R2 =

20 PA

2

VCS

10V/Div

VSNS

100 mV/Div

ILED

100 mA/Div

400 ns/DIV

LM3401

www.ti.com

SNVS516B – AUGUST 2007 – REVISED OCTOBER 2011

Figure 3. Hysteretic Switching Waveforms

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)

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