TPS62290DRVT datasheet(15/25 Pages) TI

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OUTPUT VOLTAGE SETTING

OUT + VREF

1 )

2 with an internal reference voltage V

REF typical 0.6V.

OUTPUT FILTER DESIGN (INDUCTOR AND OUTPUT CAPACITOR)

Inductor Selection

DIL + Vout

1 *

Vout

Vin

(1)

Lmax + Ioutmax )

DIL

(2)

TPS62290, TPS62291, TPS62293

SLVS764C – JUNE 2007 – REVISED MARCH 2008

The output voltage can be calculated to:

To minimize the current through the feedback divider network, R2 should be 180 kΩ or 360 kΩ. The sum of R1

and R2 should not exceed ~1MΩ, to keep the network robust against noise. An external feed forward capacitor

C1 is required for optimum load transient response. The value of C1 should be in the range between 22pF and

33pF.

Route the FB line away from noise sources, such as the inductor or the SW line.

The TPS6229x is designed to operate with inductors in the range of 1.5

µH to 4.7µH and with output capacitors in

the range of 4.7

µF to 22µF. The part is optimized for operation with a 2.2µH inductor and 10µF output capacitor.

Larger or smaller inductor values can be used to optimize the performance of the device for specific operation

conditions. For stable operation, the L and C values of the output filter may not fall below 1

µH effective

inductance and 3.5

µF effective capacitance.

The inductor value has a direct effect on the ripple current. The selected inductor has to be rated for its dc

resistance and saturation current. The inductor ripple current (

ΔIL) decreases with higher inductance and

increases with higher VI or VO.

The inductor selection has also impact on the output voltage ripple in PFM mode. Higher inductor values will lead

to lower output voltage ripple and higher PFM frequency, lower inductor values will lead to a higher output

voltage ripple but lower PFM frequency.

Equation 1 calculates the maximum inductor current under static load conditions. The saturation current of the

inductor should be rated higher than the maximum inductor current as calculated with Equation 2. This is

recommended because during heavy load transient the inductor current will rise above the calculated value.

With:

f = Switching Frequency (2.25MHz typical)

L = Inductor Value

ΔIL = Peak to Peak inductor ripple current

ILmax = Maximum Inductor current

A more conservative approach is to select the inductor current rating just for the maximum switch current of the

corresponding converter.

Accepting larger values of ripple current allows the use of low inductance values, but results in higher output

voltage ripple, greater core losses, and lower output current capability.

The total losses of the coil have a strong impact on the efficiency of the dc/dc conversion and consist of both the

losses in the dc resistance (R(DC)) and the following frequency-dependent components:

• The losses in the core material (magnetic hysteresis loss, especially at high switching frequencies)

• Additional losses in the conductor from the skin effect (current displacement at high frequencies)

• Magnetic field losses of the neighboring windings (proximity effect)

• Radiation losses

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