LT3748 datasheet(23/30 Pages) LINER

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LT3748

3748fa

APPLICATIONS INFORMATION

Table 3. Voltage Stresses, Output Capability and Diode Current vs Turns Ratio in 48VIN to 12V, 2A Application

NPS

VDS(MAX)

VR(DIODE)

D (VIN = 48V)

D (VIN = 36V)

ILIM (2A OUT AT VIN = 36V)

IDIODE(RMS) (VIN = 48V)

0.21

0.26

3.3

0.34

0.41

3.7

120

0.51

0.58

4.6

144

0.61

0.68

5.2

DESIGN EXAMPLE: 48VIN TO 12V, 2A OUT

The second example is a telecom application shown on the

front page of the datasheet. The focus of this application

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 VOUT 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 RDS(ON) is of minimum benefit. However, since

the current for the gate drive is pulled from a high VIN,

minimizing both QG and operating frequency is essential

unless a third winding is added. The Vishay Si7464DP, with

a 200V VDS(MAX) and low gate charge, keeps the INTVCC

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 VIN.

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 13 and 14 for the DC1557A demo board layout.

Note the proximity of the RREF and RFB 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 VIN 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 INTVCC 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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