RMS _ IN

OUT

I

I

D(1 D)

=

´

-

2

2

RMS _ IN

OUT

i

I

D I

(1 D)

3

é

ù

D

-

+

ê

ú

ê

ú

ë

û

S

S

1

T

f

=

S

IN

OUT

L

DT

L

V

V

2 i

æ

ö

=

´

-

ç

÷

D

è

ø

LM2831

SNVS422D – AUGUST 2006 – REVISED SEPTEMBER 2015

www.ti.com

In general,

ΔiL = 0.1 × (IOUT) → 0.2 × (IOUT)

(6)

If

ΔiL = 20% of 1.50 A, the peak current in the inductor will be 1.8 A. The minimum ensured current limit over all

operating conditions is 1.8 A. One can either reduce

ΔiL, or make the engineering judgment that zero margin will

be safe enough. The typical current limit is 2.5 A.

The LM2831 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:

(7)

Where:

(8)

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 1 A

and the peak current is 1.25 A, then the inductor should be specified with a saturation current limit of > 1.25 A.

There is no need to specify the saturation or peak current of the inductor at the 2.5-A typical switch current limit.

The difference in inductor size is a factor of 5. Because of the operating frequency of the LM2831, 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 (RDCR) will provide better operating efficiency. For

recommended inductors, see LM2831X Design Example 2 through LM2831X Buck Converter and Voltage

Double Circuit With LDO Follower Design Example 9.

8.2.1.2.2

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 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 (IRMS-IN) must be greater than:

(9)

Neglecting inductor ripple simplifies the above equation to:

(10)

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 LM2831, leaded

capacitors may have an ESL so large that the resulting impedance (2

πfL) 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, 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. Consult capacitor manufacturer

data sheets to see how rated capacitance varies over operating conditions.

14

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