User manual

V+

R22

LM3448

6VCC

Design Guide

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Figure 33. Bias Supply Circuit

Input Capacitance

The input capacitors C1 and C10 have to be able to provide energy during the worst-case switching period

at the peak of the AC voltage input. They should be high frequency, high stability capacitors (usually

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

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

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

capacitance is calculated given the same ripple specification,

(27)

Output Capacitance

C3 should be a high quality electrolytic capacitor with a voltage rating greater than the specified LED stack

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

(28)

13.3 Input Filter

Background

Since the LM3448 is used for AC to DC systems, electromagnetic interference (EMI) filtering is critical to

pass the necessary standards for both conducted and radiated EMI. This filter will vary depending on the

output power, the switching frequencies, and the layout of the PCB. There are two major components to

EMI: differential noise and common-mode noise. Differential noise is typically represented in the EMI

spectrum below approximately 500kHz while common-mode noise shows up at higher frequencies.

AN-2127 LM3448 A19 Edison Retrofit Evaluation Board SNOA559B–October 2011–Revised May 2013

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