Datasheet
L =
V
IN
2 x 'i
L
x DT
S
¨
©
§
¸
¹
·
¸
¸
¹
·
¨
¨
©
§
=
2L
IN
V
L
Âi
x
S
DT
¨
¨
©
§
=
L
V
IN
'i2 L
DT
S
¸
¸
¹
·
LMR62421
SNVS734B –OCTOBER 2011–REVISED APRIL 2013
www.ti.com
(4)
A good design practice is to design the inductor to produce 10% to 30% ripple of maximum load. From the
previous equations, the inductor value is then obtained.
where
• 1/T
S
= F
SW
= switching frequency (5)
One must also ensure that the minimum current limit (2.1A) is not exceeded, so the peak current in the inductor
must be calculated. The peak current (I
LPK
) in the inductor is calculated by:
IL
pk
= I
IN
+ ΔI
L
(6)
or
IL
pk
= I
OUT
/ D' + ΔI
L
(7)
When selecting an inductor, make sure that it is capable of supporting the peak input current without saturating.
Inductor saturation will result in a sudden reduction in inductance and prevent the regulator from operating
correctly. Because of the speed of the internal current limit, the peak current of the inductor need only be
specified for the required maximum input current. For example, if the designed maximum input current is 1.5A
and the peak current is 1.75A, then the inductor should be specified with a saturation current limit of >1.75A.
There is no need to specify the saturation or peak current of the inductor at the 3A typical switch current limit.
Because of the operating frequency of the LMR62421, ferrite based inductors are preferred to minimize core
losses. This presents little restriction since the variety of ferrite-based inductors is huge. Lastly, inductors with
lower series resistance (DCR) will provide better operating efficiency. For recommended inductors see Example
Circuits.
INPUT CAPACITOR
An input capacitor is necessary to ensure that V
IN
does not drop excessively during switching transients. The
primary specifications of the input capacitor are capacitance, voltage, RMS current rating, and ESL (Equivalent
Series Inductance). The recommended input capacitance is 10 µF to 44 µF depending on the application. The
capacitor manufacturer specifically states the input voltage rating. Make sure to check any recommended
deratings and also verify if there is any significant change in capacitance at the operating input voltage and the
operating temperature. The ESL of an input capacitor is usually determined by the effective cross sectional area
of the current path. At the operating frequencies of the LMR62421, certain capacitors may have an ESL so large
that the resulting impedance (2πfL) will be higher than that required to provide stable operation. As a result,
surface mount capacitors are strongly recommended. Multilayer ceramic capacitors (MLCC) are good choices for
both input and output capacitors and have very low ESL. For MLCCs it is recommended to use X7R or X5R
dielectrics. Consult capacitor manufacturer datasheet to see how rated capacitance varies over operating
conditions.
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