Datasheet

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OUTPUT VOLTAGE SETTING
V
OUT
+ V
REF
ǒ
1 )
R
1
R
2
Ǔ
with an internal reference voltage V
REF
typical 0.6V.
OUTPUT FILTER DESIGN (INDUCTOR AND OUTPUT CAPACITOR)
Inductor Selection
DI
L
+ Vout
1 *
Vout
Vin
L ƒ
(1)
I
Lmax
+ I
outmax
)
DI
L
2
(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, R
2
should be 180 k Ω or 360 k Ω . The sum of R
1
and R
2
should not exceed ~1M Ω , to keep the network robust against noise. An external feed forward capacitor
C
1
is required for optimum load transient response. The value of C
1
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 µ H to 4.7 µ H and with output capacitors in
the range of 4.7 µ F to 22 µ F. The part is optimized for operation with a 2.2 µ H inductor and 10 µ F 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 µ H effective
inductance and 3.5 µ F 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 ( Δ I
L
) decreases with higher inductance and
increases with higher V
I
or V
O
.
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
Δ I
L
= Peak to Peak inductor ripple current
I
Lmax
= 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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