Datasheet

R23

C20

COFF

Design Guide

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12.4 COFF Current Source

The current source used to establish the constant off-time is shown in Figure 32.

Figure 32. COFF Current Source Circuit

Capacitor C20 will be charged with current from the V

supply through resistor R23. The COFF pin

threshold will therefore be tripped based on the following capacitor equation,

(36)

where,

(37)

Solving for off-time t

OFF

results in,

(38)

and we also know that the t

OFF

is calculated where T

is the switching period,

(39)

Re-arranging and substituting equations results in the following equation where COFF is typically chosen

as value around 330pF,

(40)

12.5 TRIAC Holding Circuit

An optional TRIAC holding current circuit is also provided on the evaluation board as shown in Figure 33.

The DIM pin signal is applied through an RC filter as a varying DC voltage to Q4 such that the voltage on

the FLTR2 pin is adjusted and additional holding current can be sinked.

AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver SNOA555C–April 2011–Revised May 2013

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