LR8304 datasheet(8/13 Pages) LRC

8 / 13 page

APPLICATION INFORMATION

Inductor Selection, Maximum Load Current

Because the PFM peak current control scheme is

inherently stable, the inductor value does not

affect the stability of the regulator. The selection of

the inductor together with the nominal load current,

input and output voltage of the application

determines

the

switching

frequency

the

converter. Depending on the application, inductor

values

between

2.2 H

and

47 H

are

recommended. The maximum inductor value is

determined by the maximum on time of the switch,

typically 6 s. The peak current limit of 400mA

(TYP) should be reached within this 6 s period for

proper operation. The inductor value determines

the maximum switching frequency of the converter.

Therefore, select the inductor value that ensures

the maximum switching frequency at the converter

maximum load current is not exceeded. The

maximum switching frequency is calculated by the

following formula:

Where:

IP = Peak current

L = Selected inductor value

VIN(MIN) = The highest switching frequency occurs

at the minimum input voltage

If the selected inductor value does not exceed the

maximum switching frequency of the converter,

the next step is to calculate the switching

frequency at the nominal load current using the

following formula:

Where:

IP = Peak current

L = Selected inductor value

ILOAD = Nominal load current

Vd = Rectifier diode forward voltage (typically

0.3V)

A smaller inductor value gives a higher converter

switching frequency, but lowers the efficiency.

The inductor value has less effect on the

maximum available load current and is only of

secondary order.

The best way to calculate the maximum available

load current under certain operating conditions is

to estimate the expected converter efficiency at

the maximum load current. The maximum load

current can then be estimated as follows:

Where:

IP = Peak current

L = Selected inductor value

fS(MAX)

Maximum

switching

frequency

calculated previously

Expected converter efficiency. Typically 70% to

85%

The maximum load current of the converter is the

current at the operation point where the converter

starts to enter the continuous conduction mode.

Usually the converter should always operate in

discontinuous conduction mode.

Last,

the

selected

inductor

should

have

saturation current that meets the maximum peak

current of the converter.

Another important inductor parameter is the DC

resistance. The lower the DC resistance, the

higher the efficiency of the converter. See Table 1

and the typical applications for the inductor

selection.

Table 1. Recommended Inductor for Typical

LCD Bias Supply

INDUCTOR

COMPONENT

10 H

Sumida

CR32-100

High efficiency

10 H

Sumida

CDRH3D16-100

High efficiency

10 H

Murata

LQH4C100K04

High efficiency

4.7 H

Sumida

CDRH3D16-4R7

Small solution

size

4.7 H

Murata

LQH3C4R7M24

Small solution

size

LESHAN RADIO COMPANY, LTD.

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