VSW = IOUT x RDSON
The diode forward drop (VD) can range from 0.3V to 0.7V de-
pending on the quality of the diode. The lower the VD, the
higher the operating efficiency of the converter. The inductor
value determines the output ripple current. Lower inductor
values decrease the size of the inductor, but increase the
output ripple current. An increase in the inductor value will
decrease the output ripple current.
One must ensure that the minimum current limit (1.8A) is not
exceeded, so the peak current in the inductor must be calcu-
lated. The peak current (ILPK) in the inductor is calculated by:
ILPK = IOUT + ΔiL
20174805
FIGURE 3. Inductor Current
In general,
ΔiL = 0.1 x (IOUT) → 0.2 x (IOUT)
If ΔiL = 20% of 1.50A, the peak current in the inductor will be
1.8A. The minimum guaranteed current limit over all operating
conditions is 1.8A. One can either reduce ΔiL, or make the
engineering judgment that zero margin will be safe enough.
The typical current limit is 2.5A.
The LM2831 operates at frequencies allowing the use of ce-
ramic output capacitors without compromising transient re-
sponse. Ceramic capacitors allow higher inductor ripple
without significantly increasing output ripple. See the output
capacitor section for more details on calculating output volt-
age ripple. Now that the ripple current is determined, the
inductance is calculated by:
Where
When selecting an inductor, make sure that it is capable of
supporting the peak output current without saturating. Induc-
tor 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 1.0A and the peak current is 1.25A, then the induc-
tor should be specified with a saturation current limit of >
1.25A. There is no need to specify the saturation or peak cur-
rent of the inductor at the 2.5A typical switch current limit. The
difference in inductor size is a factor of 5. Because of the op-
erating frequency of the LM2831, ferrite based inductors are
preferred to minimize core losses. This presents little restric-
tion since the variety of ferrite-based inductors is huge. Lastly,
inductors with lower series resistance (RDCR) will provide bet-
ter operating efficiency. For recommended inductors see Ex-
ample Circuits.
INPUT CAPACITOR
An input capacitor is necessary to ensure that VIN 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 22 µF.The input volt-
age 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 (IRMS-IN)
must be greater than:
Neglecting inductor ripple simplifies the above equation to:
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 effec-
tive cross sectional area of the current path. A large leaded
capacitor will have high ESL and a 0805 ceramic chip capac-
itor will have very low ESL. At the operating frequencies of the
LM2831, leaded capacitors may have an ESL so large that
the resulting impedance (2πfL) will be higher than that re-
quired to provide stable operation. As a result, surface mount
capacitors are strongly recommended.
Sanyo POSCAP, Tantalum or Niobium, Panasonic SP, 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 type capacitors
due to their tolerance and temperature characteristics. Con-
sult capacitor manufacturer datasheets to see how rated
capacitance varies over operating conditions.
OUTPUT CAPACITOR
The output capacitor is selected based upon the desired out-
put ripple and transient response. The initial current of a load
transient is provided mainly by the output capacitor. The out-
put ripple of the converter is:
When using MLCCs, the ESR is typically so low that the ca-
pacitive 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
LM2831, there is really no need to review any other capacitor
technologies. Another benefit of ceramic capacitors is their
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LM2831