Datasheet

( )
IN OUT
ON
V V
I = T
L
-
D ´
L
L
Lmax outmax
ΔI
I = I +
2
TPS62120
TPS62122
SLVSAD5 –JULY 2010 www.ti.com
INDUCTOR SELECTION
The inductor value affects its peak-to-peak ripple current, the PWM-to-PFM transition point, the output voltage
ripple and the efficiency. 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
and can be
estimated according Equation 8.
Equation 9 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 9. This is
recommended because during heavy load transient the inductor current will rise above the calculated value. A
more conservative way is to select the inductor saturation current according to the high-side MOSFET switch
current limit I
LIMF
.
(8)
(9)
With:
T
ON
= see equation (3)
L = Inductor Value
ΔI
L
= Peak to Peak inductor ripple current
I
Lmax
= Maximum Inductor current
In DC/DC converter applications, the efficiency is essentially affected by the inductor AC resistance (i.e. quality
factor) and by the inductor DCR value. To achieve high efficiency operation, care should be taken in selecting
inductors featuring a quality factor above 25 at the switching frequency. Increasing the inductor value produces
lower RMS currents, but degrades transient response. For a given physical inductor size, increased inductance
usually results in an inductor with lower saturation current.
The total losses of the coil 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
The following inductor series from different suppliers have been used with the TPS6212X converters.
Table 2. List of Inductors
INDUCTANCE [µH] DIMENSIONS [mm
3
] INDUCTOR TYPE SUPPLIER
22 3 × 3 × 1.5 LQH3NPN Murata
18/22 3 × 3 × 1.5 LPS3015 Coilcraft
OUTPUT CAPACITOR SELECTION
The unique hysteretic PFM/PWM control scheme of the TPS6212x allows the use of ceramic capacitors.
Ceramic capacitors with low ESR values have the lowest output voltage ripple and are recommended. The
output capacitor requires either an X7R or X5R dielectric. Y5V and Z5U dielectric capacitors, aside from their
wide variation in capacitance over temperature, become resistive at high frequencies.
At light load currents the converter operates in Power Save Mode and the output voltage ripple is dependent on
the output capacitor value and the PFM peak inductor current. Higher output capacitor values minimize the
voltage ripple in PFM Mode and tighten DC output accuracy in PFM Mode.
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