TPS54560
SLVSBN0 –MARCH 2013
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L1
7.2uH
VOUT
5V, 5A
0.1uF
C4
C6
47uF
C7
C9
U1
TPS54560DDA
D1
47uF
47uF
B560C
8
7
6
5
1
2
3
4
R5
53.6k
BOOT
SW
GND
COMP
FB
VIN
7V to 60V
VIN
EN
C10
C3
C1
C2
R1
442k
FB
FB
RT/CLK
2.2uF
2.2uF
2.2uF
2.2uF
R4
16.9k
9
C8
47pF
R6
10.2k
R2
90.9k
R3
243k
C5
4700pF
Figure 35. 5 V Output TPS54560 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 inductor value is calculated to be 7.6 μH. The nearest standard value
is 7.2 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 5 A and the peak inductor current is 5.8 A. The chosen inductor is a WE 7447798720, which
has a saturation current rating of 7.9 A and an RMS current rating of 6 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 TPS54560 which is nominally 7.5 A.
22
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