Datasheet
f − Frequency − Hz
0
5
10
15
20
25
30
35
40
45
50
Gain − dB
Phase − °
Gain
Phase
100 1k 10k 1M100k
200
180
160
140
120
100
80
40
0
G027
20
60
Component Selection
LOUT +
V
OUT
V
IN(max)
V
IN(max)
* V
OUT
f
s
I
RIPPLE
(18)
I
LOUT_RMS
+ I
OUT
2
)
I
RIPPLE
2
12
Ǹ
+ 10.02 A
(19)
I
PK
+ I
OUT
)
I
RIPPLE
2
+ 11.03 A
(20)
TPS40077
www.ti.com
..................................................................................................................................................... SLUS714D – JANUARY 2007 – REVISED APRIL 2009
Figure 30. Bode Plot Showing 57 ° Phase Margin at Crossover Frequency of 54 kHz
Power Train Components
Output Inductor, LOUT
The output inductor is one of the most important components to select. It stores the energy necessary to keep
the output regulated when the switch FET is turned off. The value of the output inductor dictates the peak and
RMS currents in the converter. These currents are important when selecting other components. Equation (1) can
be used to calculate a value for LOUT for this module which operates at a switching frequency (f) of 300 kHz.
I
RIPPLE
is the allowable ripple in the inductor. Select I
RIPPLE
to be between 20% and 30% of maximum I
OUT
. For
this design, I
RIPPLE
of 2.5 A was selected. Calculated LOUT is 2.13 µ H. A standard inductor with value of 2.5 µ H
was chosen. This will reduce I
RIPPLE
by about 17% to 2.07 A.
This I
RIPPLE
value can be used calculate the rms and peak current flowing in LOUT. Note that this peak current is
also seen by the switching FET and synchronous rectifier.
The power loss from the selected inductor DCR is 357 mW. The ac core loss for this Coilcraft inductor may be
found from the Coilcraft Web site, where there is a loss calculator . The loss is 179 mW.
The inductor is selected with a saturation current higher than this current plus the current that is developed
charging the output capacitance during the soft-start interval.
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