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APPLICATION INFORMATION

INDUCTOR SELECTION, MAXIMUM LOAD CURRENT

fSmax +

Vin

min

(Vout–Vin)

I

P

L

Vout

(2)

fS I

load +

2

I

load

(Vout–Vin ) Vd)

P

L

(3)

I

load max +

h

P

L

fSmax

2

(Vout * Vin)

(4)

TPS61040

TPS61041

SLVS413B – OCTOBER 2002 – REVISED MAY 2004

Since 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 of the converter. Depending on the application, inductor values

between 2.2 µH up to 47 µH are recommended. The maximum inductor value is determined by the maximum on

time of the switch, typically 6 µs. The peak current limit of 400 mA/250mA (typically) should be reached within

this 6-µs 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:

L = Selected inductor value

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:

L = Selected inductor value

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. This number can be taken out of the efficiency

graphs shown in Figures 1, 2, 3, and 4. The maximum load current can then be estimated as follows:

Where:

L = Selected inductor value

η = Expected converter efficiency. Typically 70% to 85%

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