Datasheet

LT3748
24
3748fb
For more information www.linear.com/LT3748
applications inForMation
Table 3. Voltage Stresses, Output Capability and Diode Current vs Turns Ratio in 48V
IN
to 12V, 2A Application
N
PS
V
DS(MAX)
V
R(DIODE)
D (V
IN
= 48V) D (V
IN
= 36V) I
LIM
(2A OUT AT V
IN
= 36V) I
DIODE(RMS)
(V
IN
= 48V)
1 84 84 0.21 0.26 6 3.3
2 96 48 0.34 0.41 4 3.7
4 120 30 0.51 0.58 3 4.6
6 144 24 0.61 0.68 2 5.2
DESIGN EXAMPLE: 48V
IN
TO 12V, 2A OUT
The second example is a telecom application shown on
the front page of the datasheet. The focus of this applica
-
tion is a cheap, small and simple solution. Table 3 shows
the results of the initial step for selecting the turns ratio.
In this example, the output diode is a much smaller ef
-
ficiency loss due to the smaller voltage drop across it in
ratio to V
OUT
so minimizing output diode current is not
as important. Of greater importance is minimizing the
stresses on the MOSFET and output diode and the 4:1
case seems to be the best compromise for that to avoid
using a snubber on either device.
20µH of primary inductance is required for minimum
off-time while selecting the transformer, but in order to
minimize output ripple at maximum load a 60.8µH trans
-
former is selected
. To
meet the saturation current (12A,
peak, on the secondary windings), a Versa-Pak VP4-0047-R
provides a compact and efficient solution.
For the MOSFET switch, since the input voltage is so high,
resistive losses on the primary side will be very low so
minimizing R
DS(ON)
is of minimum benefit. However, since
the current for the gate drive is pulled from a high V
IN
,
minimizing both Q
G
and operating frequency is essential
unless a third winding is added. The Vishay Si7464DP, with
a 200V V
DS(MAX)
and low gate charge, keeps the INTV
CC
current to just over 3mA, worst-case, which when added
to quiescent current will keep power dissipation in the
LT3748 to just over 1/4W at 72V V
IN
.
The output diode only nominally has 30V of reverse bias
but a B360 diode is selected to ensure enough margin that
a snubber will not be required. A more expensive diode
with lower forward drop might recover several percent
efficiency and if high temperature operation is required
a diode rated for more average current at temperature
might be needed, but the B360 is small and inexpensive.
The rest of the design and
component selection
is straight-
forward.
Suggested Layout
See
Figures 14 and 15 for the DC1557A demo board lay
-
out. Note the proximity of the R
REF
and R
FB
resistors (R9,
R5) to the LT3748 for optimal regulation. The location of
these two resistors as close to the physical pins of the
LT3748 is critical for accurate regulation. In addition, the
high frequency current path from the V
IN
bypass capacitor
(C2) through the primary-side winding, the MOSFET switch
and sense resistor (R10) is a very tight loop. Similarly,
the high frequency current path for the MOSFET gate
switching from the INTV
CC
capacitor through the source of
the MOSFET and sense resistor is similarly small in area.
For improved regulation it is recommended that the user
ensure that the high current ground is kept separate or
at least physically isolated from the small-signal ground
used by the other ground-referenced pins.
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