L =

x (1-D)

r =

D =

LM2734Z

SNVS334D – JANUARY 2005 – REVISED JUNE 2008

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The catch diode (D1) forward voltage drop and the voltage drop across the internal NMOS must be included to

calculate a more accurate duty cycle. Calculate D by using the following formula:

(14)

(15)

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.

The ratio of ripple current (

ratio r is defined as:

(16)

One must also ensure that the minimum current limit (1.2A) is not exceeded, so the peak current in the inductor

(17)

If r = 0.5 at an output of 1A, the peak current in the inductor will be 1.25A. The minimum guaranteed current limit

over all operating conditions is 1.2A. One can either reduce r to 0.4 resulting in a 1.2A peak current, or make the

engineering judgement that 50mA over will be safe enough with a 1.7A typical current limit and 6 sigma limits.

When the designed maximum output current is reduced, the ratio r can be increased. At a current of 0.1A, r can

be made as high as 0.9. The ripple ratio can be increased at lighter loads because the net ripple is actually quite

low, and if r remains constant the inductor value can be made quite large. An equation empirically developed for

the maximum ripple ratio at any current below 2A is:

-0.3667

(18)

Note that this is just a guideline.

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 voltage ripple.

Now that the ripple current or ripple ratio is determined, the inductance is calculated by:

(19)

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 0.5A and the peak current is 0.7A, then the inductor should

be specified with a saturation current limit of >0.7A. There is no need to specify the 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

primary specifications of the input capacitor are capacitance, voltage, RMS current rating, and ESL (Equivalent

Series Inductance). The recommended input capacitance is 10µF, although 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 deratings and also verify if there is any significant change in capacitance at the operating input

greater than:

10

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