14
X
6
6
7
1
1
C
2
F
R
(R || R )
=
´
´ p ´
´
X
OUT
OUT
4.35
f
V
C
=
´
FB
7
6
OUT
FB
V
R
R
V
V
=
´
-
(
)
sw
OUT
1
D
1
ESR
0.5
F
C
r
-
æ
ö
<
´
+
ç
÷
´
è
ø
(
)
(
)
(
) (
)
(
)
OUT
sw
OUT
OUT
1
r
C
1
1
D
1
D
1
r
F
r
V
/
I
12
æ
ö
æ
ö
>
´
´
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´ ´ D
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System Overview
25
JAJU324B – March 2015 – Revised July 2017
翻訳版
—
最新の英語版資料
http://www-s.ti.com/sc/techlit/TIDU832
Copyright © 2015–2017, Texas Instruments Incorporated
EMI/EMC
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The maximum desired output voltage ripple and the transient response to load changes are the
parameters to take into account for selecting the value of the output capacitors.
(5)
with
•
Δ
I
OUT
equal load current (2 A)
•
Δ
V
OUT
equal target output voltage undershoot (0.5 V)
•
r the inductor ripple ratio (
Δ
I
L
/I
OUT
)
indicates that C
OUT
should be higher than 10.0
μ
F. A general guideline would be to pick C
OUT
between
the minimum required output capacitance, calculated in
, and multiply this minimum required
capacitance by 10. Two 10.0
μ
F in parallel were chosen to fit the C
OUT
requirements.
The second requirement is the maximum ESR of the capacitor, which can be found using
(6)
calculates the maximum ESR of the output capacitor to meet the maximum output ripple required.
The equivalent ESR of the output capacitors C
3
and C
4
should be lower than 0.288
Ω
.
The LM46002 requires high quality ceramic (X5R or X7R) input decoupling capacitors. The recommended
value is between 4.7 to 10
μ
F. A good practice for ceramic capacitors is to choose a voltage rating of
twice the maximum input voltage. If the placement of the LM46002 additional bulk capacitance is needed,
this additional capacitor dampens voltage spikes caused by the lead inductance of the trace. The value is
not critical but it must be rated to fit the voltage requirements.
The output voltage is externally adjusted with a resistor divider network. The divider network is comprised
of top feedback resistor R
6
and bottom feedback resistor R
7
.
is used to determine the output voltage
of the converter.
(7)
R
6
is chosen to be 1 M
Ω
to minimize quiescent current, which improves light load efficiency in this
application. With the desired output voltage set to be 5 V and the V
FB
is equal to 1.01 V, the R
7
value can
then be calculated using
. The equation gives the value 253.4
Ω
. The chosen value for this design is a
249-k
Ω
resistor. For more details on the adjustable output voltage, see the LM46002 data sheet
.
The LM46002 is internally compensated and the internal R-C values are 400 k
Ω
and 50 pF, respectively.
Depending on the V
OUT
and the frequency F
S
, if the output capacitor C
OUT
is dominated by low ESR
(ceramic types) capacitors, it could result in low-phase margin. To improve the phase boost, an external
feed forward capacitor C
14
can be added in parallel with R
6
. C
14
is chosen such that the phase margin is
boosted at the crossover frequency without C
14
. A simple estimation for the crossover frequency without
C
14
(F
X
) is shown in
(8)
The following equation for C
14
was tested:
(9)
