Datasheet

LT3742
14
3742fa
value calculated above, and then recheck the amount of
ripple current:
I
RIPPLE
=
V
IN
– V
OUT
L
•
V
OUT
V
IN
•
1
500kHz
The DC resistance (DCR) of the inductor can have a sig-
nificant impact on total system efficiency, as it causes an
I
2
R
DCR
power loss. Consider inductance value, DCR, and
current rating when choosing an inductor. Table 1 shows
several recommended inductor vendors. Each offers
numerous devices in a wide variety of values, current
ratings, and package sizes.
Table 1. Recommended Inductor Manufacturers
VENDOR WEBSITE
Sumida www.sumida.com
Toko www.toko.com
Würth www.we-online.com
NEC-Tokin www.nec-tokinamerica.com
TDK www.tdk.com
Maximum Output Current (R
SENSE
Value Selection)
Maximum output current is determined largely by the
values of the current sense resistor, RSENSE (which sets
the inductor peak current), and the inductor (which sets
the inductor ripple current). The LT3742 current compara-
tor has a guaranteed minimum threshold of 50mV, which
does not vary with duty cycle. The maximum output cur-
rent is calculated:
I
OUT(MAX)
=
50mV
R
SENSE
–
I
RIPPLE
2
Rearranging the equation above to solve for RSENSE
gives:
R
SENSE
=
50mV
I
OUT(MAX)
+
I
RIPPLE
2
⎛
⎝
⎜
⎞
⎠
⎟
Inductor, Catch Diode and MOSFET Current Rating
Once the inductor and R
SENSE
values have been chosen, the
current ratings of the inductor, catch diode and MOSFET
can then be determined. The LT3742 current comparator
has a guaranteed maximum threshold of 70mV, and there
is a small amount of current overshoot resulting from
the response time of the current sense comparator. The
components should be rated to handle:
I
RATED
≥
70mV
R
SENSE
+
V
IN
L
• 100ns
⎛
⎝
⎜
⎞
⎠
⎟
Schottky Catch Diode Selection
During output short-circuits, the diode will conduct current
most of the time, so it is important to choose a device
with a sufficient current rating. In addition, the diode must
have a reverse voltage rating greater than the maximum
input voltage. Many surface mount Schottky diodes are
available in very small packages. Read their data sheets
carefully as they typically must be temperature derated.
Basically, excessive heating prevents them from being
used effectively at their rated maximum current. A few
recommended diodes are listed in Table 2.
Table 2. Recommended Schottky Diodes
VENDOR DEVICE
Diodes, Inc.
www.diodes.com
PDS540 (5A, 40V)
SBM1040 (10A, 40V)
Microsemi
www.microsemi.com
UPS340 (3A, 40V)
UPS840 (8A, 40V)
On Semiconductor
www.onsemi.com
MBRD320 (3A, 20V)
MBRD340 (3A, 40V)
Power MOSFET Selection
There are several important parameters to consider when
choosing an N-channel power MOSFET: drain current
(maximum I
D
); breakdown voltage (maximum V
DS
and
V
GS
); threshold voltage (V
GS(TH)
); on-resistance (R
DS(ON)
);
reverse transfer capacitance (C
RSS
); and total gate charge
applicaTions inForMaTion