LM3676SD-ADJEV National Semiconductor, LM3676SD-ADJEV Datasheet - Page 14

• Manufacturer Part Number: LM3676SD-ADJEV
• Description: BOARD EVALUATION LM3676SD-ADJ
• Manufacturer: National Semiconductor

Specifications of LM3676SD-ADJEV

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1 ~ 3.3V
• Current - Output: 600mA
• Voltage - Input: 2.9 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 2MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM3676
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Not Compliant

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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. The minimum value of inductance to
guarantee good performance is 1.76µH at I
current over the ambient temperature range. Shielded in-
ductors 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
•
•
•
•
•
•
Method 2:
A more conservative and recommended approach is to
choose an inductor that has a saturation current rating greater
than the maximum current limit of 1200mA.
I
I
V
L : min inductor value including worst case tolerances
(30% drop can be considered for method 1)
f : minimum switching frequency (1.6Mhz)
V
RIPPLE
OUTMAX
IN
OUT
: maximum input voltage in application
: output voltage
: average to peak inductor current
: maximum load current (600mA)
LIM
(typ) dc
14
A 2.2 µH inductor with a saturation current rating of at least
1200 mA is recommended for most applications.The
inductor’s resistance should be less than 0.3Ω for good effi-
ciency. Table one lists suggested inductors and suppliers. 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 models
is unacceptable.
INPUT CAPACITOR SELECTION
A ceramic input capacitor of 4.7 µF, 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 minimum input capacitance to guarantee good per-
formance is 2.2µF at 3V dc bias; 1.5µF at 5V dc bias
including tolerances and over ambient temperature
range. The input filter capacitor supplies current to the PFET
switch of the LM3676 in the first half of each cycle and re-
duces 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 cur-
rent. Select a capacitor with sufficient ripple current rating.
The input current ripple can be calculated as:
IN
pin of the device. A larger value may be used for im-