1

i

2

3

1

L =

LM2832

www.ti.com

SNVS455 – AUGUST 2006

The LM2832 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 1.0A

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

There is no need to specify the saturation or peak current of the inductor at the 3.25A typical switch current limit.

The difference in inductor size is a factor of 5. Because of the operating frequency of the LM2832, ferrite based

inductors are preferred to minimize core losses. This presents little restriction since the variety of ferrite-based

recommended inductors see Example Circuits.

INPUT CAPACITOR

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

(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 LM2832, 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

datasheets 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:

(11)

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 LM2832, 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

Copyright © 2006, Texas Instruments Incorporated

Submit Documentation Feedback

11

Product Folder Links: LM2832