Datasheet

OUT D INI
N
L
SW S W
SW
O UT D IN IN
O UT
IN est
(V + V V ) (1 D)V D
1
I = = =
L L
1 1
L +
V + V V V
P
RPL%
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est IN
S
W OUT
OUT D IN IN
η V
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TPS61175
SLVS892B –DECEMBER 2008–REVISED FEBRUARY 2012 www.ti.com
Unless otherwise stated, the design equations that follow assume that the converter is running in continuous
mode.
SELECTING THE INDUCTOR
The selection of the inductor affects steady state operation as well as transient behavior and loop stability. These
factors make it the most important component in power regulator design. There are three important inductor
specifications, inductor value, DC resistance and saturation current. Considering inductor value alone is not
enough.
Inductor values can have ±20% tolerance with no current bias. When the inductor current approaches saturation
level, its inductance can fall to some percentage of its 0-A value depending on how the inductor vendor defines
saturation current. For CCM operation, the rule of thumb is to choose the inductor so that its inductor ripple
current (ΔI
L
) is no more than a certain percentage (RPL% = 20–40%) of its average DC value (I
IN(AVG)
= I
L(AVG)
)
(4)
Rearranging and solving for L gives
(5)
Choosing the inductor ripple current to closer to 20% of the average inductor current results in a larger
inductance value, maximizes the converter’s potential output current and minimizes EMI. Choosing the inductor
ripple current closer to 40% of I
L(AVG)
results in a smaller inductance value, and a physically smaller inductor,
improves transient response but results in potentially higher EMI and lower efficiency if the DCR of the smaller
packaged inductor is significantly higher. Using an inductor with a smaller inductance value than computed
above may result in the converter operating in DCM. This reduces the boost converter’s maximum output current,
causes larger input voltage and output ripple and typically reduces efficiency. Table 3 lists the recommended
inductor for the TPS61175.
Table 3. Recommended Inductors for TPS61175
L DCR MAX SATURATION CURRENT SIZE
PART NUMBER VENDOR
(μH) (mΩ) (A) (L × W × H mm)
D104C2 10 44 3.6 10.4x10.4x4.8 TOKO
VLF10040 15 42 3.1 10.0x9.7x4.0 TDK
CDRH105RNP 22 61 2.9 10.5x10.3x5.1 Sumida
MSS1038 15 50 3.8 10.0x10.2x3.8 Coilcraft
The device has built-in slope compensation to avoid subharmonic oscillation associated with current mode
control. If the inductor value is lower than 4.7μH, the slope compensation may not be adequate, and the loop can
be unstable. Applications requiring inductors above 47μH have not been evaluated. Therefore, the user is
responsible for verifying operation if they select an inductor that is outside the 4.7μH–47μH recommended range.
COMPUTING THE MAXIMUM OUTPUT CURRENT
The over-current limit for the integrated power FET limits the maximum input current and thus the maximum input
power for a given input voltage. Maximum output power is less than maximum input power due to power
conversion losses. Therefore, the current limit setting, input voltage, output voltage and efficiency can all change
the maximum current output (I
OUT(MAX)
). The current limit clamps the peak inductor current, therefore the ripple
has to be subtracted to derive maximum DC current.
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