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LT3748

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6. Select the Feedback Resistor for Proper Output

Voltage

Using the iterative process laid out earlier in the Applications

Information section, select the feedback resistor RFB and

program the output voltage to 5V. Adjust the RTC resistor

for temperature compensation of the output voltage. RREF

is selected as 6.04k.

7. Select the Output Capacitor

The output capacitor should be chosen to minimize the

output voltage ripple while considering the increase in

size and cost of a larger capacitor. The following equation

calculates the output voltage ripple:

ΔVMAX =

LPRI •ILIM2

2• COUT •VOUT

8. Add Snubber Circuitry as Necessary

With the primary components selected, the application

should be constructed to evaluate ringing at the drain of the

MOSFET switch and to evaluate step response to optimize

the compensation network. If using an RC snubber, the

equations from the Applications Information section can

be used or a rough estimate of component values may

come from using the published leakage inductance of the

transformer and selecting a snubber capacitor ranging

from 1 to 3 times larger than the published MOSFET output

capacitance. In this application, the peak MOSFET drain

voltage was measured at maximum load from minimum

VIN and exceeded the 150V rating of the Si7738. A DZ

APPLICATIONS INFORMATION

clamp was considered in order to maximize efficiency but

was unable to turn on fast enough to sufficiently clamp the

very fast leakage spike. The final solution is an RC snubber,

implemented iteratively, that decreases efficiency by less

than 1% across the majority of the output load range while

reducing the worst-case drain voltage spike to just 80V.

Similarly, the anode of the output diode is probed to look

at potential ringing when the MOSFET switch turns on and

a peak of 45V is measured across the diode. Therefore,

no snubber circuitry is required.

9. Optimize the Compensation Network

To set the compensation, the application is first configured

with a 22nF capacitor and 10k resistor as a starting point. A

load step is applied at both light and heavy loads at the 60V

maximum input voltage and the capacitance is decreased

until damping decreases to the desired limit, in this case

with a compensation capacitance of 2.2nF and a response

implying about 60˚ of phase margin. After verifying stability

at the minimum input voltage, as well, the compensation

capacitance is doubled for safety margin. The series resis-

tance is varied from 5k to 50k but the optimal response is

observed with 24.7k. For best ripple performance, select

a compensation capacitor not less than 1nF, and select a

compensation resistor not greater than 50k.

10. Soft-Start Capacitor and UVLO Resistor Divider

A soft-start capacitor helps during the start-up of the

flyback converter. Select the UVLO resistor divider for

the intended input operation range. These equations are

aforementioned.

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