LM2832 datasheet(12/27 Pages) NSC

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Design Guide

INDUCTOR SELECTION

The Duty Cycle (D) can be approximated quickly using the

ratio of output voltage (V

O) to input voltage (VIN):

The catch diode (D1) forward voltage drop and the voltage

drop across the internal PMOS must be included to calculate

a more accurate duty cycle. Calculate D by using the follow-

ing formula:

SW can be approximated by:

SW =IOUT xRDSON

The diode forward drop (V

D) can range from 0.3V to 0.7V

depending on the quality of the diode. The lower the V

D, 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 (2.4A) is not

exceeded, so the peak current in the inductor must be

calculated. The peak current (I

LPK) in the inductor is calcu-

lated by:

LPK =IOUT +

∆i

In general,

∆i

L =0.1x(IOUT) → 0.2x(IOUT)

∆i

L = 20% of 2A, the peak current in the inductor will be

2.4A. The minimum guaranteed current limit over all operat-

ing conditions is 2.4A. One can either reduce

∆i

L, or make

the engineering judgment that zero margin will be safe

enough. The typical current limit is 3.25A.

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 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 maxi-

mum 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 pre-

sents little restriction since the variety of ferrite-based induc-

tors is huge. Lastly, inductors with lower series resistance

DCR) will provide better operating efficiency. For recom-

mended 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, volt-

age, RMS current rating, and ESL (Equivalent Series Induc-

tance). The recommended input capacitance is 22 µF.The

input voltage rating is specifically stated by the capacitor

manufacturer. Make sure to check any recommended derat-

ings 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 cur-

rent rating (I

RMS-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 calcu-

late 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 frequen-

cies 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.

20197505

FIGURE 3. Inductor Current

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