Texas Instruments LM5019 Isolated Bias Supply Evaluation Board LM5019ISOEVAL/NOPB LM5019ISOEVAL/NOPB Datenbogen
Produktcode
LM5019ISOEVAL/NOPB
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
SNVS788E – 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
100 mA
Switching Frequency
400 kHz
R
FB1
, R
FB2
:
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 LM5019 is restricted by the forced minimum
off-time (T
off-time (T
OFF(MIN)
) as given by:
(6)
Similarly, at maximum input voltage, the maximum switching frequency of LM5019 is restricted by the minimum
T
T
ON
as given by:
(7)
Resistor R
ON
sets the nominal switching frequency based on the following equations:
where
•
K = 1 x 10
–10
(8)
Operation at high switching frequency results in lower efficiency while providing the smallest solution. For this
example a 400 kHz was selected, resulting in R
example a 400 kHz was selected, resulting in R
ON
= 246 k
Ω. Selecting a standard value for R
ON
= 237 k
Ω results
in a nominal frequency of 416 kHz.
Inductor Selection:
The inductance selection is a compromise between solution size, output ripple, and efficiency. The peak inductor
current at maximum load current should be smaller than the minimum current limit of 150 mA. The maximum
permissible peak to peak inductor ripple is:
current at maximum load current should be smaller than the minimum current limit of 150 mA. The maximum
permissible peak to peak inductor ripple is:
ΔIL = 2*(I
LIM(min)
- I
OUT(max)
= 2*50 = 100mA
(9)
The minimum inductance is given by:
(10)
Resulting in L = 215 µH. A standard value of 22 0µH is selected. For proper operation the inductor saturation
current should be higher than the peak encountered in the application. For robust short circuit protection, the
inductor saturation current should be higher than the maximum current limit of 300 mA.
current should be higher than the peak encountered in the application. For robust short circuit protection, the
inductor saturation current should be higher than the maximum current limit of 300 mA.
12
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