Texas Instruments LM5017 Isolated Bias Supply Evaluation Board LM5017ISOEVAL/NOPB LM5017ISOEVAL/NOPB 데이터 시트
제품 코드
LM5017ISOEVAL/NOPB
û
I
L(MAX)
I
LI
(peak) = I
OUT
+
2
= 602 mA
V
IN
- V
OUT
û
I
L
=
L1 x
g
SW
V
OUT
V
IN
x
V
OUT
g
SW
=
K x R
ON
D
MIN
g
SW(MAX)
=
T
ON(MIN)
10/95
100 ns
=
= 1.05 MHz
1 - D
MAX
g
SW(MAX)
=
T
OFF(MIN)
1 - 10/12.5
200 ns
=
= 1 MHz
SNVS783G – JANUARY 2012 – REVISED DECEMBER 2013
APPLICATION INFORMATION
Selection of External Components
Selection of external components is illustrated through a design example. The design example specifications are:
Buck Converter Design Specifications
Input voltage range
12.5 V to 95 V
Output voltage
10 V
Maximum Load current
500 mA
Switching Frequency
200 kHz
RFB1, RFB2:
V
OUT
= V
FB
x (R
FB2
/R
FB1
+ 1), and since V
FB
= 1.225 V, the ratio of R
FB2
to R
FB1
calculates as 7:1. Standard
values of 6.98 k
Ω and 1.00 kΩ are chosen. Other values could be used as long as the 7:1 ratio is maintained.
Frequency Selection:
At the minimum input voltage, the maximum switching frequency of LM5017 is restricted by the forced minimum
off-time (T
off-time (T
OFF(MIN)
) as given by:
Similarly, at maximum input voltage, the maximum switching frequency of LM5017 is restricted by the minimum
T
T
ON
as given by:
Resistor R
ON
sets the nominal switching frequency based on the following equations:
(1)
where K = 1 x 10
–10
. Operation at high switching frequency results in lower efficiency while providing the smallest
solution. For this example a conservative 200 kHz was selected, resulting in R
ON
= 504 k
Ω. Selecting a standard
value for R
ON
= 499 k
Ω results in a nominal frequency of 202 kHz.
Inductor Selection:
The minimum inductance is selected to limit the output ripple to 20 to 40 percent of the maximum load current. In
addition, the peak inductor current at maximum load should be smaller than the minimum current limit as given in
addition, the peak inductor current at maximum load should be smaller than the minimum current limit as given in
table. The inductor current ripple is given by:
The maximum ripple is observed at maximum input voltage. Substituting V
IN
= 95 V and
Δ
IL
= 40 percent x I
OUT
(max)
results in L1 = 198.4
μH. The next higher standard value of 220 μH is chosen. The peak-to-peak minimum
and maximum inductor current ripples of 35 mA and 204 mA are given at minimum and maximum input voltages
respectively. The peak inductor and switch current is given by
respectively. The peak inductor and switch current is given by
which is smaller than the minimum current limit. The inductor should be able to withstand the maximum current
limit of 1.3 A, which can be reached during startup and overload conditions.
limit of 1.3 A, which can be reached during startup and overload conditions.
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13
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