Datasheet

'V
OUT
= 'I
L
R
ESR
+
8 x f
SW
x C
OUT
1
I
RMS-IN
= I
OUT
x
D x
1 - D
I
RMS-IN
= I
OUT
x
D x
r
2
12
1 - D +
L =
V
OUT
+ V
D
I
OUT
x r x f
SW
x (1-D)
LMR10530
www.ti.com SNVS814A –JUNE 2012–REVISED APRIL 2013
The LMR10530 operates at frequencies allowing the use of ceramic output capacitors without compromising
transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple.
See the Output Capacitor section for more details on calculating output voltage ripple.
Now that the ripple current is determined, the inductance is calculated by:
where
• f
SW
is the switching frequency. (9)
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
properly. Because of the operating frequency of the LMR10530, ferrite based inductors are preferred to minimize
core losses. This presents little restriction since the variety and availability of ferrite-based inductors is large.
Lastly, inductors with lower series resistance (DCR) will provide better operating efficiency. For recommended
inductor selection, refer to Design Examples.
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 rating, RMS current rating, and ESL
(Equivalent Series Inductance). 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:
(10)
Neglecting inductor ripple simplifies the above equation to:
(11)
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. As a rule of thumb, a large leaded capacitor
will have high ESL and a 1206 ceramic chip capacitor will have very low ESL. At the operating frequencies of the
LMR10530, leaded capacitors may have an ESL so large that the resulting impedance (2πfL) will be higher than
that required to provide stable operation. It is strongly recommended to use ceramic capacitors due to their low
ESR and low ESL. A 22µF multilayer ceramic capacitor (MLCC) is a good choice for most applications. In cases
where large capacitance is required, use surface mount capacitors such as Tantalum capacitors and place at
least a 4.7µF ceramic capacitor close to the V
IN
pin. 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:
(12)
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 LMR10530, 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 of 22µF output capacitance. In the case of low output voltage, a
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