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APPLICATION INFORMATION

BOOST CONVERTER DESIGN PROCEDURE

D +

Vout ) VD * Vin

Vout ) VD * Vsw

+

10 V ) 0.8 V * 3.3 V

10 V ) 0.8 V * 0.5 V

+ 0.73

I

L +

Iout

1 * D

+

300 mA

1 * 0.73

+ 1.11 A

Di

L +

V

in *

Vsw

D

fs

L

+

(3.3 V * 0.5 V)

0.73

1.6 MHz

4.2 mH

+ 304 mA

I

swpeak +

I

L )

DiL

2

+ 1.11 A )

304 mA

2

+ 1.26 A

Inductor Selection

TPS65140

TPS65145

SLVS497B – SEPTEMBER 2003 – REVISED MARCH 2004

The first step in the design procedure is to calculate the maximum possible output current of the main boost

converter under certain input and output voltage conditions. Below is an example for a 3.3-V to 10-V conversion:

1. Duty cycle:

2. Average inductor current:

3. Inductor peak-to-peak ripple current:

4. Peak switch current:

The integrated switch, the inductor, and the external Schottky diode must be able to handle the peak switch

current. The calculated peak switch current has to be equal or lower to the minimum N-MOSFET switch current

limit as specified in the electrical characteristics table (1.6 A for the TPS65140 and 0.96 A for the TPS65145). If

the peak switch current is higher, then the converter cannot support the required load current. This calculation

must be done for the minimum input voltage where the peak switch current is highest. The calculation includes

losses, inductor core and winding losses, etc., require a slightly higher peak switch current in the actual

application. The above calculation still allows for a good design and component selection.

Several inductors work with the TPS65140. Especially with the external compensation, the performance can be

adjusted to the specific application requirements. The main parameter for the inductor selection is the saturation

current of the inductor which should be higher than the peak switch current as calculated above with additional

margin to cover for heavy load transients and extreme start-up conditions. Another method is to choose the

inductor with a saturation current at least as high as the minimum switch current limit of 1.6 A for the TPS65140

and 0.96 A for the TPS65145. The different switch current limits allow selection of a physically smaller inductor

when less output current is required. The second important parameter is the inductor dc resistance. Usually, the

lower the dc resistance, the higher the efficiency. However, the inductor dc resistance is not the only parameter

determining the efficiency. Especially for a boost converter where the inductor is the energy storage element, the

type and material of the inductor influences the efficiency as well. Especially at high switching frequencies of 1.6

MHz, inductor core losses, proximity effects, and skin effects become more important. Usually, an inductor with a

larger form factor yields higher efficiency. The efficiency difference between different inductors can vary between

2% to 10%. For the TPS65140, inductor values between 3.3 µH and 6.8 µH are a good choice but other values

can be used as well. Possible inductors are shown in Table 1.

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