LM3661TL-1.25 NSC [National Semiconductor], LM3661TL-1.25 Datasheet - Page 14

• Manufacturer Part Number: LM3661TL-1.25
• Description: 450mA Subminiature, Micropower Step-Down DC-DC Converter for Ultra Low-Voltage Circuits
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Information
INDUCTOR SELECTION
There are a few things that one must consider when select-
ing an inductor for an application. They are the inductor DC
current rating, inductor ripple current, DC-resistance of the
inductor and value of the inductor. The DC current rating of
the inductor denotes the maximum current before the induc-
tor core enters saturation. Before selecting the DC current
rating of the inductor, an inductor ripple current must be
determined using Equation (1). The DC current of the induc-
tor should be the maximum output current of the circuit plus
half of the peak to peak current ripple of the inductor using
Equation (2).
A good estimate for the inductor ripple current would be
using a operation condition or assume the inductor ripple
current to be about 30% of the maximum output current of
the device. Consider the following example for LM3661
(when I
output operates at 4.5V input and 600kHz in an application,
solving for ∆I
Therefore the maximum peak current (Equation (1)) in the
application will be 530 mA (I
with DC current rating of 600 mA or higher should suffice for
the application when I
approach, it is best to select an inductor with a current rating
of the maximum switch peak current of the device. Note that
If smaller inductor is used in the application, the larger the
inductor ripple current (Equation (1)). Care must be taken to
select the inductor such that the peak current rating of the
inductor accounts for minimum inductance and maximum
current for the operating condition. Equation (3) can be used
to calculate the inductor value if the application conditions
are known:
SEL
= L); a 10 µH, 450 mA load current with 1.4V
L
using Equation (2) yields ∆I
SEL
= L. For a more conservative
O
+ 1/2∆I
L
). Thus, an inductor
(Continued)
L
= 160 mA.
(1)
(2)
FIGURE 3.
14
Where f is the operating frequency, ∆I
ripple and Vo is the desired output.
Finally, the DC resistance (DCR) of the inductor also affect
the overall efficiency of the solution. Lower DCR is recom-
mended for better efficiency in handheld and battery oper-
ated applications. Consult inductor manufacture for this
specification. Table 2 lists suggested inductors and suppli-
ers.
INPUT AND OUTPUT CAPACITOR
The LM3661 is designed for ceramic capacitor for its input
and output filters. Ceramic capacitors such as X5R and X7R
are recommended to use for input and output filters. These
provide an optimal balance between small size, cost, reliabil-
ity and performance. Do not use Y5V ceramic capacitors as
they have poor dielectrics performance over temperature
and voltage characteristics for a given value. Table 2 lists
suggested capacitors and suppliers.
A 10 µF input and 22 µF output ceramic capacitors are
suggested in figure 2 (Typical application circuit) for optimal
performance.
The input filter capacitor supplies current to the PFET switch
of the LM3661 in the first part of each cycle and reduces
voltage ripple imposed on the input power source. The out-
put filter capacitor smooths out current flow from the inductor
and reduce output voltage ripple. These capacitors must be
selected with sufficient capacitance and sufficiently low ESR
Part Number
D01608C-103
P1174.103T
P0770.103T
Ell6GM100M
Table2. Suggested Inductor and Suppliers
20098833
Vendor
Coilcraft
Pulse
Panasonic
L
is the inductor current
Web
www.coilcraft.com
www.pulseeng.com
www.panasonic.com
(3)