User manual

COFF

R16

C12

www.ti.com

Design Guide

Inductor ripple current will need to be specified by the user based on desired EMI performance, inductor

size and other operating conditions. The following equations show how to calculate for maximum and

minimum inductor ripple currents respectively by basing the ripple (i.e.Δi

L(%)

as a percentage of maximum

peak inductor currents,

(6)

It is recommended that this buck converter design operate in CCM over the full range of operating peak

input voltages, and so the minimum inductor peak current at V

IN-PK(MIN)

should not go below zero,

(7)

The inductor value can be calculated based on the minimum on-time, LED output voltage and the

specified inductor ripple current Δi

L-PK(VIN-PK-MAX)

at the maximum peak input voltage as described below,

(8)

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 C12 will be charged with current from the V

supply through resistor R16. The COFF pin

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

(9)

where,

(10)

Solving for off-time t

OFF

results in,

(11)

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

Submit Documentation Feedback