Datasheet

R1 =

200 mV

LED

LED String Options

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7 LED String Options

The default LED current setting is 340 mA, typical of many 1W LEDs. To adjust this value, the current

setting resistor, R1, can be changed according to Table 1. For other values, use the equation below:

(1)

R1 must be rated to handle the power dissipation of the LED current. For example, in steady state R1 will

dissipate (0.340

x 0.59) = 68 mW, indicating that a resistor with a 1/8W power rating would be

appropriate.

The LM3401 demo board can drive a wide variety of LEDs. Ripple current and switching frequency will

vary with the LED forward voltage, number of LEDs, and input voltage. Table 2 shows the typical results

for some common LED strings when using the Table 1 settings at an input voltage of 24V. Typical

switching frequency vs input voltage is shown in Figure 5.

For some LED strings, the demo board settings given in Table 1 may not be appropriate. For example,

where the switching frequency increases above the 1.5 MHz maximum or where LED ripple current is

higher than the maximum LED rating. In this case, refer to LM3401 Hysteretic PFET Controller for High

Power LED Drive (SNVS516) to modify the demo board as needed.

The LM3401 can also drive LED currents in excess of 1A. For these applications a larger pad is provided

for a higher current PFET. An example BOM for a typical 3.5A application is shown in Table 3. The

switching frequency, ripple current, and current limit values will depend on the number and type of LEDs

installed, as well as the selected PFET. A low Vgs threshold PFET is recommended for higher current

applications, as the initial HG voltage will decrease somewhat when driving PFETs with a larger gate

capacitance. For detailed design information, refer to LM3401 Hysteretic PFET Controller for High Power

LED Drive (SNVS516).

Table 3. Bill of Materials for 3.5A Application (Vin = 5V to 20V)

Ref. # Description Part Number Size Vendor

IC1 LED Controller LM3401 VSSOP-8 Texas Instruments

L1 5.2 µH, 5A, Inductor MSS1038-522NL 10.2 × 10.4 Coilcraft

Q1 20V, 4A, 25 mΩ, PFET Si3493BDV SO-6 Vishay

D1 30V, 4A, Schottky SSB43L SMB Vishay

C1 25V, 10 µF, ceramic C3225X5R1E106K 1210 TDK

C2 50V, 33 µF, electrolytic EEEHA1H330XP 6.6 × 6.6 Panasonic

C3 25V, 220 pF, ceramic - 0805 -

R1 0.0576Ω, 1%, 1W WSL2512R0576F 2512 Vishay

R2 4.7 kΩ, 1% - 0805 -

R3 40 kΩ,1% - 0805 -

AN-1657 LM3401 Demo Board SNVA254C–August 2007–Revised April 2013

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