The LM2734Z 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 or ripple ratio is determined, the

inductance is calculated by:

where f

current. When selecting an inductor, make sure that it is

capable of supporting the peak output current without satu-

rating. Inductor saturation will result in a sudden reduction in

inductance and prevent the regulator from operating cor-

rectly. 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 de-

signed maximum output current is 0.5A and the peak current

is 0.7A, then the inductor should be specified with a satura-

saturation or peak current of the inductor at the 1.7A typical

switch current limit. The difference in inductor size is a factor

of 5. Because of the operating frequency of the LM2734Z,

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 (DCR) will provide better operating efficiency. For

recommended inductors see Example Circuits.

INPUT CAPACITOR

An input capacitor is necessary to ensure that V

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 10µF, al-

though 4.7µF works well for input voltages below 6V. 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

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 ce-

ramic chip capacitor will have very low ESL. At the operating

frequencies of the LM2734Z, certain 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 or

Cornell Dubilier ESR, and multilayer ceramic capacitors

(MLCC) are all good choices for both input and output ca-

pacitors and have very low ESL. For MLCCs it is recom-

mended 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:

When using MLCCs, the ESR is typically so low that the

capacitive ripple may dominate. When this occurs, the out-

put 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 LM2734Z, there is really no need to review any

other capacitor technologies. Another benefit of ceramic ca-

pacitors 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 ce-

ramic 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 at 10 µF of

output capacitance. Capacitance can be increased signifi-

cantly with little detriment to the regulator stability. Like the

input capacitor, recommended multilayer ceramic capacitors

are X7R or X5R. Again, verify actual capacitance at the

desired operating voltage and temperature.

Check the RMS current rating of the capacitor. The RMS

current rating of the capacitor chosen must also meet the

following condition:

CATCH DIODE

The catch diode (D1) conducts during the switch off-time. A

Schottky diode is recommended for its fast switching times

and low forward voltage drop. The catch diode should be

chosen so that its current rating is greater than:

I

The reverse breakdown rating of the diode must be at least

the maximum input voltage plus appropriate margin. To im-

prove efficiency choose a Schottky diode with a low forward

voltage drop.

BOOST DIODE

A standard diode such as the 1N4148 type is recommended.

For V

small-signal Schottky diode is recommended for greater ef-

ficiency. A good choice is the BAT54 small signal diode.

BOOST CAPACITOR

A ceramic 0.01µF capacitor with a voltage rating of at least

6.3V is sufficient. The X7R and X5R MLCCs provide the best

performance.

www.national.com

8