Datasheet

V
IN
- V
OUT
ûI
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
LM5018
SNVS787E –JANUARY 2012–REVISED DECEMBER 2013 www.ti.com
APPLICATION INFORMATION
Selection of External Components
Selection of external components is illustrated through a design example. The design example specifications are
as follows:
Buck Converter Design Specifications
Input Voltage Range 12.5 V to 95 V
Output Voltage 10 V
Maximum Load Current 300 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 kΩ 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 LM5018 is restricted by the forced minimum
off-time (T
OFF(MIN)
) as given by:
Similarly, at maximum input voltage, the maximum switching frequency of LM5018 is restricted by the minimum
T
ON
as given by:
Resistor R
ON
sets the nominal switching frequency based on the following equations:
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 400 kHz was selected, resulting in R
ON
= 246 kΩ. Selecting a standard
value for R
ON
= 237 kΩ 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 350 mA. The maximum
permissible peak to peak inductor ripple is:
ΔIL = 2*(I
LIM(min)
— I
OUT(max)
) = 2*50 = 100mA
The minimum inductance is given by:
Resulting in L = 215 µH. A standard value of 220 µH 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 700 mA.
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