R2 =

90 mV x 5

20 PA

= 22.48 k:

R1 =

200 mV

700 mA

= 286 m:

HYS

ILIM

DIM

CS

HG

SNS

VIN

GND

R2

1%

R3

+

R1

290 m

1%

0.25W

Q1

40V

1.8A

D1

40V

1A

L1

33 PH

C2*

1 PF

50V

690 mA

± 5%

46k

6k

5.

* Optional Component

C1

2.2 PF

50V

LM3401

GND

VIN

CS

LED+

SGND

IC

LM3401

SNVS516C – AUGUST 2007 – REVISED MAY 2013

www.ti.com

Figure 23. Example PCB Layout

DESIGN EXAMPLE

Figure 24. Example Circuit

The following design example is intended to illustrate the step-by-step design process described in the previous

sections. The example refers to the circuit in Figure 24, and the results are summaized in Table 1. The resulting

circuit will drive a string of 2 Luxeon V Star LEDs at 700 mA from an input voltage between 18V and 35V.

The example LEDs have a maximum DC current rating of 700 mA, a forward voltage of 5.4V to 8.3V, and a

maximum peak current rating of 1.0A.

First, set the LED DC current with R1:

(23)

And the required wattage is:

(24)

Select a standard value of 290 m

Ω, 1/4W resistor, which will result in a 690 mA LED DC current.

(25)

Which gives a maximum R2 value of:

(26)

Next, a preferred switching frequency of 1 MHz is selected for this example.

16

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