1/2

'i

2

3

iout

D =

L =

'i

x

D

f

LM3485

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SNVS178G – JANUARY 2002 – REVISED FEBRUARY 2013

−0.366726

The inductance can be calculated based upon the desired operating frequency where:

(7)

And

(8)

The inductor should be rated to the following:

Ipk = (Iout+

Δi/2)*1.1

(9)

(10)

The inductance value and the resulting ripple is one of the key parameters controlling operating frequency. The

second is the ESR.

The ESR of the output capacitor times the inductor ripple current is equal to the output ripple of the regulator.

operating frequency increases as well. If ESR is reduced then the operating frequency reduces.

The use of ceramic capacitors has become a common desire of many power supply designers. However,

ceramic capacitors have a very low ESR resulting in a 90° phase shift of the output voltage ripple. This results in

low operating frequency and increased output ripple. To fix this problem a low value resistor should be added in

series with the ceramic output capacitor. Although counter intuitive, this combination of a ceramic capacitor and

external series resistance provide highly accurate control over the output voltage ripple. The other types

capacitor, such as Sanyo POS CAP and OS-CON, Panasonic SP CAP, Nichicon "NA" series, are also

recommended and may be used without additional series resistance.

For all practical purposes, any type of output capacitor may be used with proper circuit verification.

A bypass capacitor is required between the input source and ground. It must be located near the source pin of

the external PFET. The input capacitor prevents large voltage transients at the input and provides the

instantaneous current when the PFET turns on.

The important parameters for the input capacitor are the voltage rating and the RMS current rating. Follow the

manufacturer's recommended voltage derating. For high input voltage application, low ESR electrolytic capacitor,

the Nichicon "UD" series or the Panasonic "FK" series, is available. The RMS current in the input capacitor can

be calculated.

(11)

The input capacitor power dissipation can be calculated as follows.

CIN

(12)

connected in parallel to handle large RMS currents. In some cases it may be much cheaper to use multiple

electrolytic capacitors than a single low ESR, high performance capacitor such as OS-CON or Tantalum. The

capacitance value should be selected such that the ripple voltage created by the charge and discharge of the

capacitance is less than 10% of the total ripple across the capacitor.

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