LM3678SD-1.2EV National Semiconductor, LM3678SD-1.2EV Datasheet - Page 10

• Manufacturer Part Number: LM3678SD-1.2EV
• Description: BOARD EVAL FOR LM3678SD-1.2
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3678SD-1.2EV

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 0.8V or 1.2V
• Current - Output: 1.5A
• Voltage - Input: 2.5 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 3.3MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM3678
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

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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.
There are two methods to choose the inductor saturation cur-
rent rating.
Method 1:
The saturation current should be greater than the sum of the
maximum load current and the worst case average to peak
inductor current. This can be written as
•
•
•
•
•
Output Capacitor Selection
A ceramic output capacitor of 22µF, 6.3V is sufficient for most
applications. Use X7R or X5R types; do not use Y5V. DC bias
characteristics of ceramic capacitors must be considered
when selecting case sizes like 0805 and 0603. DC bias char-
acteristics vary from manufacturer to manufacturer and dc
bias curves should be requested from them as part of the ca-
pacitor selection process.
The output filter capacitor smooths out current flow from the
inductor to the load, helps maintain a steady output voltage
during transient load changes and reduces output voltage
ripple. These capacitors must be selected with sufficient ca-
pacitance and sufficiently low ESR to perform these functions.
The output voltage ripple is caused by the charging and dis-
charging of the output capacitor and by the R
calculated as:
Voltage peak-to-peak ripple due to capacitance can be ex-
pressed as follow:
I
I
V
L : minimum inductor value including worst case
tolerances (30% drop can be considered for method 1)
f : minimum switching frequency (2.7Mhz)
RIPPLE
OUTMAX
IN
NR4012T1R0N
LPS4012-102L
LPS4012-102L
: maximum input voltage in application
: average to peak inductor current
Model
: maximum load current (1.5A)
Taiyo Yuden
Coilcraft
Coilcraft
Vendor
TABLE 1. Suggest Inductors and Their Suppliers
ESR
and can be
3.9 x 3.9 x 1.2
3.9 x 3.9 x 1.8
LxWxH (mm)
Dimensions
4 x 4 x 1.2
10
•
For a more conservative approach, a 1µH inductor with a sat-
uration current rating of at least 2.5A is recommended for
most applications.
Input Capacitor Selection
A ceramic input capacitor of 10uF, 6.3V is sufficient for most
applications. Place the input capacitor as close as possible to
the V
proved input voltage filtering. Use X7R or X5R types; do not
use Y5V. DC bias characteristics of ceramic capacitors must
be considered when selecting case sizes like 0805 and 0603.
The input filter capacitor supplies current to the PFET switch
of the LM3678 in the first half of each cycle and reduces volt-
age 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. Se-
lect a capacitor with sufficient ripple current rating. The input
current ripple can be calculated as:
Voltage peak-to-peak ripple due to ESR can be expressed as
follow:
V
Because these two components are out of phase the rms (root
mean squared) value can be used to get an approximate val-
ue of peak-to-peak ripple.
The peak-to-peak ripple voltage rms value can be expressed
as follow:
Note that the output voltage ripple is dependent on the induc-
tor current ripple and the equivalent series resistance of the
output capacitor (R
The R
dependent); make sure the value used for calculations is at
the switching frequency of the part.
PP-ESR
V
OUT
IN
ESR
pin of the device. A larger value may be used for im-
: output voltage
= (2 * I
is frequency dependent (as well as temperature
RIPPLE
D.C.R (max)
100mΩ
60mΩ
40mΩ
ESR
) * R
).
ESR
2.5A
2.5A
3.4A
I
SAT