TPS43060
TPS43061
SLVSBP4A –DECEMBER 2012–REVISED DECEMBER 2012
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For this design example, Equation 15 produces the estimated maximum input current (IIN) of 5 A. In reality this
will be higher because the simplified equations do not include the efficiency losses of the power supply. Using
KIND = 0.3 with Equation 16, the minimum inductor value is calculated to be 3.33µH. The nearest standard value
of 3.3 µH is chosen. 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 18 and Equation 19, respectively.
The calculated RMS inductor current is 5.0A and the peak inductor current is 5.73 A. The chosen inductor is a
Vishay IHLP2525CZER3R3M1 which has an RMS current rating of 6 A, a saturation current rating of 10 A and
30 mΩ DCR.
IOUT
2A
IIN
=
=
) (
= 5A
1- Dmax
1- 60%
(
)
(15)
(16)
(17)
VOUT
1
15V
1
L ³
L ³
´
=
´
IIN ´ KIND 4´ fSW 5A´ 0.3 4´750kHz
= 3.33mH
VIN
D
´
IIN ´ KIND fSW
ö2
÷
÷
ø
æ
ö2
æ
ö2
æ
ç
ç
è
IOUT
V
IN min´ Dmax
2A
1- 60%
6V ´ 60%
æ
ö2
÷
ø
ILrms =
+
=
+
= 5A
ç
ç
÷
÷
ç
ç
è
÷
1- Dmax
12 ´ L ´ fSW
12 ´3.3mH ´750kHz
ø
è
è
ø
(18)
(19)
IOUT
V
IN min´ Dmax
2A
6V ´ 60%
IL peak =
+
=
+
1- 60% 2´3.3mH ´750kHz
= 5.73A
1- Dmax
2´ L ´ fSW
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 average input current. During
power up, load faults or transient load conditions the inductor current can increase above the peak inductor
current calculated above. The above equations also do not include the efficiency of the regulator. For this reason
a more conservative design approach is to choose an inductor with a saturation current rating greater than the
typical switch current limit set by the current sense resistor or the inductor DC resistance if lossless DCR sensing
is used.
SELECTING THE CURRENT SENSE RESISTOR
The external current sense resistor sets the cycle-by-cycle peak current limit. The peak current limit should be
set to assure the maximum load current can be supported at the minimum input voltage. The typical over current
threshold voltage (VCS) with respect to duty cycle is shown in Figure 20. In this design example, the typical
current limit threshold voltage at the 60% maximum duty cycle is 68 mV.
When selecting the current limit for the design, a 20% margin is recommended from the calculated peak current
limit in Equation 19 to allow for load and line transients and the efficiency loss of the design. The recommended
current sense resistance is calculated with Figure 20. In this example the minimum resistance is calculated at
9.89 mΩ and two 20 mΩ resistors in parallel are used. The sense resistors must be rated for the power
dissipation calculated in Equation 22. Using the maximum current limit threshold of 82 mV according to the
electrical specification table, the maximum power loss in the current sense resistor is 0.672 W. Two 0.5 W rated
sense resistors are used in parallel in this design.
VCS maxtyp = 68mV
(20)
VCS maxtyp
68mV
RCS
=
=
1.2´ IL peak 1.2´5.73A
= 9.89mW
= 0.672W
(21)
(VCS max max)2
(82mV )2
10mW
P
=
=
RCS
RCS
(22)
The 10 Ω series resistors R13 and R15 with the 100 pF capacitor C12 filter high frequency switching noise from
the ISNS pins.
18
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