TPS54360
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SLVSBB4C –AUGUST 2012–REVISED OCTOBER 2012
L1
8.2uH
VOUT
0.1uF
C4
5.0V, 3.5A
C6
C7
U1
TPS54360DDA
D1
47uF
47uF
B560C
1
2
3
4
8
7
6
5
R5
53.6k
BOOT
SW
GND
COMP
FB
8.5V to 60V
VIN
VIN
EN
C1
C2
GND
FB
R1
523k
FB
RT/CLK
2.2uF
2.2uF
R4
13.0k
9
C8
39pF
R6
10.2k
R2
84.5k
R3
162k
GND
C5
GND
6800pF
GND
GND
Figure 33. 5 V Output TPS54360 Design Example.
Output Inductor Selection (LO)
To calculate the minimum value of the output inductor, use Equation 26.
KIND is a ratio that represents the amount of inductor ripple current relative to the maximum output current. The
inductor ripple current is filtered by the output capacitor. Therefore, choosing high inductor ripple currents
impacts 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 is at the discretion of the designer,
however, the following guidelines may be used.
For designs using low ESR output capacitors such as ceramics, a value as high as KIND = 0.3 may be desirable.
When using higher ESR output capacitors, KIND = 0.2 yields better results. Since the inductor ripple current is
part of the current mode PWM control system, the inductor ripple current should always be greater than 150 mA
for stable PWM operation. In a wide input voltage regulator, it is best to choose relatively large inductor ripple
current. This provides sufficienct ripple current with the input voltage at the minimum.
For this design example, KIND = 0.3 and the minimum inductor value is calculated to be 7.3 μH. The nearest
standard value is 8.2 μH. It is important that the RMS current and saturation current ratings of the inductor not be
exceeded. The RMS and peak inductor current can be found from Equation 28 and Equation 29. For this design,
the RMS inductor current is 3.5 A and the peak inductor current is 3.97 A. The chosen inductor is a WE
7447797820, which has a saturation current rating of 5.8 A and an RMS current rating of 5.05 A.
As the equation set demonstrates, lower ripple currents will reduce the output voltage ripple of the regulator but
will require a larger value of inductance. Selecting higher ripple currents will increase the output voltage ripple of
the regulator but allow for a lower inductance value.
The current flowing through the inductor is the inductor ripple current plus the output current. During power up,
faults or transient load conditions, the inductor current can increase above the peak inductor current level
calculated above. In transient conditions, the inductor current can increase up to the switch current limit of the
device. For this reason, the most conservative design approach is to choose an inductor with a saturation current
rating equal to or greater than the switch current limit of the TPS54360 which is nominally 5.5 A.
Copyright © 2012, Texas Instruments Incorporated
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