R2 =

ILED_RIP x RSNS x 5

40 PA

ILED_RIP =

SNSHYS x 2

RSNS

R2 =

SNSHYS x 5

20 PA

ILED_PK =

ILED_RIP

2

ILED +

RSNS =

VSNS

ILED

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

application schematic. The SNS resistor (RSNS) can be calculated as follows:

(1)

Where VSNS is 200 mV typically, and ILED is the DC average LED current.

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

calculated (in Watts) as follows:

WRSNS = VSNS x ILED

(2)

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

While RSNS sets the DC LED current, the AC peak LED current will be higher than the DC setting. This peak

current must not be greater than the maximum peak current rating of the LED, ILED_max. Peak LED current can be

calculated from the equation below:

(3)

The LED ripple current, ILED_RIP, is described below in the Hysteresis Adjust section.

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

must be set small enough to keep the peak LED current below its maximum rating, ILED_max.

The maximum SNS pin hysteresis can be calculated as shown:

SNSHYS_MAX = (ILED_max - ILED) x RSNS

(4)

Any SNSHYS value between 10mV and this maximum is acceptable.

The SNSHYS value is set with a single resistor from the HYS pin to GND, shown as R2 in the typical application

schematic. The HYS pin voltage, VHYS, is internally multiplied by SNSHYS_MU (0.2 typical) to generate the

hysteresis at the SNS pin, SNSHYS. The hysteresis setting resistor can be determined from the following

equation:

(5)

Where 20 µA is the typical HYS source current, and SNSHYS is the resulting SNS pin hysteresis voltage. The

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

SNSHYS value defines both the upper and lower threshold of the SNS pin. For example, with a VHYS setting of

100 mV, SNSHYS will be 20 mV. Therefore, the total hysteretic window will be 40 mV, or 20 mV above and 20mV

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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