Datasheet
APPLICATION INFORMATION
Output Filter Design (Inductor and Output Capacitor)
ƒ
c
+
1
2p L C
O
Ǹ
+
1
2p 1 mH 10 mF
Ǹ
+ 50.3 kHz
(1)
high-load transients
ǒ
di
dt
Ǔ
(e.g., ≥ 1600 mA/ µ s).
Inductor Selection
DI
L
+
V
O
V
I
V
I
* V
O
L ƒ
sw
DI
L(MAX)
+ I
O(MAX)
)
DI
L
2
(2)
TPS62350, TPS62351
TPS62352, TPS62353
TPS62354, TPS62355, TPS62356
SLVS540E – MAY 2006 – REVISED APRIL 2008 ..............................................................................................................................................................
www.ti.com
The TPS6235x step-down converter has an internal loop compensation. Therefore, the external L-C filter must
be selected to work with the internal compensation.
The device has been designed to operate with inductance values between a minimum of 0.7 µ H and maximum of
6.2 µ H. The internal compensation is optimized to operate with an output filter of L = 1 µ H and C
O
= 10 µ F. Such
an output filter has its corner frequency at:
Selecting a larger output capacitor value (e.g., 22 µ F) is less critical because the corner frequency moves to
lower frequencies with fewer stability problems. The possible output filter combinations are listed in Table 4 .
Regardless of the inductance value, operation is recommended with 10- µ F output capacitor in applications with
Table 4. Output Filter Combinations
INDUCTANCE (L) OUTPUT CAPACITANCE (C
O
) OUTPUT CAPACITANCE (C
O
)
FOR STABLE LOOP OPERATION FOR OPTIMIZED TRANSIENT PERFORMANCE
1.0 µ H ≥ 10 µ F (ceramic capacitor) ≥ 10 µ F (ceramic capacitor)
2.2 µ H ≥ 4.7 µ F (ceramic capacitor) ≥ 22 µ F (ceramic capacitor)
The inductor value also has an impact on the pulse skipping operation. The transition into power-save mode
begins when the valley inductor current drops below a level set internally. Lower inductor values result in higher
ripple current which occurs at lower load currents. This results in a dip in efficiency at light load operations.
Even though the inductor does not influence the operating frequency, 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
.
where:
f
SW
= switching frequency (3 MHz typical)
L = inductor value
Δ I
L
= peak-to-peak inductor ripple current
I
L(MAX)
= maximum inductor current
Normally, it is advisable to operate with a ripple of less than 30% of the average output current. Accepting larger
values of ripple current allows the use of low inductances, but results in higher output voltage ripple, greater core
losses, and lower output current capability.
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 TPS62350 converters.
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Product Folder Link(s): TPS62350, TPS62351 TPS62352, TPS62353 TPS62354, TPS62355, TPS62356