OUT

REF

V

R1

1

R2

V

§

·

u

¨

¸

©

¹

(5)

where

• VOUT is the regulated output voltage

• VREF is the internal reference voltage at the FB pin

For the best accuracy, should be kept R2 smaller than 300 kΩ to ensure the current flowing through R2 is

at least 100 times larger than the FB pin leakage current. Changing R2 towards a lower value increases the

immunity against noise injection. Changing the R2 towards a higher value reduces the quiescent current for

achieving highest efficiency at low load currents.

9.2.2.2 Inductor Selection

Because the selection of the inductor affects steady-state operation, transient behavior, and loop stability.

The inductor is the most important component in power regulator design. There are three important inductor

specifications, inductor value, saturation current, and dc resistance (DCR).

The TPS61033 is designed to work with inductor values between 0.37 µH and 2.9 µH. Follow Equation 6 to

Equation 8 to calculate the inductor peak current for the application. To calculate the current in the worst case,

use the minimum input voltage, maximum output voltage, and maximum load current of the application. To have

enough design margins, choose the inductor value with –30% tolerances, and low power-conversion efficiency

for the calculation.

In a boost regulator, the inductor dc current can be calculated by Equation 6.

OUT

OUT

L DC

IN

V

I

I

V

u

u K

(6)

where

• VOUT is the output voltage of the boost converter

• IOUT is the output current of the boost converter

• VIN is the input voltage of the boost converter

• η is the power conversion efficiency, use 90% for most applications

The inductor ripple current is calculated by Equation 7.

IN

L P P

SW

V

D

I

L

f

u

'

u

(7)

where

• D is the duty cycle, which can be calculated by Equation 3

• L is the inductance value of the inductor

• fSW is the switching frequency

• VIN is the input voltage of the boost converter

Therefore, the inductor peak current is calculated by Equation 8.

L P P

L P

L DC

I

I

I

2

'

(8)

Normally, it is advisable to work with an inductor peak-to-peak current of less than 40% of the average inductor

current for maximum output current. A smaller ripple from a larger valued inductor reduces the magnetic

hysteresis losses in the inductor and EMI. But in the same way, load transient response time is increased. The

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TPS61033

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