Datasheet

LM22671

SNVS589M –SEPTEMBER 2008–REVISED APRIL 2013

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(11)

where F

is the switching frequency and V

should be taken at its maximum value, for the given application.

The above formula provides a guide to select the value of the inductor L; the nearest standard value will then be

used in the circuit.

Once the inductor is selected, the actual ripple current can be found from the equation shown below:

(12)

Increasing the inductance will generally slow down the transient response but reduce the output voltage ripple.

Reducing the inductance will generally improve the transient response but increase the output voltage ripple.

The inductor must be rated for the peak current, I

, in a given application, to prevent saturation. During normal

loading conditions, the peak current is equal to the load current plus 1/2 of the inductor ripple current.

During an overload condition, as well as during certain load transients, the controller may trip current limit. In this

case the peak inductor current is given by I

, found in the Electrical Characteristics table. Good design practice

requires that the inductor rating be adequate for this overload condition. If the inductor is not rated for the

maximum expected current, it can saturate resulting in damage to the LM22671 and/or the power diode.

INPUT CAPACITOR

The input capacitor selection is based on both input voltage ripple and RMS current. Good quality input

capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the regulator current

during switch on-time. Low ESR ceramic capacitors are preferred. Larger values of input capacitance are

desirable to reduce voltage ripple and noise on the input supply. This noise may find its way into other circuitry,

sharing the same input supply, unless adequate bypassing is provided. A very approximate formula for

determining the input voltage ripple is shown below:

(13)

Where V

is the peak-to-peak ripple voltage at the switching frequency. Another concern is the RMS current

passing through this capacitor. The following equation gives an approximation to this current:

(14)

The capacitor must be rated for at least this level of RMS current at the switching frequency.

All ceramic capacitors have large voltage coefficients, in addition to normal tolerances and temperature

coefficients. To help mitigate these effects, multiple capacitors can be used in parallel to bring the minimum

capacitance up to the desired value. This may also help with RMS current constraints by sharing the current

among several capacitors. Many times it is desirable to use an electrolytic capacitor on the input, in parallel with

the ceramics. The moderate ESR of this capacitor can help to damp any ringing on the input supply caused by

long power leads. This method can also help to reduce voltage spikes that may exceed the maximum input

voltage rating of the LM22671.

It is good practice to include a high frequency bypass capacitor as close as possible to the LM22671. This small

case size, low ESR, ceramic capacitor should be connected directly to the VIN and GND pins with the shortest

possible PCB traces. Values in the range of 0.47 µF to 1 µF are appropriate. This capacitor helps to provide a

low impedance supply to sensitive internal circuitry. It also helps to suppress any fast noise spikes on the input

supply that may lead to increased EMI.

Product Folder Links: LM22671