Datasheet

2
-
IN S IN
S
OUT S
V V V
L
V I f 0.35
h
æ ö
æ ö
æ ö
= × ×
ç ÷
ç ÷
ç ÷
×
è ø
è ø
è ø
avg OUT
I I=
TPS61085
SLVS859A –JUNE 2008–REVISED APRIL 2012 www.ti.com
Inductor Selection
The TPS61085 is designed to work with a wide range of inductors. The main parameter for the inductor selection
is the saturation current of the inductor which should be higher than the peak switch current as calculated in the
Design Procedure section with additional margin to cover for heavy load transients. An alternative, more
conservative, is to choose an inductor with a saturation current at least as high as the maximum switch current
limit of 3.2 A. The other important parameter is the inductor dc resistance. Usually, the lower the dc resistance
the higher the efficiency. It is important to note that the inductor dc resistance is not the only parameter
determining the efficiency. Especially for a boost converter where the inductor is the energy storage element, the
type and core material of the inductor influences the efficiency as well. At high switching frequencies of 1.2 MHz
inductor core losses, proximity effects and skin effects become more important. Usually, an inductor with a larger
form factor gives higher efficiency. The efficiency difference between different inductors can vary between 2% to
10%. For the TPS61085, inductor values between 3 μH and 6 μH are a good choice with a switching frequency
of 1.2 MHz, typically 3.3 μH. At 650 kHz inductors between 6 μH and 13 μH, typically 6.8 μH are recommended.
Possible inductors are shown in Table 1.
Typically, it is recommended that the inductor current ripple is below 35% of the average inductor current.
Therefore, the following equation can be used to calculate the inductor value, L:
(5)
with
V
IN
Minimum input voltage
V
S
Output voltage
I
out
Maximum output current in the application
f
S
Converter switching frequency (typically 1.2 MHz or 650 kHz)
η Estimated converter efficiency (please use the number from the efficiency plots or 90% as an estimation)
Table 1. Inductor Selection
L COMPONENT SIZE DCR TYP
SUPPLIER Isat (A)
CODE
(μH) (L×W×H mm) (mΩ)
1.2 MHz
3.3 Sumida CDH38D09 4 x 4 x 1 240 1.25
4.7 Sumida CDPH36D13 5 × 5 × 1.5 155 1.36
3.3 Sumida CDPH4D19F 5.2 x 5.2 x 2 33 1.5
3.3 Sumida CDRH6D12 6.7 x 6.7 x 1.5 62 2.2
4.7 Würth Elektronik 7447785004 5.9 × 6.2 × 3.3 60 2.5
5 Coilcraft MSS7341 7.3 × 7.3 × 4.1 24 2.9
650 kHz
6.8 Sumida CDP14D19 5.2 x 5.2 x 2 50 1
10 Coilcraft LPS4414 4.3 × 4.3 × 1.4 380 1.2
6.8 Sumida CDRH6D12/LD 6.7 x 6.7 x 1.5 95 1.25
10 Sumida CDR6D23 5 × 5 × 2.4 133 1.75
10 Würth Elektronik 744778910 7.3 × 7.3 × 3.2 51 2.2
6.8 Sumida CDRH6D26HP 7 x 7 x 2.8 52 2.9
Rectifier Diode Selection
To achieve high efficiency, a Schottky type should be used for the rectifier diode. The reverse voltage rating
should be higher than the maximum output voltage of the converter. The averaged rectified forward current I
avg
,
the Schottky diode needs to be rated for, is equal to the output current I
OUT
:
(6)
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