LT3745 DESIGN GUIDE

# LT3745 Design Guide

## Overview

This is a design guide for the LT3745 LED driver.

The LT3745 LED driver by Linear Technology is a advanced driver which can individually control the current for up to 16 LED circuits. It features an on-board buck converter (except for the inductor and caps), a current regulator, and 16 PWM modules to fine tune the current for each circuit. The LT3745 LED driver by Linear Technology is a advanced driver which can individually control the current for up to 16 LED circuits. Image from http://cds.linear.com/docs/en/datasheet/3745f.pdf.

## RT And The Switching Frequency

The LT3745 datasheet gives a table of common resistances and the corresponding switching frequency, but it does not give you an equation to work it out. I grabbed the values from the datasheet, plotted them, and fitted a trend-line to get an equation, as shown in the following image (but note, Excel has rounded the co-efficient has been rounded too-much, see the equation again below the image for better accuracy): A graph of the resistance of RT, versus the switching frequency for the LT3745 LED driver IC. The power-based equation fits the data well! Note that Excel rounded the co-efficient shown by too great an amount!

The resulting power-based equation is (with better accuracy):

$$R_T = \frac{2.25167e^{11}}{f_{sw(act)}^{1.114}}$$

which had a regression coefficient of $$R^2 = 0.9994$$, not a bad fit!

## Be Careful When Probing

It could be said that the LT3745 is poorly designed when it comes to the separation of low and high-voltage pins. The design places +5V logic pins right next to the LED supply line, which can be in excess of +50V. On the small QFN-40 package, this poses a risk if you are probing pins (e.g. with an oscilloscope or multimeter), because you can quite easily fry things if you short the two together. Like this page? Upvote with shurikens!