r

12

1

)

D x

r

2

12

1-D +

LM2734Z

www.ti.com

SNVS334D – JANUARY 2005 – REVISED JUNE 2008

(20)

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 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 capacitors and have very low ESL. For MLCCs it is recommended 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:

(21)

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 LM2734Z, 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 inductor to the output. A ceramic 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

significantly 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:

(22)

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:

(23)

The reverse breakdown rating of the diode must be at least the maximum input voltage plus appropriate margin.

To improve efficiency choose a Schottky diode with a low forward voltage drop.

BOOST DIODE

3.3V, a small-signal Schottky diode is recommended for greater efficiency. 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.

OUTPUT VOLTAGE

The output voltage is set using the following equation where R2 is connected between the FB pin and GND, and

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