Datasheet

ûI
L(MAX)
I
LI
(peak) = I
OUT
+
2
= 602 mA
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
LM5017
www.ti.com 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 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 LM5017 is restricted by the forced minimum
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
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 kΩ. Selecting a standard
value for R
ON
= 499 kΩ 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
Electrical Characteristics 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
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.
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