noise shielded
inductor, in the event that noise from low-cost bobbin model
is unacceptable.
Suggest Inductors and Suppliers
Input Capacitor Selection
A ceramic input capacitor of at least 10 µF, 6.3V is sufficient
for most applications. Place the input capacitor as close as
possible to the PV
used for improved input voltage filtering. Use X7R or X5R
types; do not use Y5V. DC bias characteristics of ceramic ca-
pacitors must be considered when selecting case sizes like
0805 or 0603. The input filter capacitor supplies current to
the PFET switch of the LM3668 in the first half of each cycle
and reduces voltage ripple imposed on the input power
source. A ceramic capacitor’s low ESR provides the best
noise filtering of the input voltage spikes due to this rapidly
changing current. For applications where input voltage is 4V
or higher, it is best to use a higher voltage rating capacitor to
eliminate the DC bias affect over capacitance.
Output Capacitor Selection
A ceramic output capacitor of 22µF, 6.3V (use 10V or higher
rating for 4.5/5V output option) is sufficient for most applica-
tions. Multilayer ceramic capacitors such as X5R or X7R with
low ESR is a good choice for this as well. These capacitors
provide an ideal balance between small size, cost, reliability
and performance. Do not use Y5V ceramic capacitors as they
have poor dielectric performance over temperature and poor
voltage characteristic for a given value.
Extra attention is required if a smaller case size capacitor is
used in the application. Smaller case size capacitors typically
have less capacitance for a given bias voltage as compared
to a larger case size capacitor with the same bias voltage.
Please contact the capacitor manufacturer for detail informa-
R0M (2µF)
LPS4012-
LPS4018-
1098AS-2
Model
222L
222L
Coilcraft
Coilcraft
Vendor
TOKO
IN
SAT
pin of the device. A larger value may be
values.
Dimension
3 x 2.8x 1.2
4 x 4 x 1.2
4 x 4 x 1.8
LxWxH
(mm)
s
100 mΩ
70 mΩ
67 mΩ
D.C.R
(max)
1.8A
( lower
current
application
s)
www.national.com
2.1A
2.5A
I
SAT