Datasheet

L
L IN
I
I peak I DC
2
D
= +
( )
L
L IN
2
2
I
I rms I DC
12
D
æ ö
= +
ç ÷
è ø
IN
L
O SW
V min
Dmax
I
L
D = ´
¦
( )
OUT D
O
IN IND SW
V V
1
L min
I DC K 4
+
³ ´
´ ´ ¦
, D=50%
IN
O
IN IND SW
V
D
L min
I DC K
³ ´
´ ¦
, D 50%, V with D closest to 50%≠
IN
OUT OUT
IN
EST IN
V I
I DC
V min
´
=
h ´
TPS55340
SLVSBD4B –MAY 2012–REVISED OCTOBER 2012 www.ti.com
The minimum value of the inductor should be able to meet inductor current ripple (ΔI
L
) requirement at worst
case. In a boost converter, maximum inductor current ripple occurs at 50% duty cycle. For the applications where
duty cycle is always smaller or larger than 50%, Equation 12 should be used with the duty cycle closest to 50%
and corresponding input voltage to calculate the minimum inductance. For applications that need to operate with
50% duty cycle when input voltage is somewhere between the minimum and the maximum input voltage,
Equation 13 should be used. K
IND
is a coefficient that represents the amount of inductor ripple current relative to
the maximum input current (I
IN
DC = I
L
avg). The maximum input current can be estimated with Equation 11, with
an estimated efficiency based on similar applications (η
EST
). The inductor ripple current will be filtered by the
output capacitor. Therefore, choosing high inductor ripple currents will impact the selection of the output
capacitor since the output capacitor must have a ripple current rating equal to or greater than the inductor ripple
current. In general, the inductor ripple value (K
IND
) is at the discretion of the designer. However, the following
guidelines may be used.
For CCM operation, it is recommended to use K
IND
values in the range of 0.2 to 0.4. Choosing K
IND
closer to 0.2
results in a larger inductance value, maximizes the converter’s potential output current and minimizes EMI.
Choosing K
IND
closer to 0.4 results in a smaller inductance value, a physically smaller inductor, and improved
transient response, but potentially worse EMI and lower efficiency. Using an inductor with a smaller inductance
value may result in the converter operating in DCM. This reduces the boost converter’s maximum output current,
causes larger input voltage and output voltage ripple and reduced efficiency. For this design, choose K
IND
= 0.3
and a conservative efficiency estimate of 85% with the minimum input voltage and maximum output current.
Equation 12 is used with the maximum input voltage because this corresponds to duty cycle closest to 50%. The
maximum input current is estimated at 4.52A and the minimum inductance is 7.53 µH. A standard value of 10 µH
is chosen.
(11)
(12)
(13)
After choosing the inductance, the required current ratings can be calculated. The inductor will be closest to its
ratings with the minimum input voltage. The ripple with the chosen inductance is calculated with Equation 14.
The RMS and peak inductor current can be found with Equation 15 and Equation 16. For this design the current
ripple is 663mA, the RMS inductor current is 4.52 A, and the peak inductor current is 4.85 A. It is generally
recommended for the peak inductor current rating of the selected inductor be 20% higher to account for
transients during power up, faults or transient load conditions. The most conservative approach is to specify an
inductor with a saturation current greater than the maximum peak current limit of the TPS55340. This helps to
avoid saturation of the inductor. The chosen inductor is a Würth Elektronik 74437368100. It has a saturation
current rating of 12.5 A, RMS current rating of 5.2 A, and typical DCR of 27.0mΩ.
(14)
(15)
(16)
The TPS55340 has built-in slope compensation to avoid sub-harmonic oscillation associated with current mode
control. If the inductor value is too small, the slope compensation may not be adequate, and the loop can be
unstable.
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