Datasheet

L
Vout
1
Vin
I = Vout
L
-
D ´
´ ¦
L
Lmax outmax
I
I = I +
2
D
TPS62750, TPS62751
SLVS955A –JULY 2009–REVISED SEPTEMBER 2009 ................................................................................................................................................. www.ti.com
INDUCTOR SELECTION
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 (ΔI
L
) decreases with higher inductance and
increases with higher V
IN
or V
OUT
.
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 2 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 3. This is
recommended because during heavy load transient the inductor current will rise above the calculated value.
(2)
(3)
With:
f = Switching Frequency (2.25MHz typical)
L = Inductor Value
ΔI
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 device has
been optimized to operate with inductance values between 1.0μH and 4.7μH. It is recommended that inductance
values of at least 1.0μH is used, even if Equations 2 and 3 yield something lower.
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
Table 3. List of Inductors
MANUFACTURER INDUCTOR TYPE DIMENSIONS [mm]
Coilcraft LPS3015-222ML 3.0 x 3.0 x 1.5
TOKO 1127AS-2R2M 3.5 x 3.7 x 1.8
Murata LQH32PN1R0N0 3.2 x 2.5 x 1.7
TOKO DB3015 Series 3.2 x 3.2 x 1.5
INPUT CAPACITOR SELECTION
Because of the nature of the buck converter having a pulsating input current, a low ESR input capacitor is
required for best input voltage filtering and minimizing the interference with other circuits caused by high
input voltage spikes. For most applications a 4.7μF to 10μF ceramic capacitor is recommended. The input
capacitor can be increased without any limit for better input voltage filtering.
Take care when using only small ceramic input capacitors. When a ceramic capacitor is used at the input
and the power is being supplied through long wires, such as from a wall adapter, a load step at the output or
VIN step on the input can induce ringing at the VIN pin. This ringing can couple to the output and be
mistaken as loop instability or could even damage the part by exceeding the maximum ratings.
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