Datasheet

L
Lmax outmax
I
I = I +
2
D
L
Vout
1
Vin
I = Vout
L
-
D ´
´ ¦
kHz
FH
f
C
72
)2.22.2(2
1
=
´´´
=
mmp
TPS62732, TPS62733
TPS62730
www.ti.com SLVSAC3C –MAY 2011–REVISED DECEMBER 2012
OUTPUT FILTER DESIGN (INDUCTOR AND OUTPUT CAPACITOR)
The TPS62730 is optimized to operate with effective inductance values in the range of 1.5μH to 3μH and with
effective output capacitance in the range of 1.0μF to 10μF. The internal compensation is optimized to operate
with an output filter of L = 2.2μH and C
OUT
= 2.2μF, which gives and LC output filter corner frequency of:
(3)
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
I N
or V
O UT
. Equation 4
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 5
(4)
(5)
With:
f = Switching Frequency
L = Inductor Value
ΔI
L
= Peak to Peak inductor ripple current
I
Lmax
= Maximum Inductor current
In high-frequency converter applications, the efficiency is essentially affected by the inductor AC resistance (i.e.,
quality factor) and to a smaller extent 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 TPS62730 converters.
Table 1. List of inductors
INDUCTANCE DIMENSIONS INDUCTOR TYPE SUPPLIER
[μH] [mm3]
2.2 2.0 × 1.2 × 1.0 LQM21PN2R2NGC Murata
2.2 2.0 × 1.2 × 1.0 MIPSZ2012 FDK
DC/DC OUTPUT CAPACITOR SELECTION
The DCS-Control™ scheme of the TPS62730 allows the use of tiny 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 operate in Power
Save Mode and the output voltage ripple is dependent on the output capacitor value and the PFM peak inductor
current.
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