Datasheet

L
L Lmax outm ax
Vout
1
I
Vin
I = Vout I = I +
L 2
-
D
D ´
´ ¦
RMSCout
Vout
1
1
Vin
I = Vout
L
2 3
-
´ ´
´ ¦
´
TPS65720
TPS65721
SLVS979 –OCTOBER 2009 www.ti.com
OUTPUT FILTER DESIGN (INDUCTOR AND OUTPUT CAPACITOR)
Inductor Selection
The converter operates typically with 3.3μH output inductor. Larger or smaller inductor values can be used to
optimize the performance of the device for specific operation conditions. The selected inductor has to be rated for
its DC resistance and saturation current. The DC resistance of the inductance will influence directly the efficiency
of the converter. Therefore an inductor with lowest DC resistance should be selected for highest efficiency.
Equation 3 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
(3)
With:
f = Switching Frequency (2.25MHz typical)
L = Inductor Value
ΔI
L
= Peak to Peak inductor ripple current
I
Lmax
= Maximum Inductor current
The highest inductor current will occur at maximum Vin.
Open core inductors have a soft saturation characteristic and they can usually handle higher inductor currents
versus a comparable shielded inductor.
A more conservative approach is to select the inductor current rating just for the maximum switch current of the
corresponding converter. It must be considered, that the core material from inductor to inductor differs and will
have an impact on the efficiency especially at high switching frequencies.
Refer to Table 1 and the typical applications for possible inductors.
Table 1. Tested Inductors
INDUCTOR TYPE INDUCTOR VALUE SUPPLIER Comments
LQM21P 3.3uH Murata For TPS65720
BRC1608T2R2M 2.2uH Taiyo Yuden For TPS65720; Smallest solution size;
up to 150 mA of output current
VLS201610ET-2R2M 2.2uH TDK For TPS65720, TPS65721
GLFR1608T2R2M-LR 2.2uH TDK For TPS65720; Smallest solution size;
up to 150 mA of output current
MIPSA2520 2.2uH FDK For TPS65721; highest efficiency
Output Capacitor Selection
The advanced Fast Response voltage mode control scheme of the step-down converter allows the use of small
ceramic capacitors with a typical value of 10μF, without having large output voltage under and overshoots during
heavy load transients. Ceramic capacitors having low ESR values result in lowest output voltage ripple and are
therefore recommended. For an inductor value of 3.3μH, an output capacitor with 4.7μF can be used. Refer to
recommended components.
If ceramic output capacitors are used, the capacitor RMS ripple current rating will always meet the application
requirements. Just for completeness the RMS ripple current is calculated as:
(4)
At nominal load current the inductive converters operate in PWM mode and the overall output voltage ripple is
the sum of the voltage spike caused by the output capacitor ESR plus the voltage ripple caused by charging and
discharging the output capacitor:
44 Submit Documentation Feedback Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s): TPS65720 TPS65721