Datasheet

'V
O
= 'i
L
x (R
ESR
+
1
8 x f
S
x C
O
)
I
RMS-IN
= I
O
x
D x
r
2
12
1-D +
L =
V
O
+ V
D
I
O
x r x f
S
x (1-D)
LMR12010
SNVS731A –SEPTEMBER 2011–REVISED SEPTEMBER 2011 www.ti.com
Now that the ripple current or ripple ratio is determined, the inductance is calculated by:
where
• f
s
is the switching frequency
• I
O
is the output current (18)
When selecting an inductor, make sure that it is capable of supporting the peak output 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 output current. For example, if the designed maximum output current is 0.5A
and the peak current is 0.7A, then the inductor should be specified with a saturation current limit of >0.7A. There
is no need to specify the saturation or peak current of the inductor at the 1.7A typical switch current limit. The
difference in inductor size is a factor of 5. Because of the operating frequency of the LMR12010, 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 10µF, although 4.7µF works well for input voltages
below 6V. The input voltage rating is specifically stated by the capacitor manufacturer. 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 input capacitor maximum RMS input current rating (I
RMS-IN
) must be
greater than:
(19)
It can be shown from the above equation that maximum RMS capacitor current occurs when D = 0.5. Always
calculate the RMS at the point where the duty cycle, D, is closest to 0.5. The ESL of an input capacitor is usually
determined by the effective cross sectional area of the current path. A large leaded capacitor will have high ESL
and a 0805 ceramic chip capacitor will have very low ESL. At the operating frequencies of the LMR12010,
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. Sanyo POSCAP,
Tantalum or Niobium, Panasonic SP or Cornell Dubilier ESR, and multilayer ceramic capacitors (MLCC) are all
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.
Output Capacitor
The output capacitor is selected based upon the desired output ripple and transient response. The initial current
of a load transient is provided mainly by the output capacitor. The output ripple of the converter is:
(20)
When using MLCCs, the ESR is typically so low that the capacitive ripple may dominate. When this occurs, the
output ripple will be approximately sinusoidal and 90° phase shifted from the switching action. Given the
availability and quality of MLCCs and the expected output voltage of designs using the LMR12010, there is really
no need to review any other capacitor technologies. Another benefit of ceramic capacitors is their ability to
bypass high frequency noise. A certain amount of switching edge noise will couple through parasitic
capacitances in the inductor to the output. A ceramic capacitor will bypass this noise while a tantalum will not.
Since the output capacitor is one of the two external components that control the stability of the regulator control
loop, most applications will require a minimum at 10 µF of output capacitance. Capacitance can be increased
significantly with little detriment to the regulator stability. Like the input capacitor, recommended multilayer
ceramic capacitors are X7R or X5R. Again, verify actual capacitance at the desired operating voltage and
temperature.
14 Submit Documentation Feedback Copyright © 2011, Texas Instruments Incorporated
Product Folder Links: LMR12010