lt3742 Linear Technology Corporation, lt3742 Datasheet - Page 14

• Manufacturer Part Number: lt3742
• Description: Dual, 2-phase Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

LT3742
APPLICATIONS INFORMATION
value calculated above, and then recheck the amount of
ripple current:
The DC resistance (DCR) of the inductor can have a
signifi cant impact on total system effi ciency, as it causes
an I
and current rating when choosing an inductor. Table 1
shows several recommended inductor vendors. Each of-
fers numerous devices in a wide variety of values, current
ratings, and package sizes.
Table 1. Recommended Inductor Manufacturers
Maximum Output Current (R
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:
Rearranging the equation above to solve for RSENSE
gives:
14
I
I
R
RIPPLE
OUT MAX
2
SENSE
R
DCR
(
NEC-Tokin
=
power loss. Consider inductance value, DCR,
VENDOR
Sumida
=
Würth
)
Toko
TDK
V
=
I
OUT MAX
IN
R
50
–
SENSE
L
(
V
mV
OUT
50
)
+
–
mV
•
⎛
⎝ ⎜
I
V
RIPPLE
I
V
OUT
RIPPLE
IN
2
SENSE
2
www.nec-tokinamerica.com
•
500
⎞
⎠ ⎟
www.we-online.com
Value Selection)
www.sumida.com
www.toko.com
1
www.tdk.com
kHz
WEBSITE
Inductor, Catch Diode and MOSFET Current Rating
Once the inductor and R
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:
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 suffi cient 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
Diodes, Inc.
www.diodes.com
Microsemi
www.microsemi.com
On Semiconductor
www.onsemi.com
Power MOSFET Selection
There are several important parameters to consider when
choosing an N-channel power MOSFET: drain current
(maximum I
V
reverse transfer capacitance (C
I
RATED
GS
); threshold voltage (V
≥
R
70
SENSE
D
mV
); breakdown voltage (maximum V
+
⎛
⎝ ⎜
V
L
IN
SENSE
GS(TH)
•
100
values have been chosen, the
DEVICE
PDS540 (5A, 40V)
SBM1040 (10A, 40V)
UPS340 (3A, 40V)
UPS840 (8A, 40V)
MBRD320 (3A, 20V)
MBRD340 (3A, 40V)
ns
RSS
); on-resistance (R
⎞
⎠ ⎟
); and total gate charge
DS(ON)
DS
and
3742f
);