Datasheet

LT3741/LT3741-1
13
37411fg
For more information www.linear.com/LT3741
APPLICATIONS INFORMATION
Programming Inductor Current
The analog voltage at the CTRL1 pin is buffered and pro-
duces a reference voltage, V
CTRL
, across an internal resistor.
The regulated average inductor current is determined by:
I
O
=
V
CTRL1
30 •R
S
where R
S
is the external sense resistor and I
O
is the aver-
age inductor current, which is equal to the output current.
Figure 2
shows the maximum output current vs R
S
. The
maximum power dissipation in the resistor will be:
P
RS
=
0.05V
( )
2
R
S
Table 1 contains several resistors values, the correspond-
ing maximum current and power dissipation in the sense
resistor
. Susumu, Panasonic and Vishay offer accurate
sense resistors. Figure 3 shows the power dissipation in R
S
.
Table 1. Sense Resistor Values
MAXIMUM OUTPUT
CURRENT (A) RESISTOR, R
S
(mΩ) POWER DISSIPATION (W)
1 50 0.05
5 10 0.25
10 5 0.5
25 2 1.25
Inductor Selection
Size the inductor to have approximately 30% peak-to-peak
ripple. The overcurrent set point is equal to the high level
regulated current level set by the CTRL1 pin with an addi
-
tional 23
mV offset between the SENSE
+
and SENSE
–
pins.
The saturation current for the inductor should be at least
20% higher than the maximum regulated current. The fol
-
lowing equation sizes the inductor for best performance:
L =
V
IN
• V
O
– V
O
2
0.3• f
S
•I
O
• V
IN
where V
O
is the output voltage, I
O
is the maximum regulated
current in the inductor and f
S
is the switching frequency.
Using this equation, the inductor will have approximately
15% ripple at maximum regulated current.
Table 2. Recommended Inductor Manufacturers
VENDOR WEBSITE
Coilcraft www.coilcraft.com
Sumida www.sumida.com
Vishay www.vishay.com
Wurth Electronics www.we-online.com
NEC-Tokin www.nec-tokin.com
R
S
(mΩ)
0
0
POWER DISSIPATION (W)
0.2
0.6
0.8
1.0
1.4
2
10
14
3741 F03
0.4
1.2
8
18
20
4
6
12 16
R
S
(mΩ)
0
MAXIMUM OUTPUT CURRENT (A)
10
20
30
5
15
25
4 8 12 16
3741 F02
20
20 6 10 14 18
Figure 2. R
S
Value Selection for Regulated Output Current
Figure 3. Power Dissipation in R
S
Switching MOSFET Selection
When selecting switching MOSFETs, the following pa-
rameters are critical in determining the best devices for
a given application
: total gate charge (Q
G
), on-resistance
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