Datasheet

Design Guide

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Note that if additional TRIAC holding current is to be sourced through Q1, then the transistor will need to

be sized appropriately to handle the additional current and power dissipation requirements.

Auxiliary Winding Bias Circuit

For high efficiency during steady-state operation, an additional winding is used to establish an auxiliary

voltage V

AUX

used to provide a V

bias voltage. A minimum value of 13V is recommended for V

AUX

. An

auxiliary transformer turns ratio n

AUX

and corresponding turns calculation is used to size the primary

auxiliary winding N

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The minimum primary bias supply capacitance (C7||C8), given a minimum V

ripple specification at twice

the line frequency f

, is calculated to keep V

above UVLO at the worst-case current,

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Input Capacitance

The input capacitor of the flyback (C1) has to be able to provide energy during the worst-case switching

period at the peak of the AC voltage input. C1 should be a high frequency, high stability capacitor (usually

a metallized film capacitor, either polypropylene or polyester) with an AC voltage rating equal to the

maximum input voltage. C1 should also have a DC voltage rating exceeding the maximum peak input

voltage + half of the peak to peak input voltage ripple specification. The minimum required input

capacitance is calculated given the same ripple specification,

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Output Capacitance

C3 should be a high quality electrolytic capacitor with a voltage rating greater than the specified over-

voltage protection threshold V

OVP

. Given the desired voltage ripple, the minimum output capacitance is

calculated,

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AN-2090 LM3448 -120VAC, 6W Isolated Flyback LED Driver SNOA554C–October 2011–Revised May 2013

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