D x

r

2

12

1-D +

L =

x (1-D)

LM2736

SNVS316E – SEPTEMBER 2004 – REVISED SEPTEMBER 2011

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One must also ensure that the minimum current limit (1.0A) is not exceeded, so the peak current in the inductor

(16)

If r = 0.7 at an output of 750mA, the peak current in the inductor will be 1.0125A. The minimum guaranteed

current limit over all operating conditions is 1.0A. One can either reduce r to 0.6 resulting in a 975mA peak

current, or make the engineering judgement that 12.5mA over will be safe enough with a 1.5A 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

(17)

Note that this is just a guideline.

The LM2736 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:

(18)

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 1.5A typical switch current limit. The difference in inductor size is a factor of 5. Because of the

operating frequency of the LM2736, 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:

(19)

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 LM2736, 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:

12

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