Inductor Selection/Efficiency/Line-Transient Response
Output Capacitor Selection
Setting the Negative Output Voltage OUTN
4.6 V
R1 =
464 k
10 A
W
SLVS831A – APRIL 2008 – REVISED JULY 2008 ............................................................................................................................................................. www.ti.com
Table 1. Input Capacitor Selection
CAPACITOR
COMPONENT SUPPLIER
SIZE
10
F/10 V
Taiyo Yuden LMK212BJ106
0805
10
F/6.3 V
Taiyo Yuden JMK107BJ106
0603
The device is internally compensated and provides stable operation with either a 4.7-
H or 2.2-H inductor. For
this type of converter, the inductor selection is a key element in the design process, because it has an impact on
several application parameters. The inductor selection influences the converter efficiency, line transient
response, and maximum output current. Because the inductor ripple current is fairly large in this type of
application, the inductor has a major impact on the overall converter efficiency. Having large inductor ripple
current causes the inductor core and magnetizing losses to become dominant. Due to this, an inductor with a
larger dc winding resistance can possibly achieve higher converter efficiencies when having lower core and
magnetizing losses. Therefore, minimizing inductor ripple current also increases the overall converter efficiency.
A 4.7-
H inductor achieves a higher efficiency compared to a 2.2-H inductor, due to lower inductor ripple
current. The inductor value also influences the line transient regulation. This is because the inductor value
influences the current range entering continuous conduction mode (CCM). As discussed, the line transient
performance decreases when entering CCM. The larger the inductor value, the lower the load current when
entering CCM. The formula to calculate the current entering CCM is shown in
Equation 1. The inductors listed in
Table 2 achieve a good overall converter efficiency while having a low device profile of just 0,8 mm. The inductor
saturation current should be 900 mA, depending on the maximum output current of the application. See
Equation 2, where the converter switch current limit is calculated. The converter switch current is equal to the
peak inductor current.
Table 2. Inductor Selection
INDUCTOR VALUE
COMPONENT SUPPLIER
DIMENSIONS in mm
Isat/DCR
2.2
H
Coilcraft LPS3008-222
2,95 × 2,95 × 0,8
1.1 A/175 m
2.2
H
TOKO FDSE0312-2R2
3,3 × 3,3 × 1,2
1.2 A/160 m
2.2
H
ABCO LPF3010T-2R2
2,8 × 2,8 × 1
1.0A/100 m
2.2
H
Maruwa CXFU0208-2R2
2,65 × 2,65 × 0,8
0.85A/185 m
4.7
H
Maruwa CXFU0208-4R7
2,65 × 2,65 × 0,8
0.51A/440 m
4.7
H
Coilcraft LPS3008-472
2,95 × 2,95 × 0,8
0.8 A/350 m
4.7
H
ABCO LPF3010T-4R7
2,8 × 2,8 × 1
0.7A/280 m
A 4.7-
F output capacitor is generally sufficient for most applications, but larger values can be used as well for
improved line-transient response at higher load currents. The capacitor of
Table 3 is recommended for use with
the TPS65136.
Table 3. Output Capacitor Selection
CAPACITOR
COMPONENT SUPPLIER
SIZE
4.7
F/10V
Taiyo Yuden LMK107BJ475
0603
For highest output-voltage accuracy, the TPS65136 has an internally fixed output voltage for the positive output.
The negative output voltage is adjustable. Because the feedback FBG is regulated to ground, the voltage across
R1 is equal to the positive output voltage of 4.6 V. R1 is selected to have at least 10
A through the feedback
divider.
R2 is then calculated as:
12
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