Datasheet
h
g g
OUT
L(DC) OUT
IN
V
1
I = I
V
OUT
IN IN
L(PEAK)
OUT
I
V D V
I = + with D 1
2 f L (1 D) V
h
= -
- h
g g
g g g
TPS61253, TPS61254, TPS61256
TPS61258, TPS61259
SLVSAG8C –SEPTEMBER 2011–REVISED AUGUST 2012
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APPLICATION INFORMATION
INDUCTOR SELECTION
A boost converter normally requires two main passive components for storing energy during the conversion, an
inductor and an output capacitor are required. It is advisable to select an inductor with a saturation current rating
higher than the possible peak current flowing through the power switches.
The inductor peak current varies as a function of the load, the input and output voltages and can be estimated
using Equation 4.
(4)
Selecting an inductor with insufficient saturation performance can lead to excessive peak current in the
converter. This could eventually harm the device and reduce it's reliability.
When selecting the inductor, as well as the inductance, parameters of importance are: maximum current rating,
series resistance, and operating temperature. The inductor DC current rating should be greater (by some margin)
than the maximum input average current, refer to Equation 5 and CURRENT LIMIT OPERATION section for
more details.
(5)
The TPS6125x series of step-up converters have been optimized to operate with a effective inductance in the
range of 0.7µH to 2.9µH and with output capacitors in the range of 10µF to 47µF. The internal compensation is
optimized for an output filter of L = 1µH and C
O
= 10µF. Larger or smaller inductor values can be used to
optimize the performance of the device for specific operating conditions. For more details, see the CHECKING
LOOP STABILITY section.
In high-frequency converter applications, the efficiency is essentially affected by the inductor AC resistance (i.e.
quality factor) and to a smaller extent by the inductor DCR value. To achieve high efficiency operation, care
should be taken in selecting inductors featuring a quality factor above 25 at the switching frequency. Increasing
the inductor value produces lower RMS currents, but degrades transient response. For a given physical inductor
size, increased inductance usually results in an inductor with lower saturation current.
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 TPS6125x converters.
Table 4. List of Inductors
MANUFACTURER SERIES DIMENSIONS (in mm)
HITACHI METALS KSLI-322512BL1-1R0 3.2 x 2.5 x 1.2 max. height
LQM32PN1R0MG0 3.2 x 2.5 x 1.0 max. height
MURATA
LQM2HPN1R0MG0 2.5 x 2.0 x 1.0 max. height
TOKO DFE322512C-1R0 3.2 x 2.5 x 1.2 max. height
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