Design Procedure

1

IN

S

V

D

V

h

×

=

-

(1)

(

)

(max)

(min)

1

2

L

out

LIM

I

I

I

D

D

æ

ö

=

-

× -

ç

÷

è

ø

(2)

D

I

I

I

out

L

swpeak

-

+

D

=

1

2

(3)

IN

L

S

V

D

I

f

L

×

D

=

×

(4)

Soft-start

Inductor Selection

TPS61086

The first step in the design procedure is to verify that the maximum possible output current of the boost converter

supports the specific application requirements. A simple approach is to estimate the converter efficiency, by

taking the efficiency numbers from the provided efficiency curves or to use a worst case assumption for the

expected efficiency, e.g. 90%.

1. Duty cycle, D:

with the inductor peak-to-peak ripple current,

and

Minimum input voltage

Output voltage

Converter switch current limit (minimum switch current limit = 2.0 A)

Converter switching frequency (typically 1.2 MHz)

L

Selected inductor value

η

Estimated converter efficiency (please use the number from the efficiency plots or 90% as an estimation)

The peak switch current is the steady state peak switch current that the integrated switch, inductor and external

Schottky diode has to be able to handle. The calculation must be done for the minimum input voltage where the

peak switch current is the highest.

The boost converter has an adjustable soft-start to prevent high inrush current during start-up. To minimize the

inrush current during start-up an external capacitor, connected to the soft-start pin SS and charged with a

constant current, is used to slowly ramp up the internal current limit of the boost converter. When the EN pin is

constant current of 10

µA typically until the output of the boost converter V

available after the soft-start is completed. The larger the capacitor the slower the ramp of the current limit and the

longer the soft-start time. A 100 nF capacitor is usually sufficient for most of the applications. When the EN pin is

pulled low, the soft-start capacitor is discharged to ground.

The TPS61086 is designed to work with a wide range of inductors. 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 in the

Design Procedure section with additional margin to cover for heavy load transients. An alternative, more

conservative, is to choose an inductor with a saturation current at least as high as the maximum switch current

limit of 3.2 A. The other important parameter is the inductor DC resistance. Usually the lower the DC resistance

the higher the efficiency. It is important to note that the inductor DC resistance is not the only parameter

10

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