Datasheet

2
-
0.35
IN S IN
S
out S
V V V
L
V I f
h
æ ö
æ ö
æ ö
= × ×
ç ÷
ç ÷
ç ÷
×
è ø
è ø
è ø
(5)
Rectifier Diode Selection
avg out
I I=
(6)
D avg forward
P I V= ×
(7)
TPS61086
www.ti.com ................................................................................................................................................................................................ SLVSA05 – AUGUST 2009
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. 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 TPS61086, inductor values between 3 µ H and 6 µ H are a good choice. Possible inductors are shown in
Table 1 .
Typically, it is recommended that the inductor current ripple is below 35% of the average inductor current. The
following equation can therefore be used to calculate the inductor value, L:
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)
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 I
sat
(A)
CODE
( µ H) (L × W × H mm) (m Ω )
3.3 Sumida CDH38D09 4 x 4 x 1 240 1.25
4.7 Sumida CDPH36D13 5 x 5 x 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 W ü rth Elektronik 7447785004 5.9 x 6.2 x 3.3 60 2.5
5 Coilcraft MSS7341 7.3 x 7.3 x 4.1 24 2.9
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
:
Usually a Schottky diode with 1A maximum average rectified forward current rating is sufficient for most
applications. The Schottky rectifier can be selected with lower forward current capability depending on the output
current I
out
but has to be able to dissipate the power. The dissipated power, P
D
, is the average rectified forward
current times the diode forward voltage, V
forward
.
Typically the diode should be able to dissipate around 500mW depending on the load current and forward
voltage.
Table 2. Rectifier Diode Selection
CURRENT COMPONENT PACKAGE
V
r
V
forward
/I
avg
SUPPLIER
RATING I
avg
CODE TYPE
750 mA 20 V 0.425 V / 1 A Fairchild Semiconductor FYV0704S SOT 23
1 A 20 V 0.39 V / 1 A NXP PMEG2010AEH SOD 123
1 A 20 V 0.5 V / 1 A Vishay Semiconductor SS12 SMA
1 A 20 V 0.44 V / 1 A Vishay Semiconductor MSS1P2L µ -SMP
2 A 20 V 0.44 V / 2 A Vishay Semiconductor SL22 SMB
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