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LT3748

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LPRI≤VIN(MIN)•(VOUT+VF(DIODE))•NPS/(fSW(MIN)•ILIM•

((VOUT + VF(DIODE)) • NPS + VIN(MIN)))

LPRI ≥ (VOUT + VF(DIODE)) • RSENSE • 400ns • NPS/15mV

LPRI ≥ VIN(MAX) • RSENSE • 200ns/15mV

For this application, the primary inductance with a 2:1

transformer and a 0.016Ω sense resistor for an 6.25A

current limit is bounded by the minimum desired switch-

ing frequency and the minimum off time requirement to

be between 9.6μH and 11.5μH. Looking at Table 1, there

are no transformers that fit that exact requirement. For the

sake of prototyping, a transformer with slightly less than the

desired primary inductance is selected with the PA3177NL.

The application will need to be tested thoroughly for sta-

bility at higher input voltages and when the current limit

is at a minimum (in the middle of the output load range).

The easiest solution to ease the requirement on minimum

on-time is to reduce the maximum VIN voltage although

alternatively NPS could be increased at the expense of ef-

ficiency (and requiring a more thorough redesign).

4. Select a MOSFET Switch

The selected 2:1 transformer requires a nominal 55V rating

on the MOSFET switch, assuming no leakage inductance.

However, even a small amount of leakage inductance may

cause the drain to ring to double the anticipated voltage,

and generally this needs to be verified in the final design.

However, at currents below 10A it is fairly easy to find a

MOSFET with sufficiently low RDS(ON) to be a very small

contributor to maximum load efficiency losses while

similarly having a low enough QG to require minimum

current and minimal losses when driving the MOSFET at

lighter loads. Also, while considering the efficiency gains

and losses with a given MOSFET, it is important to real-

ize that a trade-off in RDS(ON) for VDS(MAX) may backfire

if a snubber needs to be added to the circuit to meet the

voltage requirements and dissipates more energy than the

difference in switch resistance. For that reason, a Vishay

Si7738 is selected to give lots of margin with its 150V

rating. The RMS current in the MOSFET can be calculated,

squared and multiplied by the RDS(ON) to calculate losses

and the current required to drive the FET at frequency can

be determined, by the following equations:

IMOSFET(RMS) = √ILIM2 • D/3

IINTVCC = fSW • QG

PINTVCC = IINTVCC • (VIN – VINTVCC)

In this application the MOSFET RMS current at maximum

load is about 2.7A, which into the 0.038Ω RDS(ON) will be

0.28W, or on the order of 2% loss in efficiency. Assuming

that the maximum operating frequency is around four

times higher than the maximum load frequency (at about a

quarter the output load) and reading the approximate QG at

7V operation from the Vishay data sheet, the approximate

INTVCC current is likely close to 8mA, dissipating 0.04W

when the load is on the order of 2.5W, or less than 2%,

and much less at maximum load.

5. Select the Output Diode

The output diode reverse voltage, as calculated earlier, is the

first important specification for the output diode. As with

the MOSFET, choosing a diode with enough margin should

preclude the use of a snubber. The second criterion is the

power requirement of the diode which is more difficult to

correctly ascertain—some manufacturers give direct data

about power dissipation versus duty cycle, which can be

used with the data from the table to determine. To avoid

using a snubber, a diode with a 60V reverse-bias capabil-

ity and minimal forward drop was selected—in this case,

the Diodes Inc. SBR 8U60P5. In this particular application

where maximizing efficiency is the goal, minimizing the

maximum voltage requirement on VIN may allow the use

of a diode with a lower reverse bias rating and a lower

forward drop which could further increase efficiency. Al-

ternatively, if no efficient diode is available for a particular

reverse bias rating, it may be more beneficial to increase

the windings ratio until a diode with low forward drop can

be selected and then reevaluate whether that solution with

higher RMS diode current is beneficial.

APPLICATIONS INFORMATION

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