LM3671MF-1.25 National Semiconductor, LM3671MF-1.25 Datasheet - Page 16

• Manufacturer Part Number: LM3671MF-1.25
• Description: DC/DC CONVERTER, 1.25V, SMD, 3671
• Manufacturer: National Semiconductor

Specifications of LM3671MF-1.25

• Primary Input Voltage: 2.25V
• No. Of Outputs: 1
• Output Voltage: 1.25V
• Output Current: 600mA
• Voltage Regulator Case Style: SOT-23
• No. Of Pins: 5
• Operating Temperature Range: -25°C To +85°C
• Svhc: No
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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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 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
inductors radiate less noise and should be preferred.
There are two methods to choose the inductor saturation
current 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 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
1.0
1.1
1.2
1.5
1.6
1.7
1.8
2.5
2.8
3.3
: maximum input voltage in application
: output voltage
(V)
: average to peak inductor current
: maximum load current (600mA)
R1(kΩ)
200
191
280
357
442
432
464
523
402
464
562
R2 (kΩ)
200
158
200
178
200
178
178
191
100
100
100
(Continued)
LIM
(typ) dc
C1 (pF)
8.2
8.2
8.2
6.8
8.2
8.2
6.8
18
18
12
10
16
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 4.7 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 minimum input capacitance to guarantee
good performance 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 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
OUT
33
33
IN
(Circuit of Figure 2)
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
C
IN
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
(µF)
C
OUT
10
10
10
10
10
10
10
10
10
10
10
(µF)