LM3671MFX-1.375 National Semiconductor, LM3671MFX-1.375 Datasheet - Page 13

• Manufacturer Part Number: LM3671MFX-1.375
• Description: 2MHz / 600mA Step-Down DC-DC Converter in SOT23-5
• Manufacturer: National Semiconductor

Application Information
LM3671-ADJ configurations for various V
INDUCTOR SELECTION
There are two main considerations when choosing an induc-
tor; the inductor should not saturate, and the inductor current
ripple is small enough to achieve the desired output voltage
ripple. Different saturation current rating specs are followed
by different manufacturers so attention must be given to
details. Saturation current ratings are typically specified at
25˚C so ratings at max ambient temperature of application
should be requested from manufacturer.
There are two methods to choose the inductor saturation
current rating.
Method 1:
The saturation current is greater than the sum of the maxi-
mum 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 saturation current rating greater
than the max current limit of 1150mA.
A 2.2 µH inductor with a saturation current rating of at least
1150 mA is recommended for most applications.The induc-
• I
• I
• V
• L : min inductor value including worst case tolerances
• f : minimum switching frequency (1.6Mhz)
• V
(30% drop can be considered for method 1)
V
RIPPLE
OUTMAX
IN
OUT
1.875
OUT
0.7
0.8
0.9
1.0
1.1
1.2
1.5
1.6
1.7
2.5
3.3
: maximum input voltage in application
: output voltage
(V)
: average to peak inductor current
: maximum load current (600mA)
R1(kΩ)
54.9
118
162
200
191
280
360
442
432
523
402
562
R2 (kΩ)
137
196
200
200
158
200
180
200
180
191
100
100
(Continued)
C1 (pF)
8.2
8.2
6.8
8.2
6.8
68
33
22
18
18
12
10
13
tor’s resistance should be less than 0.3Ω for good efficiency.
Table 1 lists suggested inductors and suppliers. For low-cost
applications, an unshielded bobbin inductor could be consid-
ered. For noise critical applications, a toroidal or shielded-
bobbin inductor should be used. A good practice is to lay out
the board with overlapping footprints of both types for design
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 10 µF, 6.3V is sufficient for most
applications. Place the input capacitor as close as possible
to the V
improved 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 LM3671 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. Select a capacitor with sufficient ripple current rat-
ing. The input current ripple can be calculated as:
C2 (pF)
none
none
none
none
none
none
none
none
none
none
OUT
27
33
IN
pin of the device. A larger value may be used for
L (µH)
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
C
IN
10
10
10
10
10
10
10
10
10
10
10
10
(µF)
C
OUT
www.national.com
22
22
22
22
22
22
22
22
22
22
22
22
(µF)