R2 =

40 PA

R2 =

20 PA

2

LM3401

SNVS516C – AUGUST 2007 – REVISED MAY 2013

www.ti.com

OUTPUT CURRENT SETTING

The LED average current is programmed using a resistor between SNS and GND, shown as R1 in the typical

(1)

The sense resistor power rating must be higher than its power dissipation. The required power rating can be

calculated (in Watts) as follows:

(2)

When selecting a sense resistor, thermal de-rating must also be taken into consideration.

calculated from the equation below:

(3)

HYSTERESIS ADJUST

Adjustable hysteresis (via the HYS pin) provides direct control over the LED ripple current. The HYS pin also

gives some control over the switching frequency. Although the hysteresis value can be set after the inductor is

selected, a preliminary value must be set in order to begin the frequency calculation. The hysteresis window

The maximum SNS pin hysteresis can be calculated as shown:

(4)

equation:

(5)

hysteresis voltage can be set within a range of 10 mV to 100 mV (50 mV to 500 mV at the HYS pin). The

below the 200 mV reference voltage. This directly correlates to peak-to-peak inductor and LED ripple current,

approximated by the following equation:

(6)

If LED ripple current is a design priority, the preliminary R2 value can be determined using a target LED ripple

current as shown:

(7)

A more precise equation for ripple current is given in the Inductor Selection section. Once an inductor is selected

the more precise equation should be used to determine the actual ripple current and LED peak current. Larger

hysteresis values will result in lower switching frequency and higher ripple current for a given inductor. Typical

examples are shown in Figure 16 below.

8

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